1.. SPDX-License-Identifier: GPL-2.0 2 3CEC Kernel Support 4================== 5 6The CEC framework provides a unified kernel interface for use with HDMI CEC 7hardware. It is designed to handle a multiple types of hardware (receivers, 8transmitters, USB dongles). The framework also gives the option to decide 9what to do in the kernel driver and what should be handled by userspace 10applications. In addition it integrates the remote control passthrough 11feature into the kernel's remote control framework. 12 13 14The CEC Protocol 15---------------- 16 17The CEC protocol enables consumer electronic devices to communicate with each 18other through the HDMI connection. The protocol uses logical addresses in the 19communication. The logical address is strictly connected with the functionality 20provided by the device. The TV acting as the communication hub is always 21assigned address 0. The physical address is determined by the physical 22connection between devices. 23 24The CEC framework described here is up to date with the CEC 2.0 specification. 25It is documented in the HDMI 1.4 specification with the new 2.0 bits documented 26in the HDMI 2.0 specification. But for most of the features the freely available 27HDMI 1.3a specification is sufficient: 28 29https://www.hdmi.org/spec/index 30 31 32CEC Adapter Interface 33--------------------- 34 35The struct cec_adapter represents the CEC adapter hardware. It is created by 36calling cec_allocate_adapter() and deleted by calling cec_delete_adapter(): 37 38.. c:function:: 39 struct cec_adapter *cec_allocate_adapter(const struct cec_adap_ops *ops, \ 40 void *priv, const char *name, \ 41 u32 caps, u8 available_las); 42 43.. c:function:: 44 void cec_delete_adapter(struct cec_adapter *adap); 45 46To create an adapter you need to pass the following information: 47 48ops: 49 adapter operations which are called by the CEC framework and that you 50 have to implement. 51 52priv: 53 will be stored in adap->priv and can be used by the adapter ops. 54 Use cec_get_drvdata(adap) to get the priv pointer. 55 56name: 57 the name of the CEC adapter. Note: this name will be copied. 58 59caps: 60 capabilities of the CEC adapter. These capabilities determine the 61 capabilities of the hardware and which parts are to be handled 62 by userspace and which parts are handled by kernelspace. The 63 capabilities are returned by CEC_ADAP_G_CAPS. 64 65available_las: 66 the number of simultaneous logical addresses that this 67 adapter can handle. Must be 1 <= available_las <= CEC_MAX_LOG_ADDRS. 68 69To obtain the priv pointer use this helper function: 70 71.. c:function:: 72 void *cec_get_drvdata(const struct cec_adapter *adap); 73 74To register the /dev/cecX device node and the remote control device (if 75CEC_CAP_RC is set) you call: 76 77.. c:function:: 78 int cec_register_adapter(struct cec_adapter *adap, \ 79 struct device *parent); 80 81where parent is the parent device. 82 83To unregister the devices call: 84 85.. c:function:: 86 void cec_unregister_adapter(struct cec_adapter *adap); 87 88Note: if cec_register_adapter() fails, then call cec_delete_adapter() to 89clean up. But if cec_register_adapter() succeeded, then only call 90cec_unregister_adapter() to clean up, never cec_delete_adapter(). The 91unregister function will delete the adapter automatically once the last user 92of that /dev/cecX device has closed its file handle. 93 94 95Implementing the Low-Level CEC Adapter 96-------------------------------------- 97 98The following low-level adapter operations have to be implemented in 99your driver: 100 101.. c:struct:: cec_adap_ops 102 103.. code-block:: none 104 105 struct cec_adap_ops 106 { 107 /* Low-level callbacks */ 108 int (*adap_enable)(struct cec_adapter *adap, bool enable); 109 int (*adap_monitor_all_enable)(struct cec_adapter *adap, bool enable); 110 int (*adap_monitor_pin_enable)(struct cec_adapter *adap, bool enable); 111 int (*adap_log_addr)(struct cec_adapter *adap, u8 logical_addr); 112 int (*adap_transmit)(struct cec_adapter *adap, u8 attempts, 113 u32 signal_free_time, struct cec_msg *msg); 114 void (*adap_status)(struct cec_adapter *adap, struct seq_file *file); 115 void (*adap_free)(struct cec_adapter *adap); 116 117 /* Error injection callbacks */ 118 ... 119 120 /* High-level callbacks */ 121 ... 122 }; 123 124The seven low-level ops deal with various aspects of controlling the CEC adapter 125hardware: 126 127 128To enable/disable the hardware:: 129 130 int (*adap_enable)(struct cec_adapter *adap, bool enable); 131 132This callback enables or disables the CEC hardware. Enabling the CEC hardware 133means powering it up in a state where no logical addresses are claimed. This 134op assumes that the physical address (adap->phys_addr) is valid when enable is 135true and will not change while the CEC adapter remains enabled. The initial 136state of the CEC adapter after calling cec_allocate_adapter() is disabled. 