[media] cec.txt: add CEC framework documentation

Document the new HDMI CEC framework.

[k.debski@samsung.com: add DocBook documentation by Hans Verkuil, with
Signed-off-by: NKamil Debski <kamil@wypas.org>
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/cec.txt

+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.