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Thu, 2 Apr 2026 17:01:09 +0000 Message-ID: <1ed28250-b72a-4aa2-8eed-aeaff687c1d5@amd.com> Date: Thu, 2 Apr 2026 13:01:00 -0400 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 0/3] Add "link bpc" DRM property To: Daniel Stone Cc: Pekka Paalanen , =?UTF-8?Q?Michel_D=C3=A4nzer?= , Nicolas Frattaroli , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Leo Li , Rodrigo Siqueira , Alex Deucher , =?UTF-8?Q?Christian_K=C3=B6nig?= , =?UTF-8?B?VmlsbGUgU3lyasOkbMOk?= , Daniel Stone , Dmitry Baryshkov , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, amd-gfx@lists.freedesktop.org, kernel@collabora.com, Derek Foreman , Marius Vlad References: <20260319-link-bpc-v5-0-5306cd04a708@collabora.com> <4265353.aeNJFYEL58@workhorse> <254c20a4-cce3-4c8e-9902-514586f3e694@mailbox.org> <5416161.aeNJFYEL58@workhorse> <792c4540-d690-4453-a32e-62e23e78d628@mailbox.org> <9d525fe4-b091-4cd9-b977-de19ffe4b957@amd.com> <20260331155028.71246d7a@fluorite> Content-Language: en-US From: Harry Wentland In-Reply-To: Content-Type: text/plain; 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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. > I agree with you. But I wanted to respond to the statement that "policy belongs in userspace" and add a bit more nuance. It depends on the use-case and in any case, it's important drivers have the ability to set a good default policy for a given HW. We're still missing a lot of toggles to allow userspace to select a desired policy and I'm sure we'll be able to add those where needed but I agree with Michel and Ville that we need to get the end-to-end implementation right. Harry > Cheers, > Daniel