On Fri, 2 Feb 2024 17:02:20 -0300 Arthur Grillo <arthurgrillo@xxxxxxxxxx> wrote: > On 02/02/24 12:49, Pekka Paalanen wrote: > > On Fri, 2 Feb 2024 13:13:22 +0100 > > Miquel Raynal <miquel.raynal@xxxxxxxxxxx> wrote: > > > >> Hello Maxime, > >> > >> + Arthur > >> > >> mripard@xxxxxxxxxx wrote on Fri, 2 Feb 2024 10:53:37 +0100: > >> > >>> Hi Miquel, > >>> > >>> On Fri, Feb 02, 2024 at 10:26:01AM +0100, Miquel Raynal wrote: > >>>> pekka.paalanen@xxxxxxxxxxxxx wrote on Fri, 2 Feb 2024 10:55:22 +0200: > >>>> > >>>>> On Thu, 01 Feb 2024 18:31:32 +0100 > >>>>> Louis Chauvet <louis.chauvet@xxxxxxxxxxx> wrote: > >>>>> > >>>>>> Change the composition algorithm to iterate over pixels instead of lines. > >>>>>> It allows a simpler management of rotation and pixel access for complex formats. > >>>>>> > >>>>>> This new algorithm allows read_pixel function to have access to x/y > >>>>>> coordinates and make it possible to read the correct thing in a block > >>>>>> when block_w and block_h are not 1. > >>>>>> The iteration pixel-by-pixel in the same method also allows a simpler > >>>>>> management of rotation with drm_rect_* helpers. This way it's not needed > >>>>>> anymore to have misterious switch-case distributed in multiple places. > >>>>> > >>>>> Hi, > >>>>> > >>>>> there was a very good reason to write this code using lines: > >>>>> performance. Before lines, it was indeed operating on individual pixels. > >>>>> > >>>>> Please, include performance measurements before and after this series > >>>>> to quantify the impact on the previously already supported pixel > >>>>> formats, particularly the 32-bit-per-pixel RGB variants. > >>>>> > >>>>> VKMS will be used more and more in CI for userspace projects, and > >>>>> performance actually matters there. > >>>>> > >>>>> I'm worrying that this performance degradation here is significant. I > >>>>> believe it is possible to keep blending with lines, if you add new line > >>>>> getters for reading from rotated, sub-sampled etc. images. That way you > >>>>> don't have to regress the most common formats' performance. > >>>> > >>>> While I understand performance is important and should be taken into > >>>> account seriously, I cannot understand how broken testing could be > >>>> considered better. Fast but inaccurate will always be significantly > >>>> less attractive to my eyes. > >>> > >>> AFAIK, neither the cover letter nor the commit log claimed it was fixing > >>> something broken, just that it was "better" (according to what > >>> criteria?). > >> > >> Better is probably too vague and I agree the "fixing" part is not > >> clearly explained in the commit log. The cover-letter however states: > >> > >>> Patch 2/2: This patch is more complex. My main target was to solve issues > >>> I found in [1], but as it was very complex to do it "in place", I choose > >>> to rework the composition function. > >> ... > >>> [1]: https://lore.kernel.org/dri-devel/20240110-vkms-yuv-v2-0-952fcaa5a193@xxxxxxxxxx/ > >> > >> If you follow this link you will find all the feedback and especially > >> the "broken" parts. Just to be clear, writing bugs is totally expected > >> and review/testing is supposed to help on this regard. I am not blaming > >> the author in any way, just focusing on getting this code in a more > >> readable shape and hopefully reinforce the testing procedure. > >> > >>> If something is truly broken, it must be stated what exactly is so we > >>> can all come up with a solution that will satisfy everyone. > >> > >> Maybe going through the series pointed above will give more context > >> but AFAIU: the YUV composition is not totally right (and the tests used > >> to validate it need to be more complex as well in order to fail). > >> > >> Here is a proposal. > >> > >> Today's RGB implementation is only optimized in the line-by-line case > >> when there is no rotation. The logic is bit convoluted and may possibly > >> be slightly clarified with a per-format read_line() implementation, > >> at a very light performance cost. Such an improvement would definitely > >> benefit to the clarity of the code, especially when transformations > >> (especially the rotations) come into play because they would be clearly > >> handled differently instead of being "hidden" in the optimized logic. > >> Performances would not change much as this path is not optimized today > >> anyway (the pixel-oriented logic