drm/display: dp: change drm_dp_dpcd_read_link_status() return value

drm_dp_dpcd_read_link_status() follows the "return error code or number
of bytes read" protocol, with the code returning less bytes than
requested in case of some errors. However most of the drivers
interpreted that as "return error code in case of any error". Switch
drm_dp_dpcd_read_link_status() to drm_dp_dpcd_read_data() and make it
follow that protocol too.

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-v4-2-e80ff89593df@oss.qualcomm.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>

7 files changed, 28 insertions, 43 deletions

diff --git a/drivers/gpu/drm/amd/amdgpu/atombios_dp.c b/drivers/gpu/drm/amd/amdgpu/atombios_dp.c
index 521b9faab180..492813ab1b54 100644
--- a/drivers/gpu/drm/amd/amdgpu/atombios_dp.c
+++ b/drivers/gpu/drm/amd/amdgpu/atombios_dp.c
@@ -458,8 +458,8 @@ bool amdgpu_atombios_dp_needs_link_train(struct amdgpu_connector *amdgpu_connect
 	u8 link_status[DP_LINK_STATUS_SIZE];
 	struct amdgpu_connector_atom_dig *dig = amdgpu_connector->con_priv;
 
-	if (drm_dp_dpcd_read_link_status(&amdgpu_connector->ddc_bus->aux, link_status)
-	    <= 0)
+	if (drm_dp_dpcd_read_link_status(&amdgpu_connector->ddc_bus->aux,
+					 link_status) < 0)
 		return false;
 	if (drm_dp_channel_eq_ok(link_status, dig->dp_lane_count))
 		return false;
@@ -616,7 +616,7 @@ amdgpu_atombios_dp_link_train_cr(struct amdgpu_atombios_dp_link_train_info *dp_i
 		drm_dp_link_train_clock_recovery_delay(dp_info->aux, dp_info->dpcd);
 
 		if (drm_dp_dpcd_read_link_status(dp_info->aux,
-						 dp_info->link_status) <= 0) {
+						 dp_info->link_status) < 0) {
 			DRM_ERROR("displayport link status failed\n");
 			break;
 		}
@@ -681,7 +681,7 @@ amdgpu_atombios_dp_link_train_ce(struct amdgpu_atombios_dp_link_train_info *dp_i
 		drm_dp_link_train_channel_eq_delay(dp_info->aux, dp_info->dpcd);
 
 		if (drm_dp_dpcd_read_link_status(dp_info->aux,
-						 dp_info->link_status) <= 0) {
+						 dp_info->link_status) < 0) {
 			DRM_ERROR("displayport link status failed\n");
 			break;
 		}
diff --git a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
index ae1bd58975ce..96a983c970ea 100644
--- a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
+++ b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
@@ -2306,7 +2306,7 @@ static int cdns_mhdp_update_link_status(struct cdns_mhdp_device *mhdp)
 		 * If everything looks fine, just return, as we don't handle
 		 * DP IRQs.
 		 */
-		if (ret > 0 &&
+		if (!ret &&
 		    drm_dp_channel_eq_ok(status, mhdp->link.num_lanes) &&
 		    drm_dp_clock_recovery_ok(status, mhdp->link.num_lanes))
 			goto out;
diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
index e43a8f4a252d..410be0be233a 100644
--- a/drivers/gpu/drm/display/drm_dp_helper.c
+++ b/drivers/gpu/drm/display/drm_dp_helper.c
@@ -778,14 +778,13 @@ EXPORT_SYMBOL(drm_dp_dpcd_write);
  * @aux: DisplayPort AUX channel
  * @status: buffer to store the link status in (must be at least 6 bytes)
  *
- * Returns the number of bytes transferred on success or a negative error
- * code on failure.
+ * Returns a negative error code on failure or 0 on success.
  */
 int drm_dp_dpcd_read_link_status(struct drm_dp_aux *aux,
 				 u8 status[DP_LINK_STATUS_SIZE])
 {
-	return drm_dp_dpcd_read(aux, DP_LANE0_1_STATUS, status,
-				DP_LINK_STATUS_SIZE);
+	return drm_dp_dpcd_read_data(aux, DP_LANE0_1_STATUS, status,
+				     DP_LINK_STATUS_SIZE);
 }
 EXPORT_SYMBOL(drm_dp_dpcd_read_link_status);
 
