FPGA Device Feature List (DFL) Framework Overview

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The Device Feature List (DFL) FPGA framework (and drivers according to this this framework) hides the very details of low layer hardwares and provides unified interfaces to userspace. Applications could use these interfaces to configure, enumerate, open and access FPGA accelerators on platforms which implement the DFL in the device memory. Besides this, the DFL framework enables system level management functions such as FPGA reconfiguration.

Device Feature List (DFL) Overview

Device Feature List (DFL) defines a linked list of feature headers within the device MMIO space to provide an extensible way of adding features. Software can walk through these predefined data structures to enumerate FPGA features: FPGA Interface Unit (FIU), Accelerated Function Unit (AFU) and Private Features, as illustrated below:

   Header            Header            Header            Header
+----------+  +-->+----------+  +-->+----------+  +-->+----------+
|   Type   |  |   |  Type    |  |   |  Type    |  |   |  Type    |
|   FIU    |  |   | Private  |  |   | Private  |  |   | Private  |
+----------+  |   | Feature  |  |   | Feature  |  |   | Feature  |
| Next_DFH |--+   +----------+  |   +----------+  |   +----------+
+----------+      | Next_DFH |--+   | Next_DFH |--+   | Next_DFH |--> NULL
|    ID    |      +----------+      +----------+      +----------+
+----------+      |    ID    |      |    ID    |      |    ID    |
| Next_AFU |--+   +----------+      +----------+      +----------+
+----------+  |   | Feature  |      | Feature  |      | Feature  |
|  Header  |  |   | Register |      | Register |      | Register |
| Register |  |   |   Set    |      |   Set    |      |   Set    |
|   Set    |  |   +----------+      +----------+      +----------+
+----------+  |      Header
              +-->+----------+
                  |   Type   |
                  |   AFU    |
                  +----------+
                  | Next_DFH |--> NULL
                  +----------+
                  |   GUID   |
                  +----------+
                  |  Header  |
                  | Register |
                  |   Set    |
                  +----------+

FPGA Interface Unit (FIU) represents a standalone functional unit for the interface to FPGA, e.g. the FPGA Management Engine (FME) and Port (more descriptions on FME and Port in later sections).

Accelerated Function Unit (AFU) represents a FPGA programmable region and always connects to a FIU (e.g. a Port) as its child as illustrated above.

Private Features represent sub features of the FIU and AFU. They could be various function blocks with different IDs, but all private features which belong to the same FIU or AFU, must be linked to one list via the Next Device Feature Header (Next_DFH) pointer.

Each FIU, AFU and Private Feature could implement its own functional registers. The functional register set for FIU and AFU, is named as Header Register Set, e.g. FME Header Register Set, and the one for Private Feature, is named as Feature Register Set, e.g. FME Partial Reconfiguration Feature Register Set.

This Device Feature List provides a way of linking features together, it's convenient for software to locate each feature by walking through this list, and can be implemented in register regions of any FPGA device.

FIU - FME (FPGA Management Engine)

The FPGA Management Engine performs reconfiguration and other infrastructure functions. Each FPGA device only has one FME.

User-space applications can acquire exclusive access to the FME using open(), and release it using close().

The following functions are exposed through ioctls:

  • Get driver API version (DFL_FPGA_GET_API_VERSION)
  • Check for extensions (DFL_FPGA_CHECK_EXTENSION)
  • Program bitstream (DFL_FPGA_FME_PORT_PR)
  • Assign port to PF (DFL_FPGA_FME_PORT_ASSIGN)
  • Release port from PF (DFL_FPGA_FME_PORT_RELEASE)

More functions are exposed through sysfs (/sys/class/fpga_region/regionX/dfl-fme.n/):

Read bitstream ID (bitstream_id)
bitstream_id indicates version of the static FPGA region.
Read bitstream metadata (bitstream_metadata)
bitstream_metadata includes detailed information of static FPGA region, e.g. synthesis date and seed.
Read number of ports (ports_num)
one FPGA device may have more than one port, this sysfs interface indicates how many ports the FPGA device has.
Global error reporting management (errors/)
error reporting sysfs interfaces allow user to read errors detected by the hardware, and clear the logged errors.

FIU - PORT

A port represents the interface between the static FPGA fabric and a partially reconfigurable region containing an AFU. It controls the communication from SW to the accelerator and exposes features such as reset and debug. Each FPGA device may have more than one port, but always one AFU per port.

AFU

An AFU is attached to a port FIU and exposes a fixed length MMIO region to be used for accelerator-specific control registers.

User-space applications can acquire exclusive access to an AFU attached to a port by using open() on the port device node and release it using close().

