linux-kernel - RE: DMA-buf and uncached system memory

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Message-ID: <fa3f6eefc0a940b38448b0efd4b3f4e3@AcuMS.aculab.com>
Date:   Mon, 15 Feb 2021 14:41:52 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christian König' <christian.koenig@....com>,
        "Lucas Stach" <l.stach@...gutronix.de>,
        Simon Ser <contact@...rsion.fr>
CC:     "linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
        "Sharma, Shashank" <Shashank.Sharma@....com>,
        lkml <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-media <linux-media@...r.kernel.org>
Subject: RE: DMA-buf and uncached system memory

From: Christian König
> Sent: 15 February 2021 12:05
...
> Snooping the CPU caches introduces some extra latency, so what can
> happen is that the response to the PCIe read comes to late for the
> scanout. The result is an underflow and flickering whenever something is
> in the cache which needs to be flushed first.

Aren't you going to get the same problem if any other endpoints are
doing memory reads?
Possibly even ones that don't require a cache snoop and flush.

What about just the cpu doing a real memory transfer?

Or a combination of the two above happening just before your request.

If you don't have a big enough fifo you'll lose.

I did 'fix' a similar(ish) issue with video DMA latency on an embedded
system based the on SA1100/SA1101 by significantly reducing the clock
to the VGA panel whenever the cpu was doing 'slow io'.
(Interleaving an uncached cpu DRAM write between the slow io cycles
also fixed it.)
But the video was the only DMA device and that was an embedded system.
Given the application note about video latency didn't mention what was
actually happening, I'm not sure how many people actually got it working!

	David

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