On Thu, Nov 27, 2025 at 11:40:54PM +0200, Ville Syrjälä wrote: > 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? Change DP_RECEIVER_CAP_SIZE from 15 to 16 and it doesn't help. I can see it tried to read 16 bytes but it still timed out.
> > > > > 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. Right, I'll move the workaround to the drm driver, and submit v2. > > > + > > + if (dpcd[DP_DPCD_REV] == 0) > > return -EIO; > > > > return 0; > > -- > > 2.43.0 > > -- > Ville Syrjälä > Intel
