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: Chia-Lin Kao (AceLan) <[email protected]> --- v2. 1. Move the workaround from intel_dp_read_dprx_caps() to drm_dp_dpcd_read_data(), so that it applies to all DPCD reads across all DRM drivers benefit from this fix, not just i915. 2. Move the definition of drm_dp_dpcd_readb() before drm_dp_dpcd_read_data() --- include/drm/display/drm_dp_helper.h | 57 +++++++++++++++++++---------- 1 file changed, 37 insertions(+), 20 deletions(-) diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h index df2f24b950e4..14d2859f0bda 100644 --- a/include/drm/display/drm_dp_helper.h +++ b/include/drm/display/drm_dp_helper.h @@ -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 < size) - return -EPROTO; + 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 -- 2.43.0
