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Commit c197db75 authored by Thierry Reding's avatar Thierry Reding
Browse files

drm/dp: Add AUX channel infrastructure 



This is a superset of the current i2c_dp_aux bus functionality and can
be used to transfer native AUX in addition to I2C-over-AUX messages.

Helpers are provided to read and write the DPCD, either blockwise or
byte-wise. Many of the existing helpers for DisplayPort take a copy of a
portion of the DPCD and operate on that, without a way to write data
back to the DPCD (e.g. for configuration of the link).

Subsequent patches will build upon this infrastructure to provide common
functionality in a generic way.

Reviewed-by: default avatarAlex Deucher <alexander.deucher@amd.com>
Reviewed-by: default avatarJani Nikula <jani.nikula@intel.com>
Signed-off-by: default avatarThierry Reding <treding@nvidia.com>
---
Changes in v5:
- move comments partially to struct drm_dp_aux_msg in header file
- return -EPROTO on short reads in DPCD helpers

Changes in v4:
- fix a typo in a comment

Changes in v3:
- reorder drm_dp_dpcd_writeb() arguments to be more intuitive
- return number of bytes transferred in drm_dp_dpcd_write()
- factor out drm_dp_dpcd_access()
- describe error codes
parent 38dbfb59

drivers/gpu/drm/drm_dp_helper.c

+110 −0
Original line number Diff line number Diff line
@@ -346,3 +346,113 @@ int drm_dp_bw_code_to_link_rate(u8 link_bw) 
	}

}

EXPORT_SYMBOL(drm_dp_bw_code_to_link_rate);



/**

 * DOC: dp helpers

 *

 * The DisplayPort AUX channel is an abstraction to allow generic, driver-

 * independent access to AUX functionality. Drivers can take advantage of

 * this by filling in the fields of the drm_dp_aux structure.

 *

 * Transactions are described using a hardware-independent drm_dp_aux_msg

 * structure, which is passed into a driver's .transfer() implementation.

 * Both native and I2C-over-AUX transactions are supported.

 *

 * An AUX channel can also be used to transport I2C messages to a sink. A

 * typical application of that is to access an EDID that's present in the

 * sink device. The .transfer() function can also be used to execute such

 * transactions. The drm_dp_aux_register_i2c_bus() function registers an

 * I2C adapter that can be passed to drm_probe_ddc(). Upon removal, drivers

 * should call drm_dp_aux_unregister_i2c_bus() to remove the I2C adapter.

 */



static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,

			      unsigned int offset, void *buffer, size_t size)

{

	struct drm_dp_aux_msg msg;

	unsigned int retry;

	int err;



	memset(&msg, 0, sizeof(msg));

	msg.address = offset;

	msg.request = request;

	msg.buffer = buffer;

	msg.size = size;



	/*

	 * The specification doesn't give any recommendation on how often to

	 * retry native transactions, so retry 7 times like for I2C-over-AUX

	 * transactions.

	 */

	for (retry = 0; retry < 7; retry++) {

		err = aux->transfer(aux, &msg);

		if (err < 0) {

			if (err == -EBUSY)

				continue;



			return err;

		}



		if (err < size)

			return -EPROTO;



		switch (msg.reply & DP_AUX_NATIVE_REPLY_MASK) {

		case DP_AUX_NATIVE_REPLY_ACK:

			return err;



		case DP_AUX_NATIVE_REPLY_NACK:

			return -EIO;



		case DP_AUX_NATIVE_REPLY_DEFER:

			usleep_range(400, 500);

			break;

		}

	}



	DRM_ERROR("too many retries, giving up\n");

	return -EIO;

}



/**

 * drm_dp_dpcd_read() - read a series of bytes from the DPCD

 * @aux: DisplayPort AUX channel

 * @offset: address of the (first) register to read

 * @buffer: buffer to store the register values

 * @size: number of bytes in @buffer

 *

 * Returns the number of bytes transferred on success, or a negative error

 * code on failure. -EIO is returned if the request was NAKed by the sink or

 * if the retry count was exceeded. If not all bytes were transferred, this

 * function returns -EPROTO. Errors from the underlying AUX channel transfer

 * function, with the exception of -EBUSY (which causes the transaction to

 * be retried), are propagated to the caller.

