5. CEC Kernel Support

The CEC framework provides a unified kernel interface for use with HDMI CEC hardware. It is designed to handle a multiple types of hardware (receivers, transmitters, USB dongles). The framework also gives the option to decide what to do in the kernel driver and what should be handled by userspace applications. In addition it integrates the remote control passthrough feature into the kernel’s remote control framework.

5.1. The CEC Protocol

The CEC protocol enables consumer electronic devices to communicate with each other through the HDMI connection. The protocol uses logical addresses in the communication. The logical address is strictly connected with the functionality provided by the device. The TV acting as the communication hub is always assigned address 0. The physical address is determined by the physical connection between devices.

The CEC framework described here is up to date with the CEC 2.0 specification. It is documented in the HDMI 1.4 specification with the new 2.0 bits documented in the HDMI 2.0 specification. But for most of the features the freely available HDMI 1.3a specification is sufficient:


5.2. CEC Adapter Interface

The struct cec_adapter represents the CEC adapter hardware. It is created by calling cec_allocate_adapter() and deleted by calling cec_delete_adapter():

struct cec_adapter *cec_allocate_adapter(const struct cec_adap_ops *ops, void *priv,
const char *name, u32 caps, u8 available_las);
void cec_delete_adapter(struct cec_adapter *adap);

To create an adapter you need to pass the following information:

adapter operations which are called by the CEC framework and that you have to implement.
will be stored in adap->priv and can be used by the adapter ops. Use cec_get_drvdata(adap) to get the priv pointer.
the name of the CEC adapter. Note: this name will be copied.
capabilities of the CEC adapter. These capabilities determine the capabilities of the hardware and which parts are to be handled by userspace and which parts are handled by kernelspace. The capabilities are returned by CEC_ADAP_G_CAPS.
the number of simultaneous logical addresses that this adapter can handle. Must be 1 <= available_las <= CEC_MAX_LOG_ADDRS.

To obtain the priv pointer use this helper function:

void *cec_get_drvdata(const struct cec_adapter *adap);

To register the /dev/cecX device node and the remote control device (if CEC_CAP_RC is set) you call:

int cec_register_adapter(struct cec_adapter *adap, struct device *parent);

where parent is the parent device.

To unregister the devices call:

void cec_unregister_adapter(struct cec_adapter *adap);

Note: if cec_register_adapter() fails, then call cec_delete_adapter() to clean up. But if cec_register_adapter() succeeded, then only call cec_unregister_adapter() to clean up, never cec_delete_adapter(). The unregister function will delete the adapter automatically once the last user of that /dev/cecX device has closed its file handle.

5.3. Implementing the Low-Level CEC Adapter

The following low-level adapter operations have to be implemented in your driver:

struct cec_adap_ops
struct cec_adap_ops
        /* Low-level callbacks */
        int (*adap_enable)(struct cec_adapter *adap, bool enable);
        int (*adap_monitor_all_enable)(struct cec_adapter *adap, bool enable);
        int (*adap_monitor_pin_enable)(struct cec_adapter *adap, bool enable);
        int (*adap_log_addr)(struct cec_adapter *adap, u8 logical_addr);
        int (*adap_transmit)(struct cec_adapter *adap, u8 attempts,
                              u32 signal_free_time, struct cec_msg *msg);
        void (*adap_status)(struct cec_adapter *adap, struct seq_file *file);
        void (*adap_free)(struct cec_adapter *adap);

        /* High-level callbacks */

The five low-level ops deal with various aspects of controlling the CEC adapter hardware:

To enable/disable the hardware:

int (*adap_enable)(struct cec_adapter *adap, bool enable);

This callback enables or disables the CEC hardware. Enabling the CEC hardware means powering it up in a state where no logical addresses are claimed. This op assumes that the physical address (adap->phys_addr) is valid when enable is true and will not change while the CEC adapter remains enabled. The initial state of the CEC adapter after calling cec_allocate_adapter() is disabled.

Note that adap_enable must return 0 if enable is false.

