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

[Daniel Stone]

Hi Harry,

On Tue, 31 Mar 2026 at 18:47, Harry Wentland <harry.wentland@xxxxxxx> wrote:
> On 2026-03-31 08:50, Pekka Paalanen wrote:
> > People who care about the picture quality down to these levels will
> > likely want to know and learn about these techniques. They may also
> > want to explicitly control them.
> >
> > In time, when these have been used enough in the wild, compositor
> > developers will learn what makes a difference and what does not, so
> > they will adjust their reporting to end users. The most important thing
> > for the kernel is it offer an unambiguous and stable UAPI for these.
> >
> > Policy belongs in userspace.
>
> I don't like this as a blanket statement. There is a lot of policy that
> intersects with HW nuances, whether it comes to power or otherwise.
> Taking away driver vendor's abilities to optimize will hurt the Linux
> ecosystem in the long run.
>
> IMO this needs to be evaluated on a case by case basis. There are
> many places where it does make sense to give userspace a greater
> say on policy, but we don't want to push driver (HW specific) logic
> up into userspace.

It's not something that's _just_ specific to a particular
display-controller manufacturer or a particular IP generation though.
It very much depends on the usecase.

If you have a laptop and you're trying to give a presentation,
applying dithering and/or DSC makes a lot of sense: you don't want
your battery to die, and the projector's probably going to obliterate
half the colour anyway, so might as well as go for the most efficient
thing.

If your laptop is plugged into your big display at home to write code,
applying DSC to cram the highest possible resolution + refresh in
would make sense. But if dithering only results in a marginal power
saving, and your laptop is charging anyway - why bother degrading
visual acuity?

If you're a media player, then you're in a good position to know what
would be good to go over the wire, because you know (& are possibly in
control of) the format over what comes in in the first place.

But everyone's tradeoffs are different, which is why sometimes the
best choice is to ultimately leave it up to the user. If you dig into
any media playback device (STBs running Android TV, Apple TV, Fire TV,
et al), you'll see that all of them ultimately allow overrides for bpc
/ colour model / subsampling / etc. Those aren't just there for fun,
but because they are usable to real people, and it's not possible for
Amlogic or MediaTek or Rockchip or whoever to statically decide that a
certain configuration is going to be best everywhere.

Right now we have drivers making magic per-vendor/SKU decisions,
without even so much as a feedback mechanism to userspace (unless you
count debugfs, maybe) so it can even figure out what's going on, let
alone control it. To properly support some of those usecases,
userspace needs to be able to control what goes out on the wire, but
as a first step, it just wants to be informed of what the driver even
did with the properties we gave it.

The end game of this isn't Weston logging something to stdout, it's to
surface things to userspace so it can guide the kernel into making a
good decision for usecases that may not be ones the silicon vendor
decided was 'probably the best thing' however many years ago.

Cheers,
Daniel