One of the hardest parts of building your own version of the Linux kernel is determining exactly which drivers and configuration options are needed for your machine to work properly. This chapter will walk you through this process of finding and selecting the correct drivers. /proc filesystem distributionsusing distribution kernel to determine necessary modules distributionsusing distribution kernelfinding kernel configuration customizing a kernelusing distribution kenelfinding kernel configuration customizing a kernelusing distribution kenel customizing a kernel One of the easiest ways to determine which modules are necessary is to start with the kernel configuration that comes with your distribution's kernel package. It is also much easier to determine which drivers are needed on a running system, where the proper drivers are already bound to the hardware. If you do not already have a Linux distribution installed on the machine that you are building the kernel for, use a LiveCD version of a distribution. This allows you to boot Linux on the machine and determine what kernel configuration options are needed in order to get the hardware working properly. Almost all distributions provide the kernel configuration files as part of the distribution kernel package. Read the distribution-specific documentation for how to find these configurations. It is usually somewhere below the /usr/src/linux/ directory tree. If the kernel configuration is hard to find, look in the kernel itself. Most distribution kernels are built to include the configuration within the /proc filesystem. To determine if this is true for your running kernel, enter: $ ls /proc/config.gz /proc/config.gz If the /proc/config.gz filename is present, copy this file to your kernel source directory and uncompress it: $ cp /proc/config.gz ˜/linux/ $ cd ˜/linux $ gzip -dv config.gz config.gz: 74.9% -- replaced with config Copy this configuration file into your kernel directory and rename it to .config. Then use it as the basis of the kernel configuration to build the kernel as described in Chapter 4. Using this configuration file should always generate a working kernel image for your machine. The disadvantage of this kernel image is that you will have built almost every kernel module and driver that is present in the kernel source tree. This is almost never needed for a single machine, so you can start to turn off different drivers and options that are not needed. It is recommended that you disable only those options that you are sure you do not need, as there might be parts of the system that rely on specific options being enabled. sysfs (virtual filesystem) symlinkssysfs filesystem, to different portions of kernel /proc filesystem/config.gz filename images, kernelgenerating using config.gz file filesystemssysfs (virtual filesystem) distributionsusing distribution kernelfinding modules needed to drive hardware device driversfinding modules needed for your hardware customizing a kernelusing distribution kenelfinding modules needed to drive hardware config.gz file A configuration file that comes from a distribution takes a very long time to build, because of all of the different drivers being built. You want to build only the drivers for the hardware that you have, which will save time on building the kernel, and allows you to build some or all of the drivers into the kernel itself, possibly saving a bit of memory, and on some architectures, making for a faster running system. To cut your drivers down, you need to determine which modules are needed to drive your hardware. We will walk though two examples of how to find out what driver is needed to control what piece of hardware. Several locations on your system store useful information for determining which devices are bound to which drivers in a running kernel. The most important location is a virtual filesystem called sysfs. sysfs should always be mounted at the /sys location in your filesystem by the initialization scripts of your Linux distribution. sysfs provides a glimpse into how the different portions of the kernel are hooked together, with many different symlinks pointing all around the filesystem. In all of the following examples, real sysfs paths and hardware types are shown. Your machine will be different, but the relative locations of information will be the same. Do not be alarmed if the filenames in sysfs are different from your machine; it is to be expected. Additionally, the internal structure of the sysfs filesystem constantly changes around, due to the reorganization of devices and rethinking by the kernel developers about how to best display internal kernel structures to userspace. Because of this, over time, some of the symlinks previously mentioned in this chapter might not be present. However, the information is all still there, just moved around a little bit. symlinksoutput to readlink command, putting into basename symlinksfor eth0 device readlink command PCI devicesnetwork interface card, determining network driver, determining (example) menuconfig toolsearching for option to enable module Makefileskernel, searching for CONFIG_ rule that builds a module lo directory ifconfig utility find command eth0, eth1, and eth2 directories device driversfinding modules needed for your hardwaredetermining network driver (example) CONFIG_ rule that builds a module, finding