Dynamically change enumeration list of DRM enumeration property

Pekka Paalanen ppaalanen at gmail.com
Tue May 26 07:39:21 UTC 2020

On Tue, 26 May 2020 10:01:23 +0530
Yogish Kulkarni <yogishkulkarni at gmail.com> wrote:

> Hi,
> Is it possible to dynamically change enumeration list of  DRM enumeration
> property ? Motivation behind this question is to understand whether it is
> possible to create connector enum property (e.g a property which will list
> supported output encodings -  like yuv420, yuv422 etc) whose list of
> supported enum values could be changed dynamically e.g. based on which sink
> is connected.
> I think there is existing EDID connector property whose value changes based
> on connected sink. EDID is a BLOB property, I am trying to understand if
> this is also possible for ENUM type property. There is
> "drm_property_replace_blob" to replace blob but I wasn't able to find any
> API which could replace list of supported enums. Alternatively, would it be
> good idea to destroy custom enum property created by a driver and create
> new enum property with new list of supported enums e.g when there is a
> HOTPLUG event.


looking at Weston code, it *might* cope with it. A hotplug event does
cause Weston to re-discover all properties of a connector. This is
specific to connectors only.

The race exists though: userspace might be poking at KMS after you
changed the property in the kernel but before userspace handles the
hotplug event. You'd have to check that does not cause regressions. I
guess for a completely new property, the risk seems low, as userspace
does not know to poke at it (risk of using outdated property or value
IDs causing unexpected atomic commit failure). Also I'm not aware of
any KMS program that would yet attempt blind KMS state save & restore
to sanitize the KMS state after dropping and re-gaining DRM master.

You'd have to check all other KMS using programs too: every Wayland
compositor, Xorg, DRM Vulkan WSI(?), ...

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