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

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Date:   Mon, 15 Feb 2021 15:39:25 -0500
From:   Nicolas Dufresne <nicolas@...fresne.ca>
To:     Christian König <christian.koenig@....com>,
        linux-media <linux-media@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linaro-mm-sig@...ts.linaro.org, lkml <linux-kernel@...r.kernel.org>
Cc:     Sumit Semwal <sumit.semwal@...aro.org>,
        Daniel Vetter <daniel@...ll.ch>,
        "Sharma, Shashank" <Shashank.Sharma@....com>
Subject: Re: DMA-buf and uncached system memory

Le lundi 15 février 2021 à 09:58 +0100, Christian König a écrit :
> Hi guys,
> 
> we are currently working an Freesync and direct scan out from system 
> memory on AMD APUs in A+A laptops.
> 
> On problem we stumbled over is that our display hardware needs to scan 
> out from uncached system memory and we currently don't have a way to 
> communicate that through DMA-buf.
> 
> For our specific use case at hand we are going to implement something 
> driver specific, but the question is should we have something more 
> generic for this?

Hopefully I'm getting this right, but this makes me think of a long standing
issue I've met with Intel DRM and UVC driver. If I let the UVC driver allocate
the buffer, and import the resulting DMABuf (cacheable memory written with a cpu
copy in the kernel) into DRM, we can see cache artifact being displayed. While
if I use the DRM driver memory (dumb buffer in that case) it's clean because
there is a driver specific solution to that.

There is no obvious way for userspace application to know what's is right/wrong
way and in fact it feels like the kernel could solve this somehow without having
to inform userspace (perhaps).

> 
> After all the system memory access pattern is a PCIe extension and as 
> such something generic.
> 
> Regards,
> Christian.