basename program One of the most common and important devices in the system is the network interface card. It is imperative to figure out which driver is controlling this device and enable it in your kernel configuration so that networking works properly. First, work backward from the network connection name to find out which PCI device is controlling it. To do this, look at the different network names: $ ls /sys/class/net/ eth0 eth1 eth2 lo The lo directory represents the network loopback device, network loopback device and is not attached to any real network device. The eth0, eth1, and eth2 directories are what you should pay attention to, as they represent real network devices. To look further at these network devices in order to figure out which you care about, use the ifconfig utility: $ /sbin/ifconfig -a eth0 Link encap:Ethernet HWaddr 00:12:3F:65:7D:C2 inet addr:192.168.0.13 Bcast:192.168.0.255 Mask:255.255.255.0 UP BROADCAST NOTRAILERS RUNNING MULTICAST MTU:1500 Metric:1 RX packets:2720792 errors:0 dropped:0 overruns:0 frame:0 TX packets:1815488 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:100 RX bytes:3103826486 (2960.0 Mb) TX bytes:371424066 (354.2 Mb) Base address:0xdcc0 Memory:dfee0000-dff00000 eth1 Link encap:UNSPEC HWaddr 80-65-00-12-7D-C2-3F-00-00-00-00-00-00- 00-00-00 BROADCAST MULTICAST MTU:1500 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:0 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:0 (0.0 b) TX bytes:0 (0.0 b) eth2 Link encap:UNSPEC HWaddr 00-02-3C-04-11-09-D2-BA-00-00-00-00-00- 00-00-00 BROADCAST MULTICAST MTU:1500 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:0 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:0 (0.0 b) TX bytes:0 (0.0 b) lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:60 errors:0 dropped:0 overruns:0 frame:0 TX packets:60 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:13409 (13.0 Kb) TX bytes:13409 (13.0 Kb) From this list, you can tell that the eth0 device is the network device that is active and working, as can be seen by the lines: eth0 Link encap:Ethernet HWaddr 00:12:3F:65:7D:C2 inet addr:192.168.0.13 Bcast:192.168.0.255 Mask:255.255.255.0 The ouput shows this is an Ethernet device with valid IP (inet) address assigned to it. Now that we have determined that we want to make sure the eth0 device will be working in our new kernel, we need to find which driver is controlling it. This is simply a matter of walking the different links in the sysfs filesystem, which can be done in a one-line command: $ basename `readlink /sys/class/net/eth0/device/driver/module` e1000 The output shows that the module named e1000 is controlling the eth0 network device. The basename command shown compresses the following steps into a single command line: Follow the /sys/class/net/eth0/device symlink into the directory within the /sys/device/ tree that contains the information for the device that controls eth0. Note that the /sys/class/net/eth0 directory might also be a symlink on the newer versions of the kernel. Within the directory that describes the device in sysfs, there is a symlink to the driver bound to this device. That symlink is called driver, so we follow that link. Within the directory that describes the driver in sysfs, there is a symlink to the module that this driver is contained within. That symlink is called module. We want the target of that symlink. To get the target, we use the readlink command, which produces output such as: $ readlink /sys/class/net/eth0/device/driver/module ../../../../module/e1000 Because we care only about the name of the module, we want to strip the rest of the path off the output of the readlink command, and only save the rightmost portion. That is what the basename command does. Executed directly on a pathname, it would produce: $ basename ../../../../module/e1000 e1000 So we put the output of the long symlink traversal to the readlink location into the basename program, enabling the whole process to be done in one line. Now that we have the module name, we need to find the kernel configuration option that controls it. You can look through the different network device configuration menus or search the kernel source code itself to make sure you have the right option: $ cd ˜/linux/linux-2.6.17.8 $ find -type f -name Makefile | xargs grep e1000 ./drivers/net/Makefile:obj-$(CONFIG_E1000) += e1000/ ./drivers/net/e1000/Makefile:obj-$(CONFIG_E1000) += e1000.o ./drivers/net/e1000/Makefile:e1000-objs := e1000_main.o e1000_hw.o e1000_ ethtool.o e1000_param.o Remember to replace the e1000 used for this example with the name of the module that you are looking to find. The important thing to look for in the output of the previous find command is any line that has the term CONFIG_ in it. That is the configuration option that the kernel needs to have enabled in order to build the module. In the above example, the option CONFIG_E1000 is the configuration option that you are looking for. Now you have the information you need to configure the kernel. Run the menu configuration tool: $ make menuconfig Then press the / key (which initiates a search) and type in the configuration option, minus the CONFIG_ portion of the string. This process is shown in Figure 7-1.

The kernel configuration system will then tell you exactly where to select the option to enable this module. See Figure 7-2.