The following functions are exposed through ioctls:

  • Get driver API version (DFL_FPGA_GET_API_VERSION)
  • Check for extensions (DFL_FPGA_CHECK_EXTENSION)
  • Get port info (DFL_FPGA_PORT_GET_INFO)
  • Get MMIO region info (DFL_FPGA_PORT_GET_REGION_INFO)
  • Map DMA buffer (DFL_FPGA_PORT_DMA_MAP)
  • Unmap DMA buffer (DFL_FPGA_PORT_DMA_UNMAP)
  • Reset AFU (DFL_FPGA_PORT_RESET)
DFL_FPGA_PORT_RESET:
reset the FPGA Port and its AFU. Userspace can do Port reset at any time, e.g. during DMA or Partial Reconfiguration. But it should never cause any system level issue, only functional failure (e.g. DMA or PR operation failure) and be recoverable from the failure.

User-space applications can also mmap() accelerator MMIO regions.

More functions are exposed through sysfs: (/sys/class/fpga_region/<regionX>/<dfl-port.m>/):

Read Accelerator GUID (afu_id)
afu_id indicates which PR bitstream is programmed to this AFU.
Error reporting (errors/)
error reporting sysfs interfaces allow user to read port/afu errors detected by the hardware, and clear the logged errors.

DFL Framework Overview

       +----------+    +--------+ +--------+ +--------+
       |   FME    |    |  AFU   | |  AFU   | |  AFU   |
       |  Module  |    | Module | | Module | | Module |
       +----------+    +--------+ +--------+ +--------+
               +-----------------------+
               | FPGA Container Device |    Device Feature List
               |  (FPGA Base Region)   |         Framework
               +-----------------------+
------------------------------------------------------------------
             +----------------------------+
             |   FPGA DFL Device Module   |
             | (e.g. PCIE/Platform Device)|
             +----------------------------+
               +------------------------+
               |  FPGA Hardware Device  |
               +------------------------+

DFL framework in kernel provides common interfaces to create container device (FPGA base region), discover feature devices and their private features from the given Device Feature Lists and create platform devices for feature devices (e.g. FME, Port and AFU) with related resources under the container device. It also abstracts operations for the private features and exposes common ops to feature device drivers.

The FPGA DFL Device could be different hardwares, e.g. PCIe device, platform device and etc. Its driver module is always loaded first once the device is created by the system. This driver plays an infrastructural role in the driver architecture. It locates the DFLs in the device memory, handles them and related resources to common interfaces from DFL framework for enumeration. (Please refer to drivers/fpga/dfl.c for detailed enumeration APIs).

The FPGA Management Engine (FME) driver is a platform driver which is loaded automatically after FME platform device creation from the DFL device module. It provides the key features for FPGA management, including:

  1. Expose static FPGA region information, e.g. version and metadata. Users can read related information via sysfs interfaces exposed by FME driver.
  2. Partial Reconfiguration. The FME driver creates FPGA manager, FPGA bridges and FPGA regions during PR sub feature initialization. Once it receives a DFL_FPGA_FME_PORT_PR ioctl from user, it invokes the common interface function from FPGA Region to complete the partial reconfiguration of the PR bitstream to the given port.

Similar to the FME driver, the FPGA Accelerated Function Unit (AFU) driver is probed once the AFU platform device is created. The main function of this module is to provide an interface for userspace applications to access the individual accelerators, including basic reset control on port, AFU MMIO region export, dma buffer mapping service functions.

After feature platform devices creation, matched platform drivers will be loaded automatically to handle different functionalities. Please refer to next sections for detailed information on functional units which have been already implemented under this DFL framework.

Partial Reconfiguration

As mentioned above, accelerators can be reconfigured through partial reconfiguration of a PR bitstream file. The PR bitstream file must have been generated for the exact static FPGA region and targeted reconfigurable region (port) of the FPGA, otherwise, the reconfiguration operation will fail and possibly cause system instability. This compatibility can be checked by comparing the compatibility ID noted in the header of PR bitstream file against the compat_id exposed by the target FPGA region. This check is usually done by userspace before calling the reconfiguration IOCTL.

FPGA virtualization - PCIe SRIOV

This section describes the virtualization support on DFL based FPGA device to enable accessing an accelerator from applications running in a virtual machine (VM). This section only describes the PCIe based FPGA device with SRIOV support.

Features supported by the particular FPGA device are exposed through Device Feature Lists, as illustrated below:

  +-------------------------------+  +-------------+
  |              PF               |  |     VF      |
  +-------------------------------+  +-------------+
      ^            ^         ^              ^
      |            |         |              |
+-----|------------|---------|--------------|-------+
|     |            |         |              |       |
|  +-----+     +-------+ +-------+      +-------+   |
|  | FME |     | Port0 | | Port1 |      | Port2 |   |
|  +-----+     +-------+ +-------+      +-------+   |
|                  ^         ^              ^       |
|                  |         |              |       |
|              +-------+ +------+       +-------+   |
|              |  AFU  | |  AFU |       |  AFU  |   |
|              +-------+ +------+       +-------+   |
|                                                   |
|            DFL based FPGA PCIe Device             |
+---------------------------------------------------+

FME is always accessed through the physical function (PF).

