Re: [PATCH] drm/doc: describe PATH format for DP MST

[Pekka Paalanen <ppaalanen@gmail.com>]

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On Mon, 23 Oct 2023 20:36:39 +0000
Simon Ser <contact@emersion.fr> wrote:

> 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>
> Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
> Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
>  drivers/gpu/drm/drm_connector.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index c3725086f413..392bec1355a3 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -1198,6 +1198,11 @@ 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 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

Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>

with or without the suggested "guaranteed" wording.

Are DP MST port numbers guaranteed to be tied to the physical hardware
configuration (e.g. how cables are connected) and therefore stable
across reboots? What about stable across kernel upgrades?

If I knew that, I could perhaps manufacture a stable identifier in
userspace by replacing the parent connector ID with a stable connector
designator.


Thanks,
pq

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