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