drm/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 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>

diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h
index df2f24b950e4c226f71abfffe35c7c19ea4a92c1..14d2859f0bdab788836e0a1c4725ac2cc839d70f 100644 (file)
--- 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