The first item in the display exactly matches what you searched for. The location information in the display tells you that to build the module E1000 into the kernel, and the following configuration option must be enabled: Device Drivers Network device support [*] Network device support Ethernet (1000 Mbit) [*] Intel(R) PRO/1000 Gigabit Ethernet support These steps will work for any type of device active in the kernel. USB (Universal Serial Bus)determining driver for USB-to-serial converter tty files, searching for device sysfs (virtual filesystem)device discovery, use in readlink command discovery of devices, summary of process device driversfinding modules needed for your hardwaresummary of device discovery device driversfinding modules needed for your hardwaredetermining USB device driver (example) class devicesysfs filesystem basename program As another example, let's look at a USB-to-serial converter that is present in our example system. It is currently connected to the /dev/ttyUSB0 port, so you need to look in the sysfs tty section: sysfs (virtual filesystem)tty section $ ls /sys/class/tty/ | grep USB ttyUSB0 You can trace through sysfs for this device to find the controlling module, as shown in the previous section: $ basename `readlink /sys/class/tty/ttyUSB0/device/driver/module` pl2303 Then search the kernel source tree to find the configuration option that you need to enable: $ cd ˜/linux/linux-2.6.17.8 $ find -type f -name Makefile | xargs grep pl2303 ./drivers/usb/serial/Makefile:obj-$(CONFIG_USB_SERIAL_PL2303) += pl2303.o Use the kernel configuration tool, as shown in Figure 7-3, to find the proper option to enable in order to set the CONFIG_USB_SERIAL_PL2303 option.

In our case, this displays the screen shown in Figure 7-4.

This shows exactly where to find the USB Prolific 2303 Single Port Serial Driver option that is needed to control this device properly. In summary, here are the steps needed to find the driver for a device that has a working driver already bound to it: Find the proper sysfs class device that the device is bound to. Network devices are listed in /sys/class/net and tty devices in /sys/class/tty. Other types of devices are listed in other directories in /sys/class, depending on the type of device. Trace through the sysfs tree to find the module name that controls this device. It will be found in the /sys/class/class_name/device_name/device/driver/module, and can be displayed using the readlink and basename applications: $ basename `readlink /sys/class/class_name/device_name/device/driver/ module` Search the kernel Makefiles for the CONFIG_ rule that builds this module name by using find and grep: $ find -type f -name Makefile | xargs grep module_name Search in the kernel configuration system for that configuration value and go to the location in the menu that it specifies to enable that driver to be built. symlinksfollowing to module names, script for modprobe program modalias files, finding Makefileskernel, searching for CONFIG_ rule that builds a module grep command find command device driversfinding modules needed for your hardwarescript to find all modules CONFIG_ rule that builds a module, finding Now that we have gone through all of the steps of poking around in sysfs and following symlinks to module names, here is a very simple script that will do all of that work, in a different way: #!/bin/bash # # find_all_modules.sh # for i in `find /sys/ -name modalias -exec cat {} \;`; do /sbin/modprobe --config /dev/null --show-depends $i ; done | rev | cut -f 1 -d '/' | rev | sort -u You can download an example file containing this script from the book's web site, provided in the "How to Contact Us" section of the Preface. This script goes through sysfs and finds all files called modalias. The modalias file contains the module alias that tells the modprobe command which module should be loaded to control this device. The module alias is made up of a combination of device manufacturer, ID, class type, and other unique identifiers for that specific type of device. All kernel driver modules have an internal list of devices that they support that is generated automatically by the list of devices the driver tells the kernel it supports. The modprobe looks through this list of devices by all drivers and tries to match it up with the alias it has. If it finds a match, it will then load the module (this procedure is how the automatic driver loading functionality in Linux works). The script has the modprobe program stop before actually loading the module, and just print out what actions it would take. This gives us a list of all of the modules that are needed to control all devices in the system. A little cleaning up of the list, by sorting it and finding the proper field to display, results in this output: $ find_all_modules.sh 8139cp.ko 8139too.ko ehci-hcd.ko firmware_class.ko i2c-i801.ko ieee80211.ko ieee80211_crypt.ko ipw2200.ko