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

Pekka Paalanen
2024-02-05T10:12:23Z

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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.

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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