 */

ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,

			 void *buffer, size_t size)

{

	return drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, offset, buffer,

				  size);

}

EXPORT_SYMBOL(drm_dp_dpcd_read);



/**

 * drm_dp_dpcd_write() - write a series of bytes to the DPCD

 * @aux: DisplayPort AUX channel

 * @offset: address of the (first) register to write

 * @buffer: buffer containing the values to write

 * @size: number of bytes in @buffer

 *

 * Returns the number of bytes transferred on success, or a negative error

 * code on failure. -EIO is returned if the request was NAKed by the sink or

 * if the retry count was exceeded. If not all bytes were transferred, this

 * function returns -EPROTO. Errors from the underlying AUX channel transfer

 * function, with the exception of -EBUSY (which causes the transaction to

 * be retried), are propagated to the caller.

 */

ssize_t drm_dp_dpcd_write(struct drm_dp_aux *aux, unsigned int offset,

			  void *buffer, size_t size)

{

	return drm_dp_dpcd_access(aux, DP_AUX_NATIVE_WRITE, offset, buffer,

				  size);

}

EXPORT_SYMBOL(drm_dp_dpcd_write);

include/drm/drm_dp_helper.h

+79 −0
Original line number Diff line number Diff line
@@ -398,4 +398,83 @@ drm_dp_enhanced_frame_cap(const u8 dpcd[DP_RECEIVER_CAP_SIZE]) 
		(dpcd[DP_MAX_LANE_COUNT] & DP_ENHANCED_FRAME_CAP);

}



/*

 * DisplayPort AUX channel

 */



/**

 * struct drm_dp_aux_msg - DisplayPort AUX channel transaction

 * @address: address of the (first) register to access

 * @request: contains the type of transaction (see DP_AUX_* macros)

 * @reply: upon completion, contains the reply type of the transaction

 * @buffer: pointer to a transmission or reception buffer

 * @size: size of @buffer

 */

struct drm_dp_aux_msg {

	unsigned int address;

	u8 request;

	u8 reply;

	void *buffer;

	size_t size;

};



/**

 * struct drm_dp_aux - DisplayPort AUX channel

 * @dev: pointer to struct device that is the parent for this AUX channel

 * @transfer: transfers a message representing a single AUX transaction

 *

 * The .dev field should be set to a pointer to the device that implements

 * the AUX channel.

 *

 * Drivers provide a hardware-specific implementation of how transactions

 * are executed via the .transfer() function. A pointer to a drm_dp_aux_msg

 * structure describing the transaction is passed into this function. Upon

 * success, the implementation should return the number of payload bytes

 * that were transferred, or a negative error-code on failure. Helpers

 * propagate errors from the .transfer() function, with the exception of

 * the -EBUSY error, which causes a transaction to be retried. On a short,

 * helpers will return -EPROTO to make it simpler to check for failure.

 */

struct drm_dp_aux {

	struct device *dev;



	ssize_t (*transfer)(struct drm_dp_aux *aux,

			    struct drm_dp_aux_msg *msg);

};



ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,

			 void *buffer, size_t size);

ssize_t drm_dp_dpcd_write(struct drm_dp_aux *aux, unsigned int offset,

			  void *buffer, size_t size);



/**

 * drm_dp_dpcd_readb() - read a single byte from the DPCD

 * @aux: DisplayPort AUX channel

 * @offset: address of the register to read

 * @valuep: location where the value of the register will be stored

 *

 * Returns the number of bytes transferred (1) on success, or a negative

 * error code on failure.

 */

static inline ssize_t drm_dp_dpcd_readb(struct drm_dp_aux *aux,

					unsigned int offset, u8 *valuep)

{

	return drm_dp_dpcd_read(aux, offset, valuep, 1);

}



/**

 * drm_dp_dpcd_writeb() - write a single byte to the DPCD

 * @aux: DisplayPort AUX channel

 * @offset: address of the register to write

 * @value: value to write to the register

 *

 * Returns the number of bytes transferred (1) on success, or a negative

 * error code on failure.

 */

static inline ssize_t drm_dp_dpcd_writeb(struct drm_dp_aux *aux,

					 unsigned int offset, u8 value)

{

	return drm_dp_dpcd_write(aux, offset, &value, 1);

}



#endif /* _DRM_DP_HELPER_H_ */