Commit a8f49a00 authored by Chia-Lin Kao (AceLan)'s avatar Chia-Lin Kao (AceLan) Committed by Mario Limonciello (AMD)
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drm/dp: Add byte-by-byte fallback for broken USB-C adapters 



Some USB-C hubs and adapters have buggy firmware where multi-byte AUX
reads consistently timeout, while single-byte reads from the same address
work correctly.

Known affected devices that exhibit this issue:
- Lenovo USB-C to VGA adapter (VIA VL817 chipset)
  idVendor=17ef, idProduct=7217
- Dell DA310 USB-C mobile adapter hub
  idVendor=413c, idProduct=c010

Analysis of the failure pattern shows:
- Single-byte probes to 0xf0000 (LTTPR) succeed
- Single-byte probes to 0x00102 (TRAINING_AUX_RD_INTERVAL) succeed
- Multi-byte reads from 0x00000 (DPCD capabilities) timeout with -ETIMEDOUT
- Retrying does not help - the failure is consistent across all attempts

The issue appears to be a firmware bug in the AUX transaction handling
that specifically affects multi-byte reads.

Add a fallback mechanism in drm_dp_dpcd_read_data() that attempts
byte-by-byte reading when the normal multi-byte read fails. This
workaround only activates for adapters that fail the standard read path,
ensuring no impact on correctly functioning hardware.

Tested with:
- Lenovo USB-C to VGA adapter (VIA VL817) - now works with fallback
- Dell DA310 USB-C hub - now works with fallback
- Dell/Analogix Slimport adapter - continues to work with normal path

Signed-off-by: default avatarChia-Lin Kao (AceLan) <acelan.kao@canonical.com>
Reviewed-by: default avatarMario Limonciello (AMD) <superm1@kernel.org>
Link: https://patch.msgid.link/20251204024647.1462866-1-acelan.kao@canonical.com


Signed-off-by: default avatarMario Limonciello (AMD) <superm1@kernel.org>
parent 9ce4aef9

include/drm/display/drm_dp_helper.h

+37 −20
Original line number Diff line number Diff line
@@ -551,6 +551,22 @@ ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset, 
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. In most of the cases you should be using

 * drm_dp_dpcd_read_byte() instead.

 */

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_read_data() - read a series of bytes from the DPCD

 * @aux: DisplayPort AUX channel (SST or MST)
@@ -570,12 +586,29 @@ static inline int drm_dp_dpcd_read_data(struct drm_dp_aux *aux, 
					void *buffer, size_t size)

{

	int ret;

	size_t i;

	u8 *buf = buffer;



	ret = drm_dp_dpcd_read(aux, offset, buffer, size);

	if (ret < 0)

		return ret;

	if (ret >= 0) {

		if (ret < size)

			return -EPROTO;

		return 0;

	}



	/*

	 * Workaround for USB-C hubs/adapters with buggy firmware that fail

	 * multi-byte AUX reads but work with single-byte reads.

	 * Known affected devices:

	 * - Lenovo USB-C to VGA adapter (VIA VL817, idVendor=17ef, idProduct=7217)

	 * - Dell DA310 USB-C hub (idVendor=413c, idProduct=c010)

	 * Attempt byte-by-byte reading as a fallback.

	 */

	for (i = 0; i < size; i++) {

		ret = drm_dp_dpcd_readb(aux, offset + i, &buf[i]);

		if (ret < 0)

			return ret;

	}



	return 0;

}
@@ -609,22 +642,6 @@ static inline int drm_dp_dpcd_write_data(struct drm_dp_aux *aux, 
	return 0;

}



/**

 * 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. In most of the cases you should be using

 * drm_dp_dpcd_read_byte() instead.

 */

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