To enable/disable the ‘monitor all’ mode:

int (*adap_monitor_all_enable)(struct cec_adapter *adap, bool enable);

If enabled, then the adapter should be put in a mode to also monitor messages that not for us. Not all hardware supports this and this function is only called if the CEC_CAP_MONITOR_ALL capability is set. This callback is optional (some hardware may always be in ‘monitor all’ mode).

Note that adap_monitor_all_enable must return 0 if enable is false.

To enable/disable the ‘monitor pin’ mode:

int (*adap_monitor_pin_enable)(struct cec_adapter *adap, bool enable);

If enabled, then the adapter should be put in a mode to also monitor CEC pin changes. Not all hardware supports this and this function is only called if the CEC_CAP_MONITOR_PIN capability is set. This callback is optional (some hardware may always be in ‘monitor pin’ mode).

Note that adap_monitor_pin_enable must return 0 if enable is false.

To program a new logical address:

int (*adap_log_addr)(struct cec_adapter *adap, u8 logical_addr);

If logical_addr == CEC_LOG_ADDR_INVALID then all programmed logical addresses are to be erased. Otherwise the given logical address should be programmed. If the maximum number of available logical addresses is exceeded, then it should return -ENXIO. Once a logical address is programmed the CEC hardware can receive directed messages to that address.

Note that adap_log_addr must return 0 if logical_addr is CEC_LOG_ADDR_INVALID.

To transmit a new message:

int (*adap_transmit)(struct cec_adapter *adap, u8 attempts,
u32 signal_free_time, struct cec_msg *msg);

This transmits a new message. The attempts argument is the suggested number of attempts for the transmit.

The signal_free_time is the number of data bit periods that the adapter should wait when the line is free before attempting to send a message. This value depends on whether this transmit is a retry, a message from a new initiator or a new message for the same initiator. Most hardware will handle this automatically, but in some cases this information is needed.

The CEC_FREE_TIME_TO_USEC macro can be used to convert signal_free_time to microseconds (one data bit period is 2.4 ms).

To log the current CEC hardware status:

void (*adap_status)(struct cec_adapter *adap, struct seq_file *file);

This optional callback can be used to show the status of the CEC hardware. The status is available through debugfs: cat /sys/kernel/debug/cec/cecX/status

To free any resources when the adapter is deleted:

void (*adap_free)(struct cec_adapter *adap);

This optional callback can be used to free any resources that might have been allocated by the driver. It’s called from cec_delete_adapter.

Your adapter driver will also have to react to events (typically interrupt driven) by calling into the framework in the following situations:

When a transmit finished (successfully or otherwise):

void cec_transmit_done(struct cec_adapter *adap, u8 status, u8 arb_lost_cnt,
u8 nack_cnt, u8 low_drive_cnt, u8 error_cnt);


void cec_transmit_attempt_done(struct cec_adapter *adap, u8 status);

The status can be one of:

the transmit was successful.
arbitration was lost: another CEC initiator took control of the CEC line and you lost the arbitration.
the message was nacked (for a directed message) or acked (for a broadcast message). A retransmission is needed.
low drive was detected on the CEC bus. This indicates that a follower detected an error on the bus and requested a retransmission.
some unspecified error occurred: this can be one of ARB_LOST or LOW_DRIVE if the hardware cannot differentiate or something else entirely.
could not transmit the message after trying multiple times. Should only be set by the driver if it has hardware support for retrying messages. If set, then the framework assumes that it doesn’t have to make another attempt to transmit the message since the hardware did that already.

The hardware must be able to differentiate between OK, NACK and ‘something else’.

The *_cnt arguments are the number of error conditions that were seen. This may be 0 if no information is available. Drivers that do not support hardware retry can just set the counter corresponding to the transmit error to 1, if the hardware does support retry then either set these counters to 0 if the hardware provides no feedback of which errors occurred and how many times, or fill in the correct values as reported by the hardware.

The cec_transmit_attempt_done() function is a helper for cases where the hardware never retries, so the transmit is always for just a single attempt. It will call cec_transmit_done() in turn, filling in 1 for the count argument corresponding to the status. Or all 0 if the status was OK.