137 138Note that adap_enable must return 0 if enable is false. 139 140 141To enable/disable the 'monitor all' mode:: 142 143 int (*adap_monitor_all_enable)(struct cec_adapter *adap, bool enable); 144 145If enabled, then the adapter should be put in a mode to also monitor messages 146that are not for us. Not all hardware supports this and this function is only 147called if the CEC_CAP_MONITOR_ALL capability is set. This callback is optional 148(some hardware may always be in 'monitor all' mode). 149 150Note that adap_monitor_all_enable must return 0 if enable is false. 151 152 153To enable/disable the 'monitor pin' mode:: 154 155 int (*adap_monitor_pin_enable)(struct cec_adapter *adap, bool enable); 156 157If enabled, then the adapter should be put in a mode to also monitor CEC pin 158changes. Not all hardware supports this and this function is only called if 159the CEC_CAP_MONITOR_PIN capability is set. This callback is optional 160(some hardware may always be in 'monitor pin' mode). 161 162Note that adap_monitor_pin_enable must return 0 if enable is false. 163 164 165To program a new logical address:: 166 167 int (*adap_log_addr)(struct cec_adapter *adap, u8 logical_addr); 168 169If logical_addr == CEC_LOG_ADDR_INVALID then all programmed logical addresses 170are to be erased. Otherwise the given logical address should be programmed. 171If the maximum number of available logical addresses is exceeded, then it 172should return -ENXIO. Once a logical address is programmed the CEC hardware 173can receive directed messages to that address. 174 175Note that adap_log_addr must return 0 if logical_addr is CEC_LOG_ADDR_INVALID. 176 177 178To transmit a new message:: 179 180 int (*adap_transmit)(struct cec_adapter *adap, u8 attempts, 181 u32 signal_free_time, struct cec_msg *msg); 182 183This transmits a new message. The attempts argument is the suggested number of 184attempts for the transmit. 185 186The signal_free_time is the number of data bit periods that the adapter should 187wait when the line is free before attempting to send a message. This value 188depends on whether this transmit is a retry, a message from a new initiator or 189a new message for the same initiator. Most hardware will handle this 190automatically, but in some cases this information is needed. 191 192The CEC_FREE_TIME_TO_USEC macro can be used to convert signal_free_time to 193microseconds (one data bit period is 2.4 ms). 194 195 196To log the current CEC hardware status:: 197 198 void (*adap_status)(struct cec_adapter *adap, struct seq_file *file); 199 200This optional callback can be used to show the status of the CEC hardware. 201The status is available through debugfs: cat /sys/kernel/debug/cec/cecX/status 202 203To free any resources when the adapter is deleted:: 204 205 void (*adap_free)(struct cec_adapter *adap); 206 207This optional callback can be used to free any resources that might have been 208allocated by the driver. It's called from cec_delete_adapter. 209 210 211Your adapter driver will also have to react to events (typically interrupt 212driven) by calling into the framework in the following situations: 213 214When a transmit finished (successfully or otherwise):: 215 216 void cec_transmit_done(struct cec_adapter *adap, u8 status, 217 u8 arb_lost_cnt, u8 nack_cnt, u8 low_drive_cnt, 218 u8 error_cnt); 219 220or:: 221 222 void cec_transmit_attempt_done(struct cec_adapter *adap, u8 status); 223 224The status can be one of: 225 226CEC_TX_STATUS_OK: 227 the transmit was successful. 228 229CEC_TX_STATUS_ARB_LOST: 230 arbitration was lost: another CEC initiator 231 took control of the CEC line and you lost the arbitration. 232 233CEC_TX_STATUS_NACK: 234 the message was nacked (for a directed message) or 235 acked (for a broadcast message). A retransmission is needed. 236 237CEC_TX_STATUS_LOW_DRIVE: 238 low drive was detected on the CEC bus. This indicates that 239 a follower detected an error on the bus and requested a 240 retransmission. 241 242CEC_TX_STATUS_ERROR: 243 some unspecified error occurred: this can be one of ARB_LOST 244 or LOW_DRIVE if the hardware cannot differentiate or something 245 else entirely. Some hardware only supports OK and FAIL as the 246 result of a transmit, i.e. there is no way to differentiate 247 between the different possible errors. In that case map FAIL 248 to CEC_TX_STATUS_NACK and not to CEC_TX_STATUS_ERROR. 