mii.ko mmc_core.ko pcmcia_core.ko rsrc_nonstatic.ko sdhci.ko snd-hda-codec.ko snd-hda-intel.ko snd-page-alloc.ko snd-pcm.ko snd-timer.ko snd.ko soundcore.ko uhci-hcd.ko usbcore.ko yenta_socket.ko This is a list of all of the modules that are needed to control the hardware in the machine. The script will also probably print out some error messages that look like: FATAL: Module pci:v00008086d00002592sv000010CFsd000012E2bc03sc00i00 not found. FATAL: Module serio:ty01pr00id00ex00 not found. Which means that it could not find a module that can control that device. Do not be concerned about this, as some devices do not have kernel drivers that will work for them. device driversdetermining correct kernel module from scratch customizing a kerneldetermining correct module from scratch Sometimes you do not have the option of getting a distribution kernel working on a machine in order to determine what kernel modules are needed to drive the hardware. Or you have added new hardware to your system, and you need to figure out what kernel configuration option needs to be enabled to get it to work properly. This section will help you determine how to find that configuration option to get the hardware up and running. The easiest way to figure out which driver controls a new device is to build all of the different drivers of that type in the kernel source tree as modules, and let the udev startup process match the driver to the device. Once this happens, you should be able to work backwards using the steps just discussed to determine the proper driver needed, and then go back and enable just that driver in the kernel configuration. But if you do not want to build all drivers, or this does not work for some reason, it will require a bit more work to determine the proper driver that is needed. The following steps are complex and require digging in the kernel source code at times. Do not be afraid of this; it will only help you understand your hardware and the kernel source better. The steps involved in matching the driver to the device differ depending on the type of device that you are working with. We will discuss the two most common forms of devices in this chapter: PCI Ethernet devicesPCI and USB devices. The methods described here will also work with other types of devices. Also, it is very important for the kernel to be able to find all of the filesystems in the system, the most important one being the root filesystem. We will go into how to do this later in "Root Filesystem." udev startup process sysfs (virtual filesystem)listing PCI device names struct pci_device_id values PCI devicesmatching to driver lspci programleading 0000 in PCI device bus ID lspci program IDsPCI devices, vendor and product PCI devices device driversdetermining correct kernel module from scratchPCI devices are distinguished by vendor ID and device ID; each combination of vendor and device ID could require a unique driver. This is the basis for the research this section shows you. For this example, let's use a PCI network card that is not working with the currently running kernel version. This example will be different from your situation, with different PCI device and bus ID values, but the steps involved should be relevant to any type of PCI device you wish to find a working driver for. First, find the PCI device in the system that is not working. To get a list of all PCI devices, use the lspci program. Because we care only about Ethernet PCI devices, we will narrow our search of the PCI devices by searching only for strings containing the term Ethernet (case-insensitive): $ /usr/sbin/lspci | grep -i ethernet 06:04.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-8139/ 8139C/8139C+ (rev 10) This is the device we wish to get working.Note that you can just try searching through the kernel configuration for a device that matches the string described here, a device from Realtek Semiconductor with a product name of RTL-8139/8139C/8139C+, but this does not always work. That is why we are taking the long way around in this chapter. Almost all distributions place the lspci program in the /usr/sbin/ directory, but some place it in other locations. To find out where it is located, enter: $ which lspci /usr/sbin/lspci If you are using a distribution that puts it somewhere else, please use that path whenever we discuss using lspci. The first few bits of the lspci output show the PCI bus ID for this device, 06:04.0. That is the value we will use when looking through sysfs in order to find out more information about this device. Go into sysfs where all of the different PCI devicesvendor IDsPCI devices product IDsPCI devices device IDsPCI devices are listed, and look at their names: $ cd /sys/bus/pci/devices/ $ ls 0000:00:00.0 0000:00:1d.0 0000:00:1e.0 0000:00:1f.3 0000:06:03.3 0000:00:02.0 0000:00:1d.1 0000:00:1f.0 0000:06:03.0 0000:06:03.4 0000:00:02.1 0000:00:1d.2 0000:00:1f.1 0000:06:03.1 0000:06:04.0 