diff --git a/drivers/gpu/drm/hisilicon/hibmc/dp/dp_link.c b/drivers/gpu/drm/hisilicon/hibmc/dp/dp_link.c
index f6355c16cc0a..a3b78b0fd53e 100644
--- a/drivers/gpu/drm/hisilicon/hibmc/dp/dp_link.c
+++ b/drivers/gpu/drm/hisilicon/hibmc/dp/dp_link.c
@@ -188,7 +188,7 @@ static int hibmc_dp_link_training_cr(struct hibmc_dp_dev *dp)
 		drm_dp_link_train_clock_recovery_delay(&dp->aux, dp->dpcd);
 
 		ret = drm_dp_dpcd_read_link_status(&dp->aux, lane_status);
-		if (ret != DP_LINK_STATUS_SIZE) {
+		if (ret) {
 			drm_err(dp->dev, "Get lane status failed\n");
 			return ret;
 		}
@@ -236,7 +236,7 @@ static int hibmc_dp_link_training_channel_eq(struct hibmc_dp_dev *dp)
 		drm_dp_link_train_channel_eq_delay(&dp->aux, dp->dpcd);
 
 		ret = drm_dp_dpcd_read_link_status(&dp->aux, lane_status);
-		if (ret != DP_LINK_STATUS_SIZE) {
+		if (ret) {
 			drm_err(dp->dev, "get lane status failed\n");
 			break;
 		}
diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c b/drivers/gpu/drm/msm/dp/dp_ctrl.c
index 9c463ae2f8fa..925b176b66c7 100644
--- a/drivers/gpu/drm/msm/dp/dp_ctrl.c
+++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c
@@ -1099,20 +1099,6 @@ static bool msm_dp_ctrl_train_pattern_set(struct msm_dp_ctrl_private *ctrl,
 	return ret == 1;
 }
 
-static int msm_dp_ctrl_read_link_status(struct msm_dp_ctrl_private *ctrl,
-				    u8 *link_status)
-{
-	int ret = 0, len;
-
-	len = drm_dp_dpcd_read_link_status(ctrl->aux, link_status);
-	if (len != DP_LINK_STATUS_SIZE) {
-		DRM_ERROR("DP link status read failed, err: %d\n", len);
-		ret = -EINVAL;
-	}
-
-	return ret;
-}
-
 static int msm_dp_ctrl_link_train_1(struct msm_dp_ctrl_private *ctrl,
 			int *training_step)
 {
@@ -1139,7 +1125,7 @@ static int msm_dp_ctrl_link_train_1(struct msm_dp_ctrl_private *ctrl,
 	for (tries = 0; tries < maximum_retries; tries++) {
 		drm_dp_link_train_clock_recovery_delay(ctrl->aux, ctrl->panel->dpcd);
 
-		ret = msm_dp_ctrl_read_link_status(ctrl, link_status);
+		ret = drm_dp_dpcd_read_link_status(ctrl->aux, link_status);
 		if (ret)
 			return ret;
 
@@ -1251,7 +1237,7 @@ static int msm_dp_ctrl_link_train_2(struct msm_dp_ctrl_private *ctrl,
 	for (tries = 0; tries <= maximum_retries; tries++) {
 		drm_dp_link_train_channel_eq_delay(ctrl->aux, ctrl->panel->dpcd);
 
-		ret = msm_dp_ctrl_read_link_status(ctrl, link_status);
+		ret = drm_dp_dpcd_read_link_status(ctrl->aux, link_status);
 		if (ret)
 			return ret;
 
@@ -1804,7 +1790,7 @@ static bool msm_dp_ctrl_channel_eq_ok(struct msm_dp_ctrl_private *ctrl)
 	u8 link_status[DP_LINK_STATUS_SIZE];
 	int num_lanes = ctrl->link->link_params.num_lanes;
 
-	msm_dp_ctrl_read_link_status(ctrl, link_status);
+	drm_dp_dpcd_read_link_status(ctrl->aux, link_status);
 
 	return drm_dp_channel_eq_ok(link_status, num_lanes);
 }
@@ -1862,7 +1848,7 @@ int msm_dp_ctrl_on_link(struct msm_dp_ctrl *msm_dp_ctrl)
 			if (!msm_dp_catalog_link_is_connected(ctrl->catalog))
 				break;
 