249 250CEC_TX_STATUS_MAX_RETRIES: 251 could not transmit the message after trying multiple times. 252 Should only be set by the driver if it has hardware support for 253 retrying messages. If set, then the framework assumes that it 254 doesn't have to make another attempt to transmit the message 255 since the hardware did that already. 256 257The hardware must be able to differentiate between OK, NACK and 'something 258else'. 259 260The \*_cnt arguments are the number of error conditions that were seen. 261This may be 0 if no information is available. Drivers that do not support 262hardware retry can just set the counter corresponding to the transmit error 263to 1, if the hardware does support retry then either set these counters to 2640 if the hardware provides no feedback of which errors occurred and how many 265times, or fill in the correct values as reported by the hardware. 266 267Be aware that calling these functions can immediately start a new transmit 268if there is one pending in the queue. So make sure that the hardware is in 269a state where new transmits can be started *before* calling these functions. 270 271The cec_transmit_attempt_done() function is a helper for cases where the 272hardware never retries, so the transmit is always for just a single 273attempt. It will call cec_transmit_done() in turn, filling in 1 for the 274count argument corresponding to the status. Or all 0 if the status was OK. 275 276When a CEC message was received: 277 278.. c:function:: 279 void cec_received_msg(struct cec_adapter *adap, struct cec_msg *msg); 280 281Speaks for itself. 282 283Implementing the interrupt handler 284---------------------------------- 285 286Typically the CEC hardware provides interrupts that signal when a transmit 287finished and whether it was successful or not, and it provides and interrupt 288when a CEC message was received. 289 290The CEC driver should always process the transmit interrupts first before 291handling the receive interrupt. The framework expects to see the cec_transmit_done 292call before the cec_received_msg call, otherwise it can get confused if the 293received message was in reply to the transmitted message. 294 295Optional: Implementing Error Injection Support 296---------------------------------------------- 297 298If the CEC adapter supports Error Injection functionality, then that can 299be exposed through the Error Injection callbacks: 300 301.. code-block:: none 302 303 struct cec_adap_ops { 304 /* Low-level callbacks */ 305 ... 306 307 /* Error injection callbacks */ 308 int (*error_inj_show)(struct cec_adapter *adap, struct seq_file *sf); 309 bool (*error_inj_parse_line)(struct cec_adapter *adap, char *line); 310 311 /* High-level CEC message callback */ 312 ... 313 }; 314 315If both callbacks are set, then an ``error-inj`` file will appear in debugfs. 316The basic syntax is as follows: 317 318Leading spaces/tabs are ignored. If the next character is a ``#`` or the end of the 319line was reached, then the whole line is ignored. Otherwise a command is expected. 320 321This basic parsing is done in the CEC Framework. It is up to the driver to decide 322what commands to implement. The only requirement is that the command ``clear`` without 323any arguments must be implemented and that it will remove all current error injection 324commands. 325 326This ensures that you can always do ``echo clear >error-inj`` to clear any error 327injections without having to know the details of the driver-specific commands. 328 329Note that the output of ``error-inj`` shall be valid as input to ``error-inj``. 330So this must work: 331 332.. code-block:: none 333 334 $ cat error-inj >einj.txt 335 $ cat einj.txt >error-inj 336 337The first callback is called when this file is read and it should show the 338current error injection state:: 339 340 int (*error_inj_show)(struct cec_adapter *adap, struct seq_file *sf); 341 342It is recommended that it starts with a comment block with basic usage 343information. It returns 0 for success and an error otherwise. 344 345The second callback will parse commands written to the ``error-inj`` file:: 346 347 bool (*error_inj_parse_line)(struct cec_adapter *adap, char *line); 348 349The ``line`` argument points to the start of the command. Any leading 350spaces or tabs have already been skipped. It is a single line only (so there 351are no embedded newlines) and it is 0-terminated. The callback is free to 352modify the contents of the buffer. It is only called for lines containing a 353command, so this callback is never called for empty lines or comment lines. 354 355Return true if the command was valid or false if there were syntax errors. 