Try to address the DMA-buf coherency problem

[Nicolas Dufresne]

Le mercredi 23 novembre 2022 à 17:33 +0100, Daniel Vetter a écrit :
> On Wed, Nov 23, 2022 at 10:33:38AM +0200, Pekka Paalanen wrote:
> > On Tue, 22 Nov 2022 18:33:59 +0100
> > Christian König <christian.koenig at amd.com> wrote:
> > 
> > > We should have come up with dma-heaps earlier and make it clear that 
> > > exporting a DMA-buf from a device gives you something device specific 
> > > which might or might not work with others.
> > > 
> > > Apart from that I agree, DMA-buf should be capable of handling this. 
> > > Question left is what documentation is missing to make it clear how 
> > > things are supposed to work?
> > 
> > Perhaps somewhat related from Daniel Stone that seems to have been
> > forgotten:
> > https://lore.kernel.org/dri-devel/20210905122742.86029-1-daniels@collabora.com/
> > 
> > It aimed mostly at userspace, but sounds to me like the coherency stuff
> > could use a section of its own there?
> 
> Hm yeah it would be great to land that and then eventually extend. Daniel?

There is a lot of things documented in this document that have been said to be
completely wrong user-space behaviour in this thread. But it seems to pre-date
the DMA Heaps. The document also assume that DMA Heaps completely solves the CMA
vs system memory issue. But it also underline a very important aspect, that
userland is not aware which one to use. What this document suggest though seems
more realist then what has been said here.

Its overall a great document, it unfortunate that it only makes it into the DRM
mailing list.

Nicolas