Ports (and related AFUs) are accessed via PF by default, but could be exposed through virtual function (VF) devices via PCIe SRIOV. Each VF only contains 1 Port and 1 AFU for isolation. Users could assign individual VFs (accelerators) created via PCIe SRIOV interface, to virtual machines.

The driver organization in virtualization case is illustrated below:

 +-------++------++------+             |
 | FME   || FME  || FME  |             |
 | FPGA  || FPGA || FPGA |             |
 |Manager||Bridge||Region|             |
 +-------++------++------+             |
 +-----------------------+  +--------+ |             +--------+
 |          FME          |  |  AFU   | |             |  AFU   |
 |         Module        |  | Module | |             | Module |
 +-----------------------+  +--------+ |             +--------+
       +-----------------------+       |       +-----------------------+
       | FPGA Container Device |       |       | FPGA Container Device |
       |  (FPGA Base Region)   |       |       |  (FPGA Base Region)   |
       +-----------------------+       |       +-----------------------+
         +------------------+          |         +------------------+
         | FPGA PCIE Module |          | Virtual | FPGA PCIE Module |
         +------------------+   Host   | Machine +------------------+
-------------------------------------- | ------------------------------
          +---------------+            |          +---------------+
          | PCI PF Device |            |          | PCI VF Device |
          +---------------+            |          +---------------+

FPGA PCIe device driver is always loaded first once a FPGA PCIe PF or VF device is detected. It:

  • Finishes enumeration on both FPGA PCIe PF and VF device using common interfaces from DFL framework.
  • Supports SRIOV.

The FME device driver plays a management role in this driver architecture, it provides ioctls to release Port from PF and assign Port to PF. After release a port from PF, then it's safe to expose this port through a VF via PCIe SRIOV sysfs interface.

To enable accessing an accelerator from applications running in a VM, the respective AFU's port needs to be assigned to a VF using the following steps:

  1. The PF owns all AFU ports by default. Any port that needs to be reassigned to a VF must first be released through the DFL_FPGA_FME_PORT_RELEASE ioctl on the FME device.

  2. Once N ports are released from PF, then user can use command below to enable SRIOV and VFs. Each VF owns only one Port with AFU.

    echo N > $PCI_DEVICE_PATH/sriov_numvfs
    
  3. Pass through the VFs to VMs

  4. The AFU under VF is accessible from applications in VM (using the same driver inside the VF).

Note that an FME can't be assigned to a VF, thus PR and other management functions are only available via the PF.

Device enumeration

This section introduces how applications enumerate the fpga device from the sysfs hierarchy under /sys/class/fpga_region.

In the example below, two DFL based FPGA devices are installed in the host. Each fpga device has one FME and two ports (AFUs).

FPGA regions are created under /sys/class/fpga_region/:

/sys/class/fpga_region/region0
/sys/class/fpga_region/region1
/sys/class/fpga_region/region2
...

Application needs to search each regionX folder, if feature device is found, (e.g. "dfl-port.n" or "dfl-fme.m" is found), then it's the base fpga region which represents the FPGA device.

Each base region has one FME and two ports (AFUs) as child devices:

/sys/class/fpga_region/region0/dfl-fme.0
/sys/class/fpga_region/region0/dfl-port.0
/sys/class/fpga_region/region0/dfl-port.1
...

/sys/class/fpga_region/region3/dfl-fme.1
/sys/class/fpga_region/region3/dfl-port.2
/sys/class/fpga_region/region3/dfl-port.3
...

In general, the FME/AFU sysfs interfaces are named as follows:

/sys/class/fpga_region/<regionX>/<dfl-fme.n>/
/sys/class/fpga_region/<regionX>/<dfl-port.m>/

with 'n' consecutively numbering all FMEs and 'm' consecutively numbering all ports.

The device nodes used for ioctl() or mmap() can be referenced through:

/sys/class/fpga_region/<regionX>/<dfl-fme.n>/dev
/sys/class/fpga_region/<regionX>/<dfl-port.n>/dev

Add new FIUs support

It's possible that developers made some new function blocks (FIUs) under this DFL framework, then new platform device driver needs to be developed for the new feature dev (FIU) following the same way as existing feature dev drivers (e.g. FME and Port/AFU platform device driver). Besides that, it requires modification on DFL framework enumeration code too, for new FIU type detection and related platform devices creation.

Add new private features support

In some cases, we may need to add some new private features to existing FIUs (e.g. FME or Port). Developers don't need to touch enumeration code in DFL framework, as each private feature will be parsed automatically and related mmio resources can be found under FIU platform device created by DFL framework. Developer only needs to provide a sub feature driver with matched feature id. FME Partial Reconfiguration Sub Feature driver (see drivers/fpga/dfl-fme-pr.c) could be a reference.

Open discussion

FME driver exports one ioctl (DFL_FPGA_FME_PORT_PR) for partial reconfiguration to user now. In the future, if unified user interfaces for reconfiguration are added, FME driver should switch to them from ioctl interface.