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

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Date:   Mon, 15 Feb 2021 09:06:01 +0000
From:   Simon Ser <contact@...rsion.fr>
To:     Christian König <christian.koenig@....com>
Cc:     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>,
        "Sharma, Shashank" <Shashank.Sharma@....com>
Subject: Re: DMA-buf and uncached system memory

On Monday, February 15th, 2021 at 9:58 AM, Christian König <christian.koenig@....com> wrote:

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

Intel also needs uncached system memory if I'm not mistaken?

Where are the buffers allocated? If GBM, then it needs to allocate memory that
can be scanned out if the USE_SCANOUT flag is set or if a scanout-capable
modifier is picked.

If this is about communicating buffer constraints between different components
of the stack, there were a few proposals about it. The most recent one is [1].

Simon

[1]: https://xdc2020.x.org/event/9/contributions/615/