linux-kernel - [PATCH] drm/i915/dp: Add byte-by-byte fallback for broken USB-C adapters

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Message-ID: <20251127044406.618543-1-acelan.kao@canonical.com>
Date: Thu, 27 Nov 2025 12:44:06 +0800
From: "Chia-Lin Kao (AceLan)" <acelan.kao@...onical.com>
To: Jani Nikula <jani.nikula@...ux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@...el.com>,
	Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
	Tvrtko Ursulin <tursulin@...ulin.net>,
	David Airlie <airlied@...il.com>,
	Simona Vetter <simona@...ll.ch>,
	intel-gfx@...ts.freedesktop.org,
	intel-xe@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] drm/i915/dp: Add byte-by-byte fallback for broken USB-C adapters

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) <acelan.kao@...onical.com>
---
 .../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