drm/dp_mst: Make drm_dp_mst_dpcd_write() consistent with drm_dp_dpcd_write()

Noticed this while having some problems with hubs sometimes not being
detected on the first plug. Every single dpcd read or write function
returns the number of bytes transferred on success or a negative error
code, except apparently for drm_dp_mst_dpcd_write() - which returns 0 on
success.

There's not really any good reason for this difference that I can tell,
and having the two functions give differing behavior means that
drm_dp_dpcd_write() will end up returning 0 on success for MST devices,
but the number of bytes transferred for everything else.

So, fix that and update the kernel doc.

Signed-off-by: Lyude Paul <[email protected]>
Fixes: 2f221a5efed4 ("drm/dp_mst: Add MST support to DP DPCD R/W functions")
Cc: Hans de Goede <[email protected]>
Cc: Mikita Lipski <[email protected]>
Cc: Sean Paul <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Alex Deucher <[email protected]>

1 files changed, 4 insertions, 7 deletions

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 61e7beada832..6485ecaddbcb 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -2063,7 +2063,7 @@ ssize_t drm_dp_mst_dpcd_read(struct drm_dp_aux *aux,
  * sideband messaging as drm_dp_dpcd_write() does for local
  * devices via actual AUX CH.
  *
- * Return: 0 on success, negative error code on failure.
+ * Return: number of bytes written on success, negative error code on failure.
  */
 ssize_t drm_dp_mst_dpcd_write(struct drm_dp_aux *aux,
 			      unsigned int offset, void *buffer, size_t size)
@@ -3428,12 +3428,9 @@ static int drm_dp_send_dpcd_write(struct drm_dp_mst_topology_mgr *mgr,
 	drm_dp_queue_down_tx(mgr, txmsg);
 
 	ret = drm_dp_mst_wait_tx_reply(mstb, txmsg);
-	if (ret > 0) {
-		if (txmsg->reply.reply_type == DP_SIDEBAND_REPLY_NAK)
-			ret = -EIO;
-		else
-			ret = 0;
-	}
+	if (ret > 0 && txmsg->reply.reply_type == DP_SIDEBAND_REPLY_NAK)
+		ret = -EIO;
+
 	kfree(txmsg);
 fail_put:
 	drm_dp_mst_topology_put_mstb(mstb);