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) <[email protected]> > --- > .../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
