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. 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. 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