0000:00:1b.0 0000:00:1d.7 0000:00:1f.2 0000:06:03.2 0000:06:05.0 The kernel numbers PCI devices with a leading 0000: that do not show up in the output of the lspci program.Some 64-bit processors will show the leading bus number for PCI devices in the output of lspci, but for the majority of the common Linux machines, it will not show up by default. So add the leading 0000: onto the number that you found using lspci and go into that directory: $ cd 0000:06:04.0 In this directory, you want to know the values of the vendor and device filenames: $ cat vendor 0x10ec $ cat device 0x8139 These are the vendor and device IDs for this PCI device. The kernel uses these values to match a driver to a device properly. PCI drivers tell the kernel which vendor and device IDs they will support so that the kernel knows how to bind the driver to the proper device. Write them down somewhere, as we will refer to them later. Now that we know the vendor and product ID for this PCI device, we need to find the proper kernel driver that advertises that it supports this device. Go back to the kernel source directory: $ cd ˜/linux/linux-2.6.17.8/ The most common location for PCI IDs in the kernel source tree is include/linux/pci_ids.h. Search that file for our vendor product number: $ grep -i 0x10ec include/linux/pci_ids.h #define PCI_VENDOR_ID_REALTEK 0x10ec The defined value here, PCI_VENDOR_ID_REALTEK is what will probably be used in any kernel driver that purports to support devices from this manufacturer. To be safe, also look in this file for our device ID, as it is also sometimes described there: $ grep -i 0x8139 include/linux/pci_ids.h #define PCI_DEVICE_ID_REALTEK_8139 0x8139 That definition will be useful later. Now look for driver source files referring to this vendor definition: $ grep -Rl PCI_VENDOR_ID_REALTEK * include/linux/pci_ids.h drivers/net/r8169.c drivers/net/8139too.c drivers/net/8139cp.c We don't need to look at the first file listed here, pci_ids.h, because that is where we found the original definition. But the files r8139.c, 8139too.c, and 8169cp.c in the drivers/net/ subdirectory should be examined more closely. Open one of these files in an editor and search for PCI_VENDOR_ID_REALTEK. In the file drivers/net/r8169.c, it shows up in this section of code: static struct pci_device_id rtl8169_pci_tbl[] = { { PCI_DEVICE(PCI_VENDOR_ID_REALTEK, 0x8169), }, { PCI_DEVICE(PCI_VENDOR_ID_REALTEK, 0x8129), }, { PCI_DEVICE(PCI_VENDOR_ID_DLINK, 0x4300), }, { PCI_DEVICE(0x16ec, 0x0116), }, { PCI_VENDOR_ID_LINKSYS, 0x1032, PCI_ANY_ID, 0x0024, }, {0,}, }; All PCI drivers contain a list of the different devices that they support. That list is contained in a structure of struct pci_device_id values, just like this one. That is what we need to look at in order to determine whether our device is supported by this driver. The vendor value matches here, but the second value after the vendor is the device value. Our device has the value 0x8139, while this driver supports the device values of 0x8169 and 0x8129 for devices with the vendor ID of PCI_VENDOR_ID_REALTEK. So this driver will not support our device. Moving on to the next file, drivers/net/8139too.c, we find the string PCI_VENDOR_ID_REALTEK in the following bit of code: if (pdev->vendor == PCI_VENDOR_ID_REALTEK && pdev->device == PCI_DEVICE_ID_REALTEK_8139 && pci_rev >= 0x20) { dev_info(&pdev->dev, "This (id %04x:%04x rev %02x) is an enhanced 8139C+ chip\n", pdev->vendor, pdev->device, pci_rev); dev_info(&pdev->dev, "Use the \"8139cp\" driver for improved performance and stability.\n"); } The use of the PCI_VENDOR_ID_REALTEK value here also corresponds with the code that checks whether the PCI device ID matches the PCI_DEVICE_ID_REALTEK_8139 value. If it does, the driver is to print out a message that says: "Use the 8139cp driver for improved performance and stability." Perhaps we should look at that driver next. Even if we did not have such a visible clue, the 8139too.c driver does not have the vendor and device ID pair that we are looking for in a struct pci_device_id variable, so that gives us the clue that it will not support our device. Finally, look at the drivers/net/8139cp.c file. It uses the PCI_VENDOR_ID_REALTEK definition in the following code segment: static struct pci_device_id cp_pci_tbl[] = { { PCI_VENDOR_ID_REALTEK, PCI_DEVICE_ID_REALTEK_8139, PCI_ANY_ID, PCI_ANY_ID, 0, 0, }, { PCI_VENDOR_ID_TTTECH, PCI_DEVICE_ID_TTTECH_MC322, PCI_ANY_ID, PCI_ANY_ID, 0, 0, }, { }, }; MODULE_DEVICE_TABLE(pci, cp_pci_tbl); Here is a use of both our vendor and device ID values in a struct pci_device_id variable. This driver should support our device. Now that we have the driver name, we can work backward, as shown in the first section in this chapter, to find the proper kernel configuration value that should be enabled to build this driver. In summary, here are the steps needed in order to find which PCI driver can control a