drm/doc: describe PATH format for DP MST

This is already uAPI, xserver parses it. It's useful to document
since user-space might want to lookup the parent connector.

Additionally, people (me included) have misunderstood the PATH
property for being stable across reboots, but since a KMS object
ID is baked in there that's not the case. So PATH shouldn't be
used as-is in config files and such.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20231023203629.198109-1-contact@emersion.fr

diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
index c3725086f41325afd6dd805a14f12dc9c9755a19..b0516505f7ae928ad39c7a383264507f6efd7531 100644 (file)
--- a/drivers/gpu/drm/drm_connector.c
+++ b/drivers/gpu/drm/drm_connector.c
@@ -1198,6 +1198,12 @@ static const u32 dp_colorspaces =
  *     drm_connector_set_path_property(), in the case of DP MST with the
  *     path property the MST manager created. Userspace cannot change this
  *     property.
+ *
+ *     In the case of DP MST, the property has the format
+ *     ``mst:<parent>-<ports>`` where ``<parent>`` is the KMS object ID of the
+ *     parent connector and ``<ports>`` is a hyphen-separated list of DP MST
+ *     port numbers. Note, KMS object IDs are not guaranteed to be stable
+ *     across reboots.
  * TILE:
  *     Connector tile group property to indicate how a set of DRM connector
  *     compose together into one logical screen. This is used by both high-res