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

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Date:   Mon, 27 Jun 2022 10:30:30 -0400
From:   Nicolas Dufresne <nicolas@...fresne.ca>
To:     Lucas Stach <l.stach@...gutronix.de>,
        Christian König <christian.koenig@....com>,
        Pekka Paalanen <ppaalanen@...il.com>
Cc:     "Sharma, Shashank" <Shashank.Sharma@....com>,
        lkml <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linaro-mm-sig@...ts.linaro.org,
        Sumit Semwal <sumit.semwal@...aro.org>,
        linux-media <linux-media@...r.kernel.org>
Subject: Re: DMA-buf and uncached system memory

Le lundi 27 juin 2022 à 16:06 +0200, Lucas Stach a écrit :
> Am Montag, dem 27.06.2022 um 09:54 -0400 schrieb Nicolas Dufresne:
> > Le jeudi 23 juin 2022 à 11:33 +0200, Lucas Stach a écrit :
> > > > 
> > > > See for example on AMD/Intel hardware most of the engines can perfectly 
> > > > deal with cache coherent memory accesses. Only the display engines can't.
> > > > 
> > > > So on import time we can't even say if the access can be coherent and 
> > > > snoop the CPU cache or not because we don't know how the imported 
> > > > DMA-buf will be used later on.
> > > > 
> > > So for those mixed use cases, wouldn't it help to have something
> > > similar to the dma_sync in the DMA-buf API, so your scanout usage can
> > > tell the exporter that it's going to do non-snoop access and any dirty
> > > cache lines must be cleaned? Signaling this to the exporter would allow
> > > to skip the cache maintenance if the buffer is in CPU uncached memory,
> > > which again is a default case for the ARM SoC world.
> > 
> > Telling the exporter for every scan is unneeded overhead. If that information is
> > made available "properly", then tracking it in attach/detach is sufficient and
> > lightweight.
> 
> That isn't sufficient. The AMD GPU is a single device, but internally
> has different engines that have different capabilities with regard to
> snooping the caches. So you will likely end up with needing the cache
> clean if the V4L2 buffer is going directly to scanout, which doesn't
> snoop, but if the usage changes to sampling you don't need any cache
> flushes.
> 
> Also I don't see a big overhead when comparing a kernel internal call
> that tells the exporter that the importer is going to access the buffer
> without snooping and thus needs the cache clean once every frame and
> the need to always clean the cache before DQBUF when a potentially non-
> snooping importer is attached.

Ack, thanks for the information.

> 
> Regards,
> Lucas
>