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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) <acelan.kao@canonical.com> Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org> Link: https://patch.msgid.link/20251204024647.1462866-1-acelan.kao@canonical.com Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>