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. The 134physical address will always be valid if CEC_CAP_NEEDS_HPD is set. If that 135capability is not set, then the physical address can change while the CEC 136hardware is enabled. CEC drivers should not set CEC_CAP_NEEDS_HPD unless 137the hardware design requires that as this will make it impossible to wake 138up displays that pull the HPD low when in standby mode. The initial 139state of the CEC adapter after calling cec_allocate_adapter() is disabled. 140 141Note that adap_enable must return 0 if enable is false. 142 143 144To enable/disable the 'monitor all' mode:: 145 146 int (*adap_monitor_all_enable)(struct cec_adapter *adap, bool enable); 147 148If enabled, then the adapter should be put in a mode to also monitor messages 149that are not for us. Not all hardware supports this and this function is only 150called if the CEC_CAP_MONITOR_ALL capability is set. This callback is optional 151(some hardware may always be in 'monitor all' mode). 152 153Note that adap_monitor_all_enable must return 0 if enable is false. 154 155 156To enable/disable the 'monitor pin' mode:: 157 158 int (*adap_monitor_pin_enable)(struct cec_adapter *adap, bool enable); 159 160If enabled, then the adapter should be put in a mode to also monitor CEC pin 161changes. Not all hardware supports this and this function is only called if 162the CEC_CAP_MONITOR_PIN capability is set. This callback is optional 163(some hardware may always be in 'monitor pin' mode). 164 165Note that adap_monitor_pin_enable must return 0 if enable is false. 166 167 168To program a new logical address:: 169 170 int (*adap_log_addr)(struct cec_adapter *adap, u8 logical_addr); 171 172If logical_addr == CEC_LOG_ADDR_INVALID then all programmed logical addresses 173are to be erased. Otherwise the given logical address should be programmed. 174If the maximum number of available logical addresses is exceeded, then it 175should return -ENXIO. Once a logical address is programmed the CEC hardware 176can receive directed messages to that address. 177 178Note that adap_log_addr must return 0 if logical_addr is CEC_LOG_ADDR_INVALID. 179 180 181To transmit a new message:: 182 183 int (*adap_transmit)(struct cec_adapter *adap, u8 attempts, 184 u32 signal_free_time, struct cec_msg *msg); 185 186This transmits a new message. The attempts argument is the suggested number of 187attempts for the transmit. 188 189The signal_free_time is the number of data bit periods that the adapter should 190wait when the line is free before attempting to send a message. This value 191depends on whether this transmit is a retry, a message from a new initiator or 192a new message for the same initiator. Most hardware will handle this 193automatically, but in some cases this information is needed. 194 195The CEC_FREE_TIME_TO_USEC macro can be used to convert signal_free_time to 196microseconds (one data bit period is 2.4 ms). 197 198 199To log the current CEC hardware status:: 200 201 void (*adap_status)(struct cec_adapter *adap, struct seq_file *file); 202 203This optional callback can be used to show the status of the CEC hardware. 204The status is available through debugfs: cat /sys/kernel/debug/cec/cecX/status 205 206To free any resources when the adapter is deleted:: 207 208 void (*adap_free)(struct cec_adapter *adap); 209 210This optional callback can be used to free any resources that might have been 211allocated by the driver. It's called from cec_delete_adapter. 212 213 214Your adapter driver will also have to react to events (typically interrupt 215driven) by calling into the framework in the following situations: 216 217When a transmit finished (successfully or otherwise):: 218 219 void cec_transmit_done(struct cec_adapter *adap, u8 status, 220 u8 arb_lost_cnt, u8 nack_cnt, u8 low_drive_cnt, 221 u8 error_cnt); 222 223or:: 224 225 void cec_transmit_attempt_done(struct cec_adapter *adap, u8 status); 226 227The status can be one of: 228 229CEC_TX_STATUS_OK: 230 the transmit was successful. 231 232CEC_TX_STATUS_ARB_LOST: 233 arbitration was lost: another CEC initiator 234 took control of the CEC line and you lost the arbitration. 235 236CEC_TX_STATUS_NACK: 237 the message was nacked (for a directed message) or 238 acked (for a broadcast message). A retransmission is needed. 239 240CEC_TX_STATUS_LOW_DRIVE: 241 low drive was detected on the CEC bus. This indicates that 242 a follower detected an error on the bus and requested a 243 retransmission. 244 245CEC_TX_STATUS_ERROR: 246 some unspecified error occurred: this can be one of ARB_LOST 247 or LOW_DRIVE if the hardware cannot differentiate or something 248 else entirely. Some hardware only supports OK and FAIL as the 249 result of a transmit, i.e. there is no way to differentiate 250 between the different possible errors. In that case map FAIL 251 to CEC_TX_STATUS_NACK and not to CEC_TX_STATUS_ERROR. 