When a CEC message was received:

void cec_received_msg(struct cec_adapter *adap, struct cec_msg *msg);

Speaks for itself.

5.4. Implementing the interrupt handler

Typically the CEC hardware provides interrupts that signal when a transmit finished and whether it was successful or not, and it provides and interrupt when a CEC message was received.

The CEC driver should always process the transmit interrupts first before handling the receive interrupt. The framework expects to see the cec_transmit_done call before the cec_received_msg call, otherwise it can get confused if the received message was in reply to the transmitted message.

5.5. Implementing the High-Level CEC Adapter

The low-level operations drive the hardware, the high-level operations are CEC protocol driven. The following high-level callbacks are available:

struct cec_adap_ops {
        /* Low-level callbacks */

        /* High-level CEC message callback */
        int (*received)(struct cec_adapter *adap, struct cec_msg *msg);

The received() callback allows the driver to optionally handle a newly received CEC message

int (*received)(struct cec_adapter *adap, struct cec_msg *msg);

If the driver wants to process a CEC message, then it can implement this callback. If it doesn’t want to handle this message, then it should return -ENOMSG, otherwise the CEC framework assumes it processed this message and it will not do anything with it.

5.6. CEC framework functions

CEC Adapter drivers can call the following CEC framework functions:

int cec_transmit_msg(struct cec_adapter *adap, struct cec_msg *msg,
bool block);

Transmit a CEC message. If block is true, then wait until the message has been transmitted, otherwise just queue it and return.

void cec_s_phys_addr(struct cec_adapter *adap, u16 phys_addr,
bool block);

Change the physical address. This function will set adap->phys_addr and send an event if it has changed. If cec_s_log_addrs() has been called and the physical address has become valid, then the CEC framework will start claiming the logical addresses. If block is true, then this function won’t return until this process has finished.

When the physical address is set to a valid value the CEC adapter will be enabled (see the adap_enable op). When it is set to CEC_PHYS_ADDR_INVALID, then the CEC adapter will be disabled. If you change a valid physical address to another valid physical address, then this function will first set the address to CEC_PHYS_ADDR_INVALID before enabling the new physical address.

void cec_s_phys_addr_from_edid(struct cec_adapter *adap,
const struct edid *edid);

A helper function that extracts the physical address from the edid struct and calls cec_s_phys_addr() with that address, or CEC_PHYS_ADDR_INVALID if the EDID did not contain a physical address or edid was a NULL pointer.

int cec_s_log_addrs(struct cec_adapter *adap,
struct cec_log_addrs *log_addrs, bool block);

Claim the CEC logical addresses. Should never be called if CEC_CAP_LOG_ADDRS is set. If block is true, then wait until the logical addresses have been claimed, otherwise just queue it and return. To unconfigure all logical addresses call this function with log_addrs set to NULL or with log_addrs->num_log_addrs set to 0. The block argument is ignored when unconfiguring. This function will just return if the physical address is invalid. Once the physical address becomes valid, then the framework will attempt to claim these logical addresses.

5.7. CEC Pin framework

Most CEC hardware operates on full CEC messages where the software provides the message and the hardware handles the low-level CEC protocol. But some hardware only drives the CEC pin and software has to handle the low-level CEC protocol. The CEC pin framework was created to handle such devices.

Note that due to the close-to-realtime requirements it can never be guaranteed to work 100%. This framework uses highres timers internally, but if a timer goes off too late by more than 300 microseconds wrong results can occur. In reality it appears to be fairly reliable.