specific PCI device: Find the PCI bus ID of the device for which you want to find the driver, using lspci. Go into the /sys/bus/pci/devices/0000:bus_id directory, where bus_id is the PCI bus ID found in the previous step. Read the values of the vendor and device files in the PCI device directory. Move back to the kernel source tree and look in include/linux/pci_ids.h for the PCI vendor and device IDs found in the previous step. Search the kernel source tree for references to those values in drivers. Both the vendor and device ID should be in a struct pci_device_id definition. Search the kernel Makefiles for the CONFIG_ rule that builds this driver by using find and grep: $ find -type f -name Makefile | xargs grep DRIVER_NAME Search in the kernel configuration system for that configuration value and go to the location in the menu that it specifies to enable that driver to be built. wirelessUSB device, finding driver vendor IDsUSB devices USB (Universal Serial Bus)finding driver for USB wireless device product IDsUSB devices PCI devicesmatching to driversteps in process lsusb program IDsUSB devices, vendor and product device IDsUSB devices device driversdetermining correct kernel module from scratchUSB devices Finding the specific driver for a USB device is much like finding the driver for a PCI device as described in the previous section, with only minor differences in finding the bus ID values. In this example, let's find the driver that is needed for a USB wireless device. As with the PCI device example, the details in this example will be different from your situation, but the steps involved should be relevant to any type of USB device for which you wish to find a working driver. As with the PCI device, the bus ID must be found for the USB device you wish to find the driver for. To do this, you can use the lsusb program that comes in the usbutils package. The lsusb program shows all USB devices attached to the system. As you do not know what the specific device you're looking for is called, start by looking at all devices: $ /usr/sbin/lsusb Bus 002 Device 003: ID 045e:0023 Microsoft Corp. Trackball Optical Bus 002 Device 001: ID 0000:0000 Bus 005 Device 003: ID 0409:0058 NEC Corp. HighSpeed Hub Bus 005 Device 001: ID 0000:0000 Bus 004 Device 003: ID 157e:300d Bus 004 Device 002: ID 045e:001c Microsoft Corp. Bus 004 Device 001: ID 0000:0000 Bus 003 Device 001: ID 0000:0000 Bus 001 Device 001: ID 0000:0000 The devices with an ID of 0000:0000 can be ignored, as they are USB host controllers that drive the bus itself. Filtering them away leaves us with four devices: $ /usr/sbin/lsusb | grep -v 0000:0000 Bus 002 Device 003: ID 045e:0023 Microsoft Corp. Trackball Optical Bus 005 Device 003: ID 0409:0058 NEC Corp. HighSpeed Hub Bus 004 Device 003: ID 157e:300d Bus 004 Device 002: ID 045e:001c Microsoft Corp. Because USB devices are easy to remove, unplug the device you want to find the driver for and run lsusb again: $ /usr/sbin/lsusb | grep -v 0000:0000 Bus 002 Device 003: ID 045e:0023 Microsoft Corp. Trackball Optical Bus 005 Device 003: ID 0409:0058 NEC Corp. HighSpeed Hub Bus 004 Device 002: ID 045e:001c Microsoft Corp. The third device is now missing, which means the device shown as: Bus 004 Device 003: ID 157e:300d is the device you want to find the driver for. If you replace the device and look at the output of lsusb again, the device number will have changed: $ /usr/sbin/lsusb | grep 157e Bus 004 Device 004: ID 157e:300d This is because the USB device numbers are not unique, but change every time a device is plugged in. What is stable is the vendor and product ID, shown here by lsusb as two four-digit values with a : between them. For this device, the vendor ID is 157e and the product ID is 300d. Write down the values you find, as you will use them in future steps. As with the PCI device, we will search the kernel source code for the USB vendor and product IDs in order to find the proper driver to control this device. Unfortunately, no single file contains all of the USB vendor IDs, as PCI has. So a search of the whole kernel source tree is necessary: $ grep -i -R -l 157e drivers/* drivers/atm/pca200e.data drivers/atm/pca200e_ecd.data drivers/atm/sba200e_ecd.data drivers/net/wireless/zd1211rw/zd_usb.c drivers/scsi/ql1040_fw.h drivers/scsi/ql1280_fw.h drivers/scsi/qlogicpti_asm.c We know this is a USB wireless device, and not an ATM or SCSI device, so we can safely ignore the files found in the atm and scsi directories. That leaves the drivers/net/wireless/zd1211rw/zd_usb.c filename to investigate. zd_usb.c shows the string 157e in the following chunk of code: static struct usb_device_id struct usb_device_id usb_ids[] = { /* ZD1211 */ { USB_DEVICE(0x0ace, 0x1211), .driver_info = DEVICE_ZD1211 }, { USB_DEVICE(0x07b8, 0x6001), .driver_info = DEVICE_ZD1211 }, { USB_DEVICE(0x126f, 0xa006), .driver_info = DEVICE_ZD1211 }, { USB_DEVICE(0x6891, 0xa727), .driver_info = DEVICE_ZD1211 }, { USB_DEVICE(0x0df6, 0x9071), .driver_info = DEVICE_ZD1211 }, { USB_DEVICE(0x157e, 0x300b), .driver_info = DEVICE_ZD1211 }, /* ZD1211B */ { USB_DEVICE(0x0ace, 0x1215), .driver_info = DEVICE_ZD1211B }, { USB_DEVICE(0x157e, 0x300d), .driver_info = DEVICE_ZD1211B }, {} }; Like PCI drivers, USB drivers tell the kernel what devices they support in order for the kernel to bind the driver to the device. This is done by using a struct usb_device_id variable, as shown here. This is a list of the different vendor and product IDs that are supported by this driver. The line: { USB_DEVICE(0x157e, 0x300b), .driver_info = DEVICE_ZD1211 }, shows that our vendor and product IDs are supported by this driver. Once you have the driver name that is necessary to control this device, work backward through the kernel Makefiles, as described earlier in the chapter, to determine how to enable this driver to be built properly. In summary, the steps needed in order to find which USB driver will control a specific USB device are: Find the USB vendor and product ID of device for which you want to find the driver, using lsusb after adding and then removing the device to see what changes in the list. Search the kernel source tree for the vendor and product ID of the USB device. Both the vendor and product ID should be in a struct usb_device_id definition. Search the kernel Makefiles for the CONFIG_ rule that builds this driver by using find and grep: $ find -type f -name Makefile | xargs grep DRIVER_NAME Search in the kernel configuration system for that configuration value and go to the location in the menu that it specifies to enable that driver to be built. root partition, filesystem type mount command The rootfilesystemsroot filesystemdevice driversdetermining correct kernel module from scratchroot filesystem customizing a kerneldetermining correct module from scratchroot filesystem boot processroot filesystem is the filesystem from which the main portion of the running system boots. It contains all of the initial programs that start up the distro, and also usually contains the entire system configuration for the machine. In short, it is very important, and must be able to be found by the kernel at boot time in order for things to work properly. If your newly configured kernel dies at boot time with an error such as: VFS: Cannot open root device hda2 (03:02) Please append a correct "root=" boot option Kernal panic: VFS: Unable to mount root fs on 03:02 then the root filesystem root filesystem wasn't found. If you are not using a ramdisk image at boot time, it is usually recommended that you build both the filesystem that you use for your root partition, and the disk controller for that disk, into the kernel, instead of having it as a module. If you use a ramdisk at boot time, you should be safe building these portions as modules. How can you determine whether you are using a ramdisk at boot time? In Chapter 5 we mention using the distribution installation script to install the kernel versus doing the installation on your own. If you are using the distribution installation script, you are probably using a ramdisk. If you are installing it on your own, you are probably not. The following subsections show how to let the kernel find the root filesystem during boot. First, the type of filesystem that the root partition is using needs to be determined. To do that, look in the output of the mount command: $ mount | grep " / " /dev/sda2 on / type ext3 (rw,noatime) We are interested in the type of the filesystem, which is shown after the word type. In this example, it is ext3. This is the type of filesystem that the root partition is using. Go into the kernel configuration system and make sure that this filesystem type is enabled, as described in Chapter 8. symlinksto logical device controlling block device sda block devicesymlink in device directory pointing to controlling logical device logical device controlling a block device In the output of the mount command shown earlier, the first portion of the line shows which block device the root filesystem is mounted on. In this example, it's /dev/sda2. Now that the filesystem is configured properly in your kernel, you must also make sure that this block device will also work correctly. To find out which drivers are needed for this, you need to look at sysfs again. All block devicessysfs (virtual filesystem)block devices block devices show up in sysfs in either /sys/block or in /sys/class/block, depending on the version of the kernel you are using. In either location, the block devices are a tree, with the different partitions being children of the main device: $ tree -d /sys/block/ | egrep "hd|sd" |-- hdc |-- hdd `-- sda |-- sda1 |-- sda2 |-- sda3 Given the information in the mount command, you need to ensure that the sda2 device is configured properly. Because this is a partition (disk partitions are numbered, while main