252 253CEC_TX_STATUS_MAX_RETRIES: 254 could not transmit the message after trying multiple times. 255 Should only be set by the driver if it has hardware support for 256 retrying messages. If set, then the framework assumes that it 257 doesn't have to make another attempt to transmit the message 258 since the hardware did that already. 259 260The hardware must be able to differentiate between OK, NACK and 'something 261else'. 262 263The \*_cnt arguments are the number of error conditions that were seen. 264This may be 0 if no information is available. Drivers that do not support 265hardware retry can just set the counter corresponding to the transmit error 266to 1, if the hardware does support retry then either set these counters to 2670 if the hardware provides no feedback of which errors occurred and how many 268times, or fill in the correct values as reported by the hardware. 269 270Be aware that calling these functions can immediately start a new transmit 271if there is one pending in the queue. So make sure that the hardware is in 272a state where new transmits can be started *before* calling these functions. 273 274The cec_transmit_attempt_done() function is a helper for cases where the 275hardware never retries, so the transmit is always for just a single 276attempt. It will call cec_transmit_done() in turn, filling in 1 for the 277count argument corresponding to the status. Or all 0 if the status was OK. 278 279When a CEC message was received: 280 281.. c:function:: 282 void cec_received_msg(struct cec_adapter *adap, struct cec_msg *msg); 283 284Speaks for itself. 285 286Implementing the interrupt handler 287---------------------------------- 288 289Typically the CEC hardware provides interrupts that signal when a transmit 290finished and whether it was successful or not, and it provides and interrupt 291when a CEC message was received. 292 293The CEC driver should always process the transmit interrupts first before 294handling the receive interrupt. The framework expects to see the cec_transmit_done 295call before the cec_received_msg call, otherwise it can get confused if the 296received message was in reply to the transmitted message. 297 298Optional: Implementing Error Injection Support 299---------------------------------------------- 300 301If the CEC adapter supports Error Injection functionality, then that can 302be exposed through the Error Injection callbacks: 303 304.. code-block:: none 305 306 struct cec_adap_ops { 307 /* Low-level callbacks */ 308 ... 309 310 /* Error injection callbacks */ 311 int (*error_inj_show)(struct cec_adapter *adap, struct seq_file *sf); 312 bool (*error_inj_parse_line)(struct cec_adapter *adap, char *line); 313 314 /* High-level CEC message callback */ 315 ... 316 }; 317 318If both callbacks are set, then an ``error-inj`` file will appear in debugfs. 319The basic syntax is as follows: 320 321Leading spaces/tabs are ignored. If the next character is a ``#`` or the end of the 322line was reached, then the whole line is ignored. Otherwise a command is expected. 323 324This basic parsing is done in the CEC Framework. It is up to the driver to decide 325what commands to implement. The only requirement is that the command ``clear`` without 326any arguments must be implemented and that it will remove all current error injection 327commands. 328 329This ensures that you can always do ``echo clear >error-inj`` to clear any error 330injections without having to know the details of the driver-specific commands. 331 332Note that the output of ``error-inj`` shall be valid as input to ``error-inj``. 333So this must work: 334 335.. code-block:: none 336 337 $ cat error-inj >einj.txt 338 $ cat einj.txt >error-inj 339 340The first callback is called when this file is read and it should show the 341current error injection state:: 342 343 int (*error_inj_show)(struct cec_adapter *adap, struct seq_file *sf); 344 345It is recommended that it starts with a comment block with basic usage 346information. It returns 0 for success and an error otherwise. 347 348The second callback will parse commands written to the ``error-inj`` file:: 349 350 bool (*error_inj_parse_line)(struct cec_adapter *adap, char *line); 351 352The ``line`` argument points to the start of the command. Any leading 353spaces or tabs have already been skipped. It is a single line only (so there 354are no embedded newlines) and it is 0-terminated. The callback is free to 355modify the contents of the buffer. It is only called for lines containing a 356command, so this callback is never called for empty lines or comment lines. 