[PATCH] drm/dp: Move byte-by-byte AUX read fallback to drm_dp_dpcd_read()

[Chia-Lin Kao (AceLan)]

Move the workaround for USB-C hubs/adapters with buggy firmware (where
multi-byte AUX reads timeout but single-byte reads work) from the
high-level drm_dp_dpcd_read_data() helper down into drm_dp_dpcd_read().

This ensures all direct and indirect callers of drm_dp_dpcd_read()
benefit from the fallback mechanism, and restores the original clean
semantics and layout of include/drm/display/drm_dp_helper.h.

Also, avoid using drm_dp_dpcd_readb() in the fallback loop since it may
return 0 when the underlying transfer returns no bytes, which would be
silently treated as success. Instead, call drm_dp_dpcd_read_byte() which
goes through drm_dp_dpcd_read_data() and maps a short transfer to
-EPROTO; check err < 0 to catch all failure cases.

Fixes: a8f49a004301 ("drm/dp: Add byte-by-byte fallback for broken USB-C adapters")
Signed-off-by: Chia-Lin Kao (AceLan) <acelan.kao@xxxxxxxxxxxxx>
---
 drivers/gpu/drm/display/drm_dp_helper.c | 21 +++++++++
 include/drm/display/drm_dp_helper.h     | 57 +++++++++----------------
 2 files changed, 41 insertions(+), 37 deletions(-)

diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
index 9c31e14cc413b..d14a8b40a1f17 100644
--- a/drivers/gpu/drm/display/drm_dp_helper.c
+++ b/drivers/gpu/drm/display/drm_dp_helper.c
@@ -762,6 +762,27 @@ ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
 		ret = drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, offset,
 					 buffer, size);
 
+	if (ret < 0 && size > 1) {
+		size_t i;
+		u8 *buf = buffer;
+		int err;
+
+		/*
+		 * 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++) {
+			err = drm_dp_dpcd_read_byte(aux, offset + i, &buf[i]);
+			if (err < 0)
+				return err;
+		}
+		ret = size;
+	}
+
 	drm_dp_dump_access(aux, DP_AUX_NATIVE_READ, offset, buffer, ret);
 	return ret;
 }
diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h
index 8c2d77a032f06..0126fb8080ab8 100644
--- a/include/drm/display/drm_dp_helper.h
+++ b/include/drm/display/drm_dp_helper.h
@@ -555,22 +555,6 @@ 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)
@@ -590,29 +574,12 @@ 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) {
-		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;
-	}
+	if (ret < 0)
+		return ret;
+	if (ret < size)
+		return -EPROTO;
 
 	return 0;
 }
@@ -646,6 +613,22 @@ 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
-- 
2.53.0