356 357Implementing the High-Level CEC Adapter 358--------------------------------------- 359 360The low-level operations drive the hardware, the high-level operations are 361CEC protocol driven. The following high-level callbacks are available: 362 363.. code-block:: none 364 365 struct cec_adap_ops { 366 /* Low-level callbacks */ 367 ... 368 369 /* Error injection callbacks */ 370 ... 371 372 /* High-level CEC message callback */ 373 int (*received)(struct cec_adapter *adap, struct cec_msg *msg); 374 }; 375 376The received() callback allows the driver to optionally handle a newly 377received CEC message:: 378 379 int (*received)(struct cec_adapter *adap, struct cec_msg *msg); 380 381If the driver wants to process a CEC message, then it can implement this 382callback. If it doesn't want to handle this message, then it should return 383-ENOMSG, otherwise the CEC framework assumes it processed this message and 384it will not do anything with it. 385 386 387CEC framework functions 388----------------------- 389 390CEC Adapter drivers can call the following CEC framework functions: 391 392.. c:function:: 393 int cec_transmit_msg(struct cec_adapter *adap, struct cec_msg *msg, \ 394 bool block); 395 396Transmit a CEC message. If block is true, then wait until the message has been 397transmitted, otherwise just queue it and return. 398 399.. c:function:: 400 void cec_s_phys_addr(struct cec_adapter *adap, u16 phys_addr, bool block); 401 402Change the physical address. This function will set adap->phys_addr and 403send an event if it has changed. If cec_s_log_addrs() has been called and 404the physical address has become valid, then the CEC framework will start 405claiming the logical addresses. If block is true, then this function won't 406return until this process has finished. 407 408When the physical address is set to a valid value the CEC adapter will 409be enabled (see the adap_enable op). When it is set to CEC_PHYS_ADDR_INVALID, 410then the CEC adapter will be disabled. If you change a valid physical address 411to another valid physical address, then this function will first set the 412address to CEC_PHYS_ADDR_INVALID before enabling the new physical address. 413 414.. c:function:: 415 void cec_s_phys_addr_from_edid(struct cec_adapter *adap, \ 416 const struct edid *edid); 417 418A helper function that extracts the physical address from the edid struct 419and calls cec_s_phys_addr() with that address, or CEC_PHYS_ADDR_INVALID 420if the EDID did not contain a physical address or edid was a NULL pointer. 421 422.. c:function:: 423 int cec_s_log_addrs(struct cec_adapter *adap, \ 424 struct cec_log_addrs *log_addrs, bool block); 425 426Claim the CEC logical addresses. Should never be called if CEC_CAP_LOG_ADDRS 427is set. If block is true, then wait until the logical addresses have been 428claimed, otherwise just queue it and return. To unconfigure all logical 429addresses call this function with log_addrs set to NULL or with 430log_addrs->num_log_addrs set to 0. The block argument is ignored when 431unconfiguring. This function will just return if the physical address is 432invalid. Once the physical address becomes valid, then the framework will 433attempt to claim these logical addresses. 434 435CEC Pin framework 436----------------- 437 438Most CEC hardware operates on full CEC messages where the software provides 439the message and the hardware handles the low-level CEC protocol. But some 440hardware only drives the CEC pin and software has to handle the low-level 441CEC protocol. The CEC pin framework was created to handle such devices. 442 443Note that due to the close-to-realtime requirements it can never be guaranteed 444to work 100%. This framework uses highres timers internally, but if a 445timer goes off too late by more than 300 microseconds wrong results can 446occur. In reality it appears to be fairly reliable. 447 448One advantage of this low-level implementation is that it can be used as 449a cheap CEC analyser, especially if interrupts can be used to detect 450CEC pin transitions from low to high or vice versa. 451 452.. kernel-doc:: include/media/cec-pin.h 453 454CEC Notifier framework 455---------------------- 456 457Most drm HDMI implementations have an integrated CEC implementation and no 458notifier support is needed. But some have independent CEC implementations 459that have their own driver. This could be an IP block for an SoC or a 460completely separate chip that deals with the CEC pin. For those cases a 461drm driver can install a notifier and use the notifier to inform the 462CEC driver about changes in the physical address. 463 464.. kernel-doc:: include/media/cec-notifier.h 465