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

[Pekka Paalanen]

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.

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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