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diff --git a/Documentation/driver-api/media/cec-core.rst b/Documentation/driver-api/media/cec-core.rst new file mode 100644 index 000000000000..ae0d20798edc --- /dev/null +++ b/Documentation/driver-api/media/cec-core.rst @@ -0,0 +1,478 @@ +.. SPDX-License-Identifier: GPL-2.0 + +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: + +https://www.hdmi.org/spec/index + + +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: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_monitor_pin_enable)(struct cec_adapter *adap, bool enable); + int (*adap_log_addr)(struct cec_adapter *adap, u8 logical_addr); + void (*adap_configured)(struct cec_adapter *adap, bool configured); + 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); + + /* Error injection callbacks */ + ... + + /* High-level callback */ + ... + }; + +The seven 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. The +physical address will always be valid if CEC_CAP_NEEDS_HPD is set. If that +capability is not set, then the physical address can change while the CEC +hardware is enabled. CEC drivers should not set CEC_CAP_NEEDS_HPD unless +the hardware design requires that as this will make it impossible to wake +up displays that pull the HPD low when in standby mode. 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 are 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. + + +Called when the adapter is fully configured or unconfigured:: + + void (*adap_configured)(struct cec_adapter *adap, bool configured); + +If configured == true, then the adapter is fully configured, i.e. all logical +addresses have been successfully claimed. If configured == false, then the +adapter is unconfigured. If the driver has to take specific actions after +(un)configuration, then that can be done through this optional callback. + + +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); + +or:: + + 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 ARB_LOST + or LOW_DRIVE if the hardware cannot differentiate or something + else entirely. Some hardware only supports OK and FAIL as the + result of a transmit, i.e. there is no way to differentiate + between the different possible errors. In that case map FAIL + to CEC_TX_STATUS_NACK and not to CEC_TX_STATUS_ERROR. + +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 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. + +Be aware that calling these functions can immediately start a new transmit +if there is one pending in the queue. So make sure that the hardware is in +a state where new transmits can be started *before* calling these functions. + +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. + +Optional: Implementing Error Injection Support +---------------------------------------------- + +If the CEC adapter supports Error Injection functionality, then that can +be exposed through the Error Injection callbacks: + +.. code-block:: none + + struct cec_adap_ops { + /* Low-level callbacks */ + ... + + /* Error injection callbacks */ + int (*error_inj_show)(struct cec_adapter *adap, struct seq_file *sf); + bool (*error_inj_parse_line)(struct cec_adapter *adap, char *line); + + /* High-level CEC message callback */ + ... + }; + +If both callbacks are set, then an ``error-inj`` file will appear in debugfs. +The basic syntax is as follows: + +Leading spaces/tabs are ignored. If the next character is a ``#`` or the end of the +line was reached, then the whole line is ignored. Otherwise a command is expected. + +This basic parsing is done in the CEC Framework. It is up to the driver to decide +what commands to implement. The only requirement is that the command ``clear`` without +any arguments must be implemented and that it will remove all current error injection +commands. + +This ensures that you can always do ``echo clear >error-inj`` to clear any error +injections without having to know the details of the driver-specific commands. + +Note that the output of ``error-inj`` shall be valid as input to ``error-inj``. +So this must work: + +.. code-block:: none + + $ cat error-inj >einj.txt + $ cat einj.txt >error-inj + +The first callback is called when this file is read and it should show the +current error injection state:: + + int (*error_inj_show)(struct cec_adapter *adap, struct seq_file *sf); + +It is recommended that it starts with a comment block with basic usage +information. It returns 0 for success and an error otherwise. + +The second callback will parse commands written to the ``error-inj`` file:: + + bool (*error_inj_parse_line)(struct cec_adapter *adap, char *line); + +The ``line`` argument points to the start of the command. Any leading +spaces or tabs have already been skipped. It is a single line only (so there +are no embedded newlines) and it is 0-terminated. The callback is free to +modify the contents of the buffer. It is only called for lines containing a +command, so this callback is never called for empty lines or comment lines. + +Return true if the command was valid or false if there were syntax errors. + +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 */ + ... + + /* Error injection 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. + + +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 |