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

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Date:   Mon, 15 Feb 2021 10:34:50 +0100
From:   Christian König <christian.koenig@....com>
To:     Simon Ser <contact@...rsion.fr>
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



Am 15.02.21 um 10:06 schrieb Simon Ser:
> 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?

No idea, that's why I'm asking. Could be that this is also interesting 
for I+A systems.

> 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].

Well the problem here is on a different level of the stack.

See resolution, pitch etc:.. can easily communicated in userspace 
without involvement of the kernel. The worst thing which can happen is 
that you draw garbage into your own application window.

But if you get the caching attributes in the page tables (both CPU as 
well as IOMMU, device etc...) wrong then ARM for example has the 
tendency to just spontaneously reboot

X86 is fortunately a bit more gracefully and you only end up with random 
data corruption, but that is only marginally better.

So to sum it up that is not something which we can leave in the hands of 
userspace.

I think that exporters in the DMA-buf framework should have the ability 
to tell importers if the system memory snooping is necessary or not.

Userspace components can then of course tell the exporter what the 
importer needs, but validation if that stuff is correct and doesn't 
crash the system must happen in the kernel.

Regards,
Christian.

>
> Simon
>
> [1]: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fxdc2020.x.org%2Fevent%2F9%2Fcontributions%2F615%2F&amp;data=04%7C01%7Cchristian.koenig%40amd.com%7Cb2824bd79bd44862b38e08d8d190f344%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637489767796900783%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=hIptfin5Xx6XlYBtGFYAAbfuNsnD6kmQEiggq9k10E8%3D&amp;reserved=0