[SRU][Q/R][PATCH 1/1] UBUNTU: SAUCE: drm/dp: Add byte-by-byte fallback for broken USB-C adapters

[AceLan Kao]

From: "Chia-Lin Kao (AceLan)" <acelan.kao at canonical.com>

BugLink: https://bugs.launchpad.net/bugs/2133143

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 at canonical.com>
(cherry picked from commit https://lore.kernel.org/lkml/20251204024647.1462866-1-acelan.kao@canonical.com/T/#u)
Signed-off-by: Chia-Lin Kao (AceLan) <acelan.kao at canonical.com>
---
 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 87caa4f1fdb86..602c5a3a3cc28 100644
--- a/include/drm/display/drm_dp_helper.h
+++ b/include/drm/display/drm_dp_helper.h
@@ -538,6 +538,22 @@ 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)
@@ -557,12 +573,29 @@ 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;
 }
@@ -596,22 +629,6 @@ 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