diff options
Diffstat (limited to 'Documentation/media/kapi')
| -rw-r--r-- | Documentation/media/kapi/cec-core.rst | 267 |
1 files changed, 267 insertions, 0 deletions
diff --git a/Documentation/media/kapi/cec-core.rst b/Documentation/media/kapi/cec-core.rst new file mode 100644 index 000000000000..75155fe37153 --- /dev/null +++ b/Documentation/media/kapi/cec-core.rst @@ -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. |