Documentation/media/kapi/cec-core.rst - arm/linux - Git at Google

 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.


 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:

 http://www.microprocessor.org/HDMISpecification13a.pdf


 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():

 .. c:function::
    struct cec_adapter *cec_allocate_adapter(const struct cec_adap_ops *ops, void *priv,
    const char *name, u32 caps, u8 available_las);

 .. c:function::
    void cec_delete_adapter(struct cec_adapter *adap);

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

 ops:
 	adapter operations which are called by the CEC framework and that you
 	have to implement.

 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.

 name:
 	the name of the CEC adapter. Note: this name will be copied.

 caps:
 	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.

 available_las:
 	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:

 .. c: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:

 .. c:function::
 	int cec_register_adapter(struct cec_adapter *adap, struct device *parent);

 where parent is the parent device.

 To unregister the devices call:

 .. c:function::
 	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.


 Implementing the Low-Level CEC Adapter
 --------------------------------------

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

 .. c:type:: struct cec_adap_ops

 .. code-block:: none

 	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_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:

 .. c:function::
 	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:

 .. c:function::
 	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 program a new logical address:

 .. c:function::
 	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:

 .. c:function::
 	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:

 .. c:function::
 	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:

 .. c:function::
 	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):

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

 or:

 .. c:function::
 	void cec_transmit_attempt_done(struct cec_adapter *adap, u8 status);

 The status can be one of:

 CEC_TX_STATUS_OK:
 	the transmit was successful.

 CEC_TX_STATUS_ARB_LOST:
 	arbitration was lost: another CEC initiator
 	took control of the CEC line and you lost the arbitration.

 CEC_TX_STATUS_NACK:
 	the message was nacked (for a directed message) or
 	acked (for a broadcast message). A retransmission is needed.

 CEC_TX_STATUS_LOW_DRIVE:
 	low drive was detected on the CEC bus. This indicates that
 	a follower detected an error on the bus and requested a
 	retransmission.

 CEC_TX_STATUS_ERROR:
 	some unspecified error occurred: this can be one of
 	the previous two if the hardware cannot differentiate or something
 	else entirely.

 CEC_TX_STATUS_MAX_RETRIES:
 	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 \*_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:

 .. c:function::
 	void cec_received_msg(struct cec_adapter *adap, struct cec_msg *msg);

 Speaks for itself.

 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.

 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:

 .. code-block:: none

 	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

 .. c:function::
 	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.


 CEC framework functions
 -----------------------

 CEC Adapter drivers can call the following CEC framework functions:

 .. c:function::
 	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.

 .. c:function::
 	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.

 .. c:function::
 	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.

 .. c:function::
 	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.

 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.

 .. kernel-doc:: include/media/cec-pin.h

 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.

 .. kernel-doc:: include/media/cec-notifier.h
	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.


	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:

	http://www.microprocessor.org/HDMISpecification13a.pdf


	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():

	.. c:function::
	struct cec_adapter cec_allocate_adapter(const struct cec_adap_ops ops, void *priv,
	const char *name, u32 caps, u8 available_las);

	.. c:function::
	void cec_delete_adapter(struct cec_adapter *adap);

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

	ops:
	adapter operations which are called by the CEC framework and that you
	have to implement.

	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.

	name:
	the name of the CEC adapter. Note: this name will be copied.

	caps:
	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.

	available_las:
	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:

	.. c: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:

	.. c:function::
	int cec_register_adapter(struct cec_adapter adap, struct device parent);

	where parent is the parent device.

	To unregister the devices call:

	.. c:function::
	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.


	Implementing the Low-Level CEC Adapter
	--------------------------------------

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

	.. c:type:: struct cec_adap_ops

	.. code-block:: none

	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_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:

	.. c:function::
	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:

	.. c:function::
	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 program a new logical address:

	.. c:function::
	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:

	.. c:function::
	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:

	.. c:function::
	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:

	.. c:function::
	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):

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

	or:

	.. c:function::
	void cec_transmit_attempt_done(struct cec_adapter *adap, u8 status);

	The status can be one of:

	CEC_TX_STATUS_OK:
	the transmit was successful.

	CEC_TX_STATUS_ARB_LOST:
	arbitration was lost: another CEC initiator
	took control of the CEC line and you lost the arbitration.

	CEC_TX_STATUS_NACK:
	the message was nacked (for a directed message) or
	acked (for a broadcast message). A retransmission is needed.

	CEC_TX_STATUS_LOW_DRIVE:
	low drive was detected on the CEC bus. This indicates that
	a follower detected an error on the bus and requested a
	retransmission.

	CEC_TX_STATUS_ERROR:
	some unspecified error occurred: this can be one of
	the previous two if the hardware cannot differentiate or something
	else entirely.

	CEC_TX_STATUS_MAX_RETRIES:
	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 \*_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:

	.. c:function::
	void cec_received_msg(struct cec_adapter adap, struct cec_msg msg);

	Speaks for itself.

	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.

	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:

	.. code-block:: none

	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

	.. c:function::
	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.


	CEC framework functions
	-----------------------

	CEC Adapter drivers can call the following CEC framework functions:

	.. c:function::
	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.

	.. c:function::
	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.

	.. c:function::
	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.

	.. c:function::
	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.

	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.

	.. kernel-doc:: include/media/cec-pin.h

	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.

	.. kernel-doc:: include/media/cec-notifier.h