block devices are not), the whole sda device must be configured. (Without the main block device, there is no way to access the individual partitions on that device.) The sda block device is represented just like the network device we looked at earlier in this chapter. There is a symlink in the device's directory called device that points to the logical device that controls this block device: $ ls -l /sys/block/sda ... device -> ../../devices/pci0000:00/0000:00:1f.2/host0/target0:0:0/0:0:0:0 ... Now you need to start walking up the chain of devices in sysfs to find out which driver is controlling this device: $ ls -l /sys/devices/pci0000:00/0000:00:1f.2/host0/target0:0:0/0:0:0:0 ... driver -> ../../../../../../bus/scsi/drivers/sd ... Here we see that the SCSI disk controller driver SCSI disk controller driver is responsible for making this device work. So we know we need to configure SCSI disk support into our kernel configuration. Continuing up the directory chain in sysfs, try to find where the driver is that controls the hardware: $ ls -l /sys/devices/pci0000:00/0000:00:1f.2/host0/target0:0:0 ... There is no link called driver in this directory, so go back up one more level: $ ls -l /sys/devices/pci0000:00/0000:00:1f.2/host0 ... Again, no driver here. Continuing on up one more level: $ ls -l /sys/devices/pci0000:00/0000:00:1f.2 ... driver -> ../../../bus/pci/drivers/ata_piix ... There! This is the disk controller we need to ensure is in our kernel configuration. So for this root filesystem, we need to enable the ext3, sd, and ata_piix drivers in our kernel configuration so that we will be able to successfully boot our kernel on this hardware. USB (Universal Serial Bus)finding drivers for USB-to-serial device (helper script) sda block devicefinding all drivers for, helper script device driversdetermining correct kernel module from scratchhelper script customizing a kerneldetermining correct module from scratchhelper script class devicescript to find all modules and drivers for block devicesfinding all drivers for sda block device, helper script As mentioned near the beginning of this chapter, files and directories within sysfs change from one release of the kernel to another. Here is a script that is handy in determining the needed kernel driver and module module name for any device node in the system. It has been developed with the kernel developers responsible for sysfs and should successfully work on all future versions of the 2.6 kernel. For instance, it makes short work of the previous example, when you had to get all of the proper drivers for the sda block device: $ get-driver.sh sda looking at sysfs device: /sys/devices/pci0000:00/0000:00:1f.2/host0/ target0:0:0/0:0:0:0 found driver: sd found driver: ata_piix I can also find all of the proper drivers needed for complex things such as USB-to-serial devices: $ get-driver.sh ttyUSB0 looking at sysfs device: /sys/devices/pci0000:00/0000:00:1d.3/usb4/4-2/4-2.3/ 4-2.3:1.0/ttyUSB0 found driver: pl2303 from module: pl2303 found driver: pl2303 from module: pl2303 found driver: usb from module: usbcore found driver: usb from module: usbcore found driver: usb from module: usbcore found driver: uhci_hcd from module: uhci_hcd You can download an example file containing this script from the book's web site, provided in the "How to Contact Us" section of the Preface. The script follows: #!/bin/sh # # Find all modules and drivers for a given class device. # if [ $# != "1" ] ; then echo echo "Script to display the drivers and modules for a specified sysfs class device" echo "usage: $0 <CLASS_NAME>" echo echo "example usage:" echo " $0 sda" echo "Will show all drivers and modules for the sda block device." echo exit 1 fi DEV=$1 if test -e "$1"; then DEVPATH=$1 else # find sysfs device directory for device DEVPATH=$(find /sys/class -name "$1" | head -1) test -z "$DEVPATH" && DEVPATH=$(find /sys/block -name "$1" | head -1) test -z "$DEVPATH" && DEVPATH=$(find /sys/bus -name "$1" | head -1) if ! test -e "$DEVPATH"; then echo "no device found" exit 1 fi fi echo "looking at sysfs device: $DEVPATH" if test -L "$DEVPATH"; then # resolve class device link to device directory DEVPATH=$(readlink -f $DEVPATH) echo "resolve link to: $DEVPATH" fi if test -d "$DEVPATH"; then # resolve old-style "device" link to the parent device PARENT="$DEVPATH"; while test "$PARENT" != "/"; do if test -L "$PARENT/device"; then DEVPATH=$(readlink -f $PARENT/device) echo "follow 'device' link to parent: $DEVPATH" break fi PARENT=$(dirname $PARENT) done fi while test "$DEVPATH" != "/"; do DRIVERPATH= DRIVER= MODULEPATH= MODULE= if test -e $DEVPATH/driver; then DRIVERPATH=$(readlink -f $DEVPATH/driver) DRIVER=$(basename $DRIVERPATH) echo -n "found driver: $DRIVER" if test -e $DRIVERPATH/module; then MODULEPATH=$(readlink -f $DRIVERPATH/module) MODULE=$(basename $MODULEPATH) echo -n " from module: $MODULE" fi echo fi DEVPATH=$(dirname $DEVPATH) done