Re: [PATCH] drm/i915/dp: Add byte-by-byte fallback for broken USB-C adapters

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Nov 27, 2025 at 12:44:06PM +0800, Chia-Lin Kao (AceLan) wrote:
> Some USB-C hubs and adapters have buggy firmware where multi-byte AUX
> reads from DPCD address 0x00000 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
> - 15-byte reads from 0x00000 (DPCD capabilities) timeout with -ETIMEDOUT
> - Retrying does not help - the failure is consistent across all attempts

I thought we changed that to the more sensible 16 bytes.
Anyone know what happened to that patch?

Anyways, does 16 bytes work better than 15 bytes?

> 
> The issue appears to be a firmware bug in the AUX transaction handling
> that specifically affects multi-byte reads from the base DPCD address.
> 
> Add a fallback mechanism that attempts byte-by-byte reading when the
> normal multi-byte drm_dp_read_dpcd_caps() fails. This workaround only
> activates for adapters that fail the standard read path, ensuring no
> impact on correctly functioning hardware.
> 
> The byte-by-byte read uses drm_dp_dpcd_readb() to read each of the 15
> DPCD capability bytes individually, working around the firmware bug
> while maintaining compatibility with all other adapters.
> 
> 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: Chia-Lin Kao (AceLan) <acelan.kao@canonical.com>
> ---
>  .../drm/i915/display/intel_dp_link_training.c | 21 ++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_link_training.c b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> index aad5fe14962f..738a5bb4adb3 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> @@ -213,6 +213,7 @@ static int intel_dp_init_lttpr(struct intel_dp *intel_dp, const u8 dpcd[DP_RECEI
>  int intel_dp_read_dprx_caps(struct intel_dp *intel_dp, u8 dpcd[DP_RECEIVER_CAP_SIZE])
>  {
>  	struct intel_display *display = to_intel_display(intel_dp);
> +	int ret, i;
>  
>  	if (intel_dp_is_edp(intel_dp))
>  		return 0;
> @@ -226,7 +227,25 @@ int intel_dp_read_dprx_caps(struct intel_dp *intel_dp, u8 dpcd[DP_RECEIVER_CAP_S
>  				      DP_LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV))
>  			return -EIO;
>  
> -	if (drm_dp_read_dpcd_caps(&intel_dp->aux, dpcd))
> +	ret = drm_dp_read_dpcd_caps(&intel_dp->aux, dpcd);
> +	if (ret == 0)
> +		return 0;
> +
> +	/*
> +	 * Workaround for USB-C hubs/adapters with buggy firmware that fail
> +	 * multi-byte AUX reads from DPCD address 0x00000 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)
> +	 * Read the DPCD capabilities byte-by-byte as a fallback.
> +	 */
> +	for (i = 0; i < DP_RECEIVER_CAP_SIZE; i++) {
> +		ret = drm_dp_dpcd_readb(&intel_dp->aux, DP_DPCD_REV + i, &dpcd[i]);
> +		if (ret < 0)
> +			return -EIO;
> +	}

Doing this in i915 specific code doesn't make sense.

> +
> +	if (dpcd[DP_DPCD_REV] == 0)
>  		return -EIO;
>  
>  	return 0;
> -- 
> 2.43.0

-- 
Ville Syrjälä
Intel