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Tue, 31 Mar 2026 17:38:00 +0000 Message-ID: Date: Tue, 31 Mar 2026 13:37:53 -0400 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 0/3] Add "link bpc" DRM property To: Pekka Paalanen Cc: =?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> <8676926.T7Z3S40VBb@workhorse> <4265353.aeNJFYEL58@workhorse> <254c20a4-cce3-4c8e-9902-514586f3e694@mailbox.org> <20260326155305.736b4e64@fluorite> <7461820c-e3ab-40f5-98d2-9878e60ba2ad@amd.com> <20260331132822.5ac57253@fluorite> Content-Language: en-US From: Harry Wentland In-Reply-To: <20260331132822.5ac57253@fluorite> Content-Type: text/plain; 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Compositors need to understand what the numbers mean, how >>> reliable they are, and act accordingly. Knowing the lower bound for >>> link precision is already useful as it guarantees a minimum precision. >>> It is up to the compositors to decide how they communicate this. >>> >>> Or course, assuming lossy compression is not too lossy. Maybe >>> lossy compression should be forbidden by default unless explicitly >>> enabled by userspace? >>> >> >> I disagree. While technically lossy, DSC is perceptually lossless, at >> least according to the designers of DSC. If I'm not mistaken this is >> all based on extensive studies. >> >> The decision to enable DSC or not has an impact on the power consumption >> of the HW, in ways that are often nuanced. Userspace has no way to know >> or understand these nuances. This should be in control of the driver. > > I guess time will tell. > > Are you saying that enabling DSC might have disadvantages aside from > image quality? > The opposite; enabling DSC might have advantages, in particular in power efficiency. And image quality impacts are negligible or (perceptually) non-existant, from my understanding. Afaik, Apple, which prides itself on image quality and being a platform for content producers, enables DSC by default on their systems. >> At most I could see a "never do DSC or dither" toggle, if one is really >> concerned about this, but I don't realistically see use-cases where this >> would improve user experience, even for users that care about color work >> and correctness. > > I'm not familiar with DSC, so I cannot criticise it. Dithering OTOH > seems to be obviously suspect though. > > Temporal dithering - what if your refresh rate is 30 Hz for some movie > playback? > > Spatial dithering - what if you have a low-resolution screen? > > I would not assume that dithering is always ok, and always achieves its > theoretical results. > Quite possibly, but we haven't really seen complaints about dither other than in scenarios where people explictly test that input pixels equal output pixels exactly. >> The YCbCr420 case is different. We probably want a way for userspace to >> understand that half 3/4 of chroma values are being tossed out. This >> would be significant for RGB content but insignificant for YCbCr420 >> content. > > Do you mean full resolution vs. chroma sub-sampled to 2x2 blocks? I > would again not assume "insignificant", because it depends on the > picture content and angular pixel density (can you see individual > pixels at your viewing distance). Gray-scale text will be fine, but > colored text is another question. > Yes. I think downgrading an output to YCbCr420 (chroma sub-sampled to 2x2 blocks) is a significant image quality impact. > I'm fine with proceeding with these assumptions, as long as it is > acknowledged that these assumptions might turn out false later and have > a contingency plan. > I guess that goes to Michel's question about what's expected of compositors here and whether there is clarity... I'm not sure I really have an answer on that other than documenting our expectations. Harry > > Thanks, > pq