DMA-BUFs always uncached on arm64, causing poor camera performance on Librem 5

[Pavel Machek <pavel@ucw.cz>]

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Hi!

It seems that DMA-BUFs are always uncached on arm64... which is a
problem.

I'm trying to get useful camera support on Librem 5, and that includes
recording vidos (and taking photos).

memcpy() from normal memory is about 2msec/1MB. Unfortunately, for
DMA-BUFs it is 20msec/1MB, and that basically means I can't easily do
760p video recording. Plus, copying full-resolution photo buffer takes
more than 200msec!

There's possibility to do some processing on GPU, and its implemented here:

https://gitlab.com/tui/tui/-/tree/master/icam?ref_type=heads

but that hits the same problem in the end -- data is in DMA-BUF,
uncached, and takes way too long to copy out.

And that's ... wrong. DMA ended seconds ago, complete cache flush
would be way cheaper than copying single frame out, and I still have
to deal with uncached frames.

So I have two questions:

1) Is my analysis correct that, no matter how I get frame from v4l and
process it on GPU, I'll have to copy it from uncached memory in the
end?

2) Does anyone have patches / ideas / roadmap how to solve that? It
makes GPU unusable for computing, and camera basically unusable for
video.

Best regards,
								Pavel
-- 
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