[PATCHv16 07/13] cec.txt: add CEC framework documentation

Fri Jun 17 07:22:05 UTC 2016

On 06/16/2016 10:12 PM, Mauro Carvalho Chehab wrote:
> Em Fri, 29 Apr 2016 15:52:22 +0200
> Hans Verkuil <hverkuil at xs4all.nl> escreveu:
> 
>> From: Hans Verkuil <hansverk at cisco.com>
>>
>> Document the new HDMI CEC framework.
> 
> As we'll be moving documentation to Sphinx/Rst, it would be good if
> you could make it work fine with sphinx, as this will likely be needed
> for Kernel 4.9. Right now it most works, although several warnings are
> produced: 

Would it be a problem if this conversion is done later in a separate patch?

I'm all for it, but I don't have the time at the moment to do this.

Regards,

	Hans

> 
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> 
> 
>>
>> Signed-off-by: Hans Verkuil <hansverk at cisco.com>
>> [k.debski at samsung.com: add DocBook documentation by Hans Verkuil, with
>> Signed-off-by: Kamil Debski <kamil at wypas.org>
>> Signed-off-by: Hans Verkuil <hans.verkuil at cisco.com>
>> ---
>>  Documentation/cec.txt | 267 ++++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 267 insertions(+)
>>  create mode 100644 Documentation/cec.txt
>>
>> diff --git a/Documentation/cec.txt b/Documentation/cec.txt
>> new file mode 100644
>> index 0000000..75155fe
>> --- /dev/null
>> +++ b/Documentation/cec.txt
>> @@ -0,0 +1,267 @@
>> +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
>> +
>> +
>> +The Kernel Interface
>> +====================
>> +
>> +CEC Adapter
>> +-----------
>> +
>> +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,
>> +	       struct device *parent);
>> +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.
>> +
>> +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.
>> +
>> +parent: the parent device.
>> +
>> +
>> +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);
>> +
>> +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.
>> +
>> +
>> +Implementing the Low-Level CEC Adapter
>> +--------------------------------------
>> +
>> +The following low-level adapter operations have to be implemented in
>> +your driver:
>> +
>> +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_log_status)(struct cec_adapter *adap);
>> +
>> +	/* High-level callbacks */
>> +	...
>> +};
>> +
>> +The three 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 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
>> +
>> +
>> +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);
>> +
>> +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.
>> +
>> +When a CEC message was received:
>> +
>> +void cec_received_msg(struct cec_adapter *adap, struct cec_msg *msg);
>> +
>> +Speaks for itself.
>> +
>> +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 no anything with it.
>> +
>> +
>> +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.
>> +
>> +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.
> 
> 
> 
> Thanks,
> Mauro
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