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

[Daniel Stone]

On Wed, 1 Apr 2026 at 14:58, Ville Syrjälä
<ville.syrjala@xxxxxxxxxxxxxxx> wrote:
> I've been musing about userspace being able to provide some kind of
> relative quality weights for each output. The driver could then use
> those to figure out how to balance the final bpc and compression
> between the outputs. Something like this would let userspace express
> its preference while still allowing the driver to decide how to
> actually get there.
>
> Simple 'desired bpc' seem somewhat insufficient because I would
> imagine userspace just sets that to max for everything at the start,
> so the driver might not be able to tell which outputs can be degraded
> harder than others.
>
> I suppose a desired+min bpc might work, but would potentially force
> userspace to tweak the parameters in some semi random fashion and
> try again if the end result isn't appealing. And exactly what to
> tweak is really hard for userspace to figure out since it has no
> idea of the possibly complex internal/tbt/mst topologies, power
> costs, etc.

I agree with everything you've written, apart from 'I would imagine
userspace just sets that to max for everything at the start'.

I've taken it as axiomatic that all of these things should have an
'auto' value, and have it as their default setting. Userspace _may_
know better than the IHV, but it's only going to know on a situational
basis.

Compositors blindly setting random tuneables to MAX_AWESOME_POWER
would be just as stupid as distros shipping ye olde Option
"AGPFastWrite" and Option "AGPMode" "8" by default. I'd expect any
userspace which blindly did that to immediately get as many bug
reports as they have users, and for them to fix it accordingly.

By analogy, we allow userspace to ignore EDID and set whatever cool
awesome mode it wants to. But it doesn't do that unless it has a very
very good reason to override the driver, and 99 times out of 100, that
reason is 'the user has figured out that this is required'.

Does that help?

Cheers,
Daniel