is already used in the rotation case). > >> > >> Arthur's YUV implementation is indeed well optimized but the added > >> complexity probably lead to small mistakes in the logic. The > >> per-format read_line() implementation mentioned above could be > >> extended to the YUV format as well, which would leverage Arthur's > >> proposal by re-using his optimized version. Louis will help on this > >> regard. However, for more complex cases such as when there is a > >> rotation, it will be easier (and not sub-optimized compared to the RGB > >> case) to also fallback to a pixel-oriented processing. > >> > >> Would this approach make sense? > > > > Hi, > > > > I think it would, if I understand what you mean. Ever since I proposed > > a line-by-line algorithm to improve the performance, I was thinking of > > per-format read_line() functions that would be selected outside of any > > loops. Extending that to support YUV is only natural. I can imagine > > rotation complicates things, and I won't oppose that resulting in a > > much heavier read_line() implementation used in those cases. They might > > perhaps call the original read_line() implementations pixel-by-pixel or > > plane-by-plane (i.e. YUV planes) per pixel. Chroma-siting complicates > > things even further. That way one could compose any > > rotation-format-siting combination by chaining function pointers. > > > > I haven't looked at VKMS in a long time, and I am disappointed to find > > that vkms_compose_row() is calling plane->pixel_read() pixel-by-pixel. > > The reading vfunc should be called with many pixels at a time when the > > source FB layout allows it. The whole point of the line-based functions > > was that they repeat the innermost loop in every function body to make > > the per-pixel overhead as small as possible. The VKMS implementations > > benchmarked before and after the original line-based algorithm showed > > that calling a function pointer per-pixel is relatively very expensive. > > Or maybe it was a switch-case. > > Hi, > > I think I'm the culprit for that, as stated on [1]. My intention with > the suggestion was to remove some code repetition and too facilitate the > rotation support implementation. Going back, I think I was to high on > DRY at the time and didn't worry about optimization, which was a > mistake. Hi Arthur, no worries. We can also blame reviewers for not minding benchmarks. ;-) > But, I agree with Miquel that the rotation logic is easier to implement > in a pixel-based way. So going pixel-by-pixel only when rotation occurs > would be great. Yes, and I think that can very well be done in the line-based framework still that existed in the old days before any rotation support was added. Essentially a plug-in line-getter function that then calls a format-specific line-getter pixel-by-pixel while applying the rotation. It would be simple, it would leave unrotated performance unharmed (use format-specific line-getter directly with lines), but it might be somewhat less performant for rotated KMS planes. I suspect that might be a good compromise. Format-specific line-getters could also be parameterized by pixel-to-pixel offset in bytes. Then they could directly traverse FB rows forward and backward, and even FB columns. It may or may not have a penalty compared to the original line-getters, so it would have to be benchmarked. Line-getters working on planar YUV FBs might delegate Y, U, V, or UV/VU reading to R8 and/or RG88 line or pixel reader functions. They might also return block-height lines instead of one line at a time. However, I wouldn't commit to any approach without benchmarking alternatives. The performance comparison must justify the code complexity, like it was seen with the initial line-based implementation. A good benchmark is key, IMO. Thanks, pq > > Best Regards, > ~Arthur Grillo > > [1]: https://lore.kernel.org/dri-devel/20230418130525.128733-2-mcanal@xxxxxxxxxx/ > > > > > Sorry, I didn't realize the optimization had already been lost. > > > > Btw. I'd suggest renaming vkms_compose_row() to vkms_fetch_row() since > > it's not composing anything and the name mislead me. > > > > I think if you inspect the compositing code as of revision > > 8356b97906503a02125c8d03c9b88a61ea46a05a you'll get a better feeling of > > what it was supposed to be. > > > > > > Thanks, > > pq
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