drm/display: dp: implement new access helpers

Existing DPCD access functions return an error code or the number of
bytes being read / write in case of partial access. However a lot of
drivers either (incorrectly) ignore partial access or mishandle error
codes. In other cases this results in a boilerplate code which compares
returned value with the size.

Implement new set of DPCD access helpers, which ignore partial access,
always return 0 or an error code.

Suggested-by: Jani Nikula <jani.nikula@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20250324-drm-rework-dpcd-access-v4-1-e80ff89593df@oss.qualcomm.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>

drivers/gpu/drm/display/drm_dp_helper.c

@@ -704,6 +704,8 @@ EXPORT_SYMBOL(drm_dp_dpcd_set_powered);
  * function returns -EPROTO. Errors from the underlying AUX channel transfer
  * function, with the exception of -EBUSY (which causes the transaction to
  * be retried), are propagated to the caller.
+ *
+ * In most of the cases you want to use drm_dp_dpcd_read_data() instead.
  */
 ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
 			 void *buffer, size_t size)
@@ -752,6 +754,8 @@ EXPORT_SYMBOL(drm_dp_dpcd_read);
  * function returns -EPROTO. Errors from the underlying AUX channel transfer
  * function, with the exception of -EBUSY (which causes the transaction to
  * be retried), are propagated to the caller.
+ *
+ * In most of the cases you want to use drm_dp_dpcd_write_data() instead.
  */
 ssize_t drm_dp_dpcd_write(struct drm_dp_aux *aux, unsigned int offset,
 			  void *buffer, size_t size)

include/drm/display/drm_dp_helper.h

@@ -527,6 +527,64 @@ 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_read_data() - read a series of bytes from the DPCD
+ * @aux: DisplayPort AUX channel (SST or MST)
+ * @offset: address of the (first) register to read
+ * @buffer: buffer to store the register values
+ * @size: number of bytes in @buffer
+ *
+ * Returns zero (0) on success, or a negative error
+ * code on failure. -EIO is returned if the request was NAKed by the sink or
+ * if the retry count was exceeded. If not all bytes were transferred, this
+ * function returns -EPROTO. Errors from the underlying AUX channel transfer
+ * function, with the exception of -EBUSY (which causes the transaction to
+ * be retried), are propagated to the caller.
+ */
+static inline int drm_dp_dpcd_read_data(struct drm_dp_aux *aux,
+					unsigned int offset,
+					void *buffer, size_t size)
+{
+	int ret;
+
+	ret = drm_dp_dpcd_read(aux, offset, buffer, size);
+	if (ret < 0)
+		return ret;
+	if (ret < size)
+		return -EPROTO;
+
+	return 0;
+}
+
+/**
+ * drm_dp_dpcd_write_data() - write a series of bytes to the DPCD
+ * @aux: DisplayPort AUX channel (SST or MST)
+ * @offset: address of the (first) register to write
+ * @buffer: buffer containing the values to write
+ * @size: number of bytes in @buffer
+ *
+ * Returns zero (0) on success, or a negative error
+ * code on failure. -EIO is returned if the request was NAKed by the sink or
+ * if the retry count was exceeded. If not all bytes were transferred, this
+ * function returns -EPROTO. Errors from the underlying AUX channel transfer
+ * function, with the exception of -EBUSY (which causes the transaction to
+ * be retried), are propagated to the caller.
+ */
+static inline int drm_dp_dpcd_write_data(struct drm_dp_aux *aux,
+					 unsigned int offset,
+					 void *buffer, size_t size)
+{
+	int ret;
+
+	ret = drm_dp_dpcd_write(aux, offset, buffer, size);
+	if (ret < 0)
+		return ret;
+	if (ret < size)
+		return -EPROTO;
+
+	return 0;
+}
+
 /**
  * drm_dp_dpcd_readb() - read a single byte from the DPCD
  * @aux: DisplayPort AUX channel
@@ -534,7 +592,8 @@ ssize_t drm_dp_dpcd_write(struct drm_dp_aux *aux, unsigned int offset,
  * @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.
+ * 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)
@@ -549,7 +608,8 @@ static inline ssize_t drm_dp_dpcd_readb(struct drm_dp_aux *aux,
  * @value: value to write to the register
  *
  * Returns the number of bytes transferred (1) on success, or a negative
- * error code on failure.
+ * error code on failure. In most of the cases you should be using
+ * drm_dp_dpcd_write_byte() instead.
  */
 static inline ssize_t drm_dp_dpcd_writeb(struct drm_dp_aux *aux,
 					 unsigned int offset, u8 value)
@@ -557,6 +617,34 @@ static inline ssize_t drm_dp_dpcd_writeb(struct drm_dp_aux *aux,
 	return drm_dp_dpcd_write(aux, offset, &value, 1);
 }
 
+/**
+ * drm_dp_dpcd_read_byte() - 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 zero (0) on success, or a negative error code on failure.
+ */
+static inline int drm_dp_dpcd_read_byte(struct drm_dp_aux *aux,
+					unsigned int offset, u8 *valuep)
+{
+	return drm_dp_dpcd_read_data(aux, offset, valuep, 1);
+}
+
+/**
+ * drm_dp_dpcd_write_byte() - write a single byte to the DPCD
+ * @aux: DisplayPort AUX channel
+ * @offset: address of the register to write
+ * @value: value to write to the register
+ *
+ * Returns zero (0) on success, or a negative error code on failure.
+ */
+static inline int drm_dp_dpcd_write_byte(struct drm_dp_aux *aux,
+					 unsigned int offset, u8 value)
+{
+	return drm_dp_dpcd_write_data(aux, offset, &value, 1);
+}
+
 int drm_dp_read_dpcd_caps(struct drm_dp_aux *aux,
 			  u8 dpcd[DP_RECEIVER_CAP_SIZE]);