Re: [PATCH v5 0/3] Add "link bpc" DRM property

[Ville Syrjälä]

On Wed, Apr 01, 2026 at 01:57:08PM +0200, Xaver Hugl wrote:
> Am Do., 19. März 2026 um 13:28 Uhr schrieb Nicolas Frattaroli
> <nicolas.frattaroli@xxxxxxxxxxxxx>:
> >
> > This series adds a new "link bpc" DRM property. It reflects the display
> > link's actual achieved output bits per component, considering any
> > degradation of the bit depth done by drivers for bandwidth or other
> > reasons. The property's value is updated during an atomic commit, which
> > is also when it fires an uevent if it changed to let userspace know.
> 
> Hi,
> I think it's a really good idea to have a property for knowing the
> actual bpc of the link... however, I do have one big concern with this
> API specifically: It only gives me this information after a modeset.
> 
> With this limitation, I can at most show the user which bpc was chosen
> after the apply display settings and have the end user manually test
> and figure things out, but I cannot show in the UI which bpc will be
> chosen with some configuration before they apply it, and I cannot do
> atomic tests to find a desired tradeoff automatically on the
> compositor side.

I think the idea of some kind of feedback properties in the atomic
commit has come up before, but no one has ever tried to implement them.

> 
> As a side note, for future patches relevant for compositors, please cc
> wayland-devel. It really shouldn't be up to chance whether or not
> compositor developers that would later use the API find out about it
> before it's merged, and keeping track of all of dri-devel is way too
> much to ask from userspace developers.
> 
> - Xaver

-- 
Ville Syrjälä
Intel