357 358Return true if the command was valid or false if there were syntax errors. 359 360Implementing the High-Level CEC Adapter 361--------------------------------------- 362 363The low-level operations drive the hardware, the high-level operations are 364CEC protocol driven. The following high-level callbacks are available: 365 366.. code-block:: none 367 368 struct cec_adap_ops { 369 /* Low-level callbacks */ 370 ... 371 372 /* Error injection callbacks */ 373 ... 374 375 /* High-level CEC message callback */ 376 int (*received)(struct cec_adapter *adap, struct cec_msg *msg); 377 }; 378 379The received() callback allows the driver to optionally handle a newly 380received CEC message:: 381 382 int (*received)(struct cec_adapter *adap, struct cec_msg *msg); 383 384If the driver wants to process a CEC message, then it can implement this 385callback. If it doesn't want to handle this message, then it should return 386-ENOMSG, otherwise the CEC framework assumes it processed this message and 387it will not do anything with it. 388 389 390CEC framework functions 391----------------------- 392 393CEC Adapter drivers can call the following CEC framework functions: 394 395.. c:function:: 396 int cec_transmit_msg(struct cec_adapter *adap, struct cec_msg *msg, \ 397 bool block); 398 399Transmit a CEC message. If block is true, then wait until the message has been 400transmitted, otherwise just queue it and return. 401 402.. c:function:: 403 void cec_s_phys_addr(struct cec_adapter *adap, u16 phys_addr, bool block); 404 405Change the physical address. This function will set adap->phys_addr and 406send an event if it has changed. If cec_s_log_addrs() has been called and 407the physical address has become valid, then the CEC framework will start 408claiming the logical addresses. If block is true, then this function won't 409return until this process has finished. 410 411When the physical address is set to a valid value the CEC adapter will 412be enabled (see the adap_enable op). When it is set to CEC_PHYS_ADDR_INVALID, 413then the CEC adapter will be disabled. If you change a valid physical address 414to another valid physical address, then this function will first set the 415address to CEC_PHYS_ADDR_INVALID before enabling the new physical address. 416 417.. c:function:: 418 void cec_s_phys_addr_from_edid(struct cec_adapter *adap, \ 419 const struct edid *edid); 420 421A helper function that extracts the physical address from the edid struct 422and calls cec_s_phys_addr() with that address, or CEC_PHYS_ADDR_INVALID 423if the EDID did not contain a physical address or edid was a NULL pointer. 424 425.. c:function:: 426 int cec_s_log_addrs(struct cec_adapter *adap, \ 427 struct cec_log_addrs *log_addrs, bool block); 428 429Claim the CEC logical addresses. Should never be called if CEC_CAP_LOG_ADDRS 430is set. If block is true, then wait until the logical addresses have been 431claimed, otherwise just queue it and return. To unconfigure all logical 432addresses call this function with log_addrs set to NULL or with 433log_addrs->num_log_addrs set to 0. The block argument is ignored when 434unconfiguring. This function will just return if the physical address is 435invalid. Once the physical address becomes valid, then the framework will 436attempt to claim these logical addresses. 437 438CEC Pin framework 439----------------- 440 441Most CEC hardware operates on full CEC messages where the software provides 442the message and the hardware handles the low-level CEC protocol. But some 443hardware only drives the CEC pin and software has to handle the low-level 444CEC protocol. The CEC pin framework was created to handle such devices. 445 446Note that due to the close-to-realtime requirements it can never be guaranteed 447to work 100%. This framework uses highres timers internally, but if a 448timer goes off too late by more than 300 microseconds wrong results can 449occur. In reality it appears to be fairly reliable. 450 451One advantage of this low-level implementation is that it can be used as 452a cheap CEC analyser, especially if interrupts can be used to detect 453CEC pin transitions from low to high or vice versa. 454 455.. kernel-doc:: include/media/cec-pin.h 456 457CEC Notifier framework 458---------------------- 459 460Most drm HDMI implementations have an integrated CEC implementation and no 461notifier support is needed. But some have independent CEC implementations 462that have their own driver. This could be an IP block for an SoC or a 463completely separate chip that deals with the CEC pin. For those cases a 464drm driver can install a notifier and use the notifier to inform the 465CEC driver about changes in the physical address. 466 467.. kernel-doc:: include/media/cec-notifier.h 468