Re: [PATCH 2/2] drm/vkms: Use a simpler composition function

Pekka Paalanen
2024-02-05T10:19:15Z

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On Mon, 5 Feb 2024 12:12:04 +0200
Pekka Paalanen <pekka.paalanen@haloniitty.fi> wrote:

> On Fri, 2 Feb 2024 17:02:20 -0300
> Arthur Grillo <arthurgrillo@riseup.net> wrote:
>
> > On 02/02/24 12:49, Pekka Paalanen wrote:
> > > On Fri, 2 Feb 2024 13:13:22 +0100
> > > Miquel Raynal <miquel.raynal@bootlin.com> wrote:
> > >
> > >> Hello Maxime,
> > >>
> > >> + Arthur
> > >>
> > >> mripard@kernel.org 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@haloniitty.fi wrote on Fri, 2 Feb 2024 10:55:22 +0200:
> > >>>>
> > >>>>> On Thu, 01 Feb 2024 18:31:32 +0100
> > >>>>> Louis Chauvet <louis.chauvet@bootlin.com> 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@riseup.net/
> > >>
> > >> 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.

Oh, actually, since the byte offset depends on format, it might be
better to parametrize by direction and compute the offset in the
format-specific line-getter before the loop.

Thanks,
pq

> 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@igalia.com/
> >
> > >
> > > 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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