-			msm_dp_ctrl_read_link_status(ctrl, link_status);
+			drm_dp_dpcd_read_link_status(ctrl->aux, link_status);
 
 			rc = msm_dp_ctrl_link_rate_down_shift(ctrl);
 			if (rc < 0) { /* already in RBR = 1.6G */
@@ -1887,7 +1873,7 @@ int msm_dp_ctrl_on_link(struct msm_dp_ctrl *msm_dp_ctrl)
 			if (!msm_dp_catalog_link_is_connected(ctrl->catalog))
 				break;
 
-			msm_dp_ctrl_read_link_status(ctrl, link_status);
+			drm_dp_dpcd_read_link_status(ctrl->aux, link_status);
 
 			if (!drm_dp_clock_recovery_ok(link_status,
 					ctrl->link->link_params.num_lanes))
diff --git a/drivers/gpu/drm/msm/dp/dp_link.c b/drivers/gpu/drm/msm/dp/dp_link.c
index 1a1fbb2d7d4f..92a9077959b3 100644
--- a/drivers/gpu/drm/msm/dp/dp_link.c
+++ b/drivers/gpu/drm/msm/dp/dp_link.c
@@ -714,21 +714,21 @@ end:
 
 static int msm_dp_link_parse_sink_status_field(struct msm_dp_link_private *link)
 {
-	int len;
+	int ret;
 
 	link->prev_sink_count = link->msm_dp_link.sink_count;
-	len = drm_dp_read_sink_count(link->aux);
-	if (len < 0) {
+	ret = drm_dp_read_sink_count(link->aux);
+	if (ret < 0) {
 		DRM_ERROR("DP parse sink count failed\n");
-		return len;
+		return ret;
 	}
-	link->msm_dp_link.sink_count = len;
+	link->msm_dp_link.sink_count = ret;
 
-	len = drm_dp_dpcd_read_link_status(link->aux,
-		link->link_status);
-	if (len < DP_LINK_STATUS_SIZE) {
+	ret = drm_dp_dpcd_read_link_status(link->aux,
+					   link->link_status);
+	if (ret < 0) {
 		DRM_ERROR("DP link status read failed\n");
-		return len;
+		return ret;
 	}
 
 	return msm_dp_link_parse_request(link);
diff --git a/drivers/gpu/drm/radeon/atombios_dp.c b/drivers/gpu/drm/radeon/atombios_dp.c
index fa78824931cc..3f3c360dce4b 100644
--- a/drivers/gpu/drm/radeon/atombios_dp.c
+++ b/drivers/gpu/drm/radeon/atombios_dp.c
@@ -501,8 +501,8 @@ bool radeon_dp_needs_link_train(struct radeon_connector *radeon_connector)
 	u8 link_status[DP_LINK_STATUS_SIZE];
 	struct radeon_connector_atom_dig *dig = radeon_connector->con_priv;
 
-	if (drm_dp_dpcd_read_link_status(&radeon_connector->ddc_bus->aux, link_status)
-	    <= 0)
+	if (drm_dp_dpcd_read_link_status(&radeon_connector->ddc_bus->aux,
+					 link_status) < 0)
 		return false;
 	if (drm_dp_channel_eq_ok(link_status, dig->dp_lane_count))
 		return false;
@@ -678,7 +678,7 @@ static int radeon_dp_link_train_cr(struct radeon_dp_link_train_info *dp_info)
 		drm_dp_link_train_clock_recovery_delay(dp_info->aux, dp_info->dpcd);
 
 		if (drm_dp_dpcd_read_link_status(dp_info->aux,
-						 dp_info->link_status) <= 0) {
+						 dp_info->link_status) < 0) {
 			DRM_ERROR("displayport link status failed\n");
 			break;
 		}
@@ -741,7 +741,7 @@ static int radeon_dp_link_train_ce(struct radeon_dp_link_train_info *dp_info)
 		drm_dp_link_train_channel_eq_delay(dp_info->aux, dp_info->dpcd);
 
 		if (drm_dp_dpcd_read_link_status(dp_info->aux,
-						 dp_info->link_status) <= 0) {
+						 dp_info->link_status) < 0) {
 			DRM_ERROR("displayport link status failed\n");
 			break;
 		}