One advantage of this low-level implementation is that it can be used as a cheap CEC analyser, especially if interrupts can be used to detect CEC pin transitions from low to high or vice versa.

struct cec_pin_ops

low-level CEC pin operations


struct cec_pin_ops {
  bool (*read)(struct cec_adapter *adap);
  void (*low)(struct cec_adapter *adap);
  void (*high)(struct cec_adapter *adap);
  bool (*enable_irq)(struct cec_adapter *adap);
  void (*disable_irq)(struct cec_adapter *adap);
  void (*free)(struct cec_adapter *adap);
  void (*status)(struct cec_adapter *adap, struct seq_file *file);
  int (*read_hpd)(struct cec_adapter *adap);


read the CEC pin. Return true if high, false if low.
drive the CEC pin low.
stop driving the CEC pin. The pull-up will drive the pin high, unless someone else is driving the pin low.
optional, enable the interrupt to detect pin voltage changes.
optional, disable the interrupt.
optional. Free any allocated resources. Called when the adapter is deleted.
optional, log status information.
read the HPD pin. Return true if high, false if low or an error if negative. If NULL or -ENOTTY is returned, then this is not supported.


These operations are used by the cec pin framework to manipulate the CEC pin.

void cec_pin_changed(struct cec_adapter * adap, bool value)

update pin state from interrupt


struct cec_adapter * adap
pointer to the cec adapter
bool value
when true the pin is high, otherwise it is low


If changes of the CEC voltage are detected via an interrupt, then cec_pin_changed is called from the interrupt with the new value.

struct cec_adapter * cec_pin_allocate_adapter(const struct cec_pin_ops * pin_ops, void * priv, const char * name, u32 caps)

allocate a pin-based cec adapter


const struct cec_pin_ops * pin_ops
low-level pin operations
void * priv
will be stored in adap->priv and can be used by the adapter ops. Use cec_get_drvdata(adap) to get the priv pointer.
const char * name
the name of the CEC adapter. Note: this name will be copied.
u32 caps
capabilities of the CEC adapter. This will be ORed with CEC_CAP_MONITOR_ALL and CEC_CAP_MONITOR_PIN.


Allocate a cec adapter using the cec pin framework.


a pointer to the cec adapter or an error pointer

5.8. CEC Notifier framework

Most drm HDMI implementations have an integrated CEC implementation and no notifier support is needed. But some have independent CEC implementations that have their own driver. This could be an IP block for an SoC or a completely separate chip that deals with the CEC pin. For those cases a drm driver can install a notifier and use the notifier to inform the CEC driver about changes in the physical address.

struct cec_notifier * cec_notifier_get(struct device * dev)

find or create a new cec_notifier for the given device.


struct device * dev
device that sends the events.


If a notifier for device dev already exists, then increase the refcount and return that notifier.

If it doesn’t exist, then allocate a new notifier struct and return a pointer to that new struct.

Return NULL if the memory could not be allocated.

void cec_notifier_put(struct cec_notifier * n)

decrease refcount and delete when the refcount reaches 0.


struct cec_notifier * n
void cec_notifier_set_phys_addr(struct cec_notifier * n, u16 pa)

set a new physical address.


struct cec_notifier * n
the CEC notifier
u16 pa
the CEC physical address


Set a new CEC physical address. Does nothing if n == NULL.

void cec_notifier_set_phys_addr_from_edid(struct cec_notifier * n, const struct edid * edid)

set parse the PA from the EDID.


struct cec_notifier * n
the CEC notifier
const struct edid * edid
the struct edid pointer


Parses the EDID to obtain the new CEC physical address and set it. Does nothing if n == NULL.

void cec_notifier_register(struct cec_notifier * n, struct cec_adapter * adap, void (*callback) (struct cec_adapter *adap, u16 pa)

register a callback with the notifier


struct cec_notifier * n
the CEC notifier
struct cec_adapter * adap
the CEC adapter, passed as argument to the callback function
void (*)(struct cec_adapter *adap, u16 pa) callback
the callback function
void cec_notifier_unregister(struct cec_notifier * n)

unregister the callback from the notifier.


struct cec_notifier * n
the CEC notifier
void cec_register_cec_notifier(struct cec_adapter * adap, struct cec_notifier * notifier)

register the notifier with the cec adapter.


struct cec_adapter * adap
the CEC adapter
struct cec_notifier * notifier
the CEC notifier
void cec_notifier_phys_addr_invalidate(struct cec_notifier * n)

set the physical address to INVALID


struct cec_notifier * n
the CEC notifier


This is a simple helper function to invalidate the physical address. Does nothing if n == NULL.