Try to address the DMA-buf coherency problem

[Nicolas Dufresne]

Le vendredi 18 novembre 2022 à 11:32 -0800, Rob Clark a écrit :
> On Thu, Nov 17, 2022 at 7:38 AM Nicolas Dufresne <nicolas at ndufresne.ca> wrote:
> > 
> > Le jeudi 17 novembre 2022 à 13:10 +0100, Christian König a écrit :
> > > > > DMA-Buf let's the exporter setup the DMA addresses the importer uses to
> > > > > be able to directly decided where a certain operation should go. E.g. we
> > > > > have cases where for example a P2P write doesn't even go to memory, but
> > > > > rather a doorbell BAR to trigger another operation. Throwing in CPU
> > > > > round trips for explicit ownership transfer completely breaks that
> > > > > concept.
> > > > It sounds like we should have a dma_dev_is_coherent_with_dev() which
> > > > accepts two (or an array?) of devices and tells the caller whether the
> > > > devices need explicit ownership transfer.
> > > 
> > > No, exactly that's the concept I'm pushing back on very hard here.
> > > 
> > > In other words explicit ownership transfer is not something we would
> > > want as requirement in the framework, cause otherwise we break tons of
> > > use cases which require concurrent access to the underlying buffer.
> > 
> > I'm not pushing for this solution, but really felt the need to correct you here.
> > I have quite some experience with ownership transfer mechanism, as this is how
> > GStreamer framework works since 2000. Concurrent access is a really common use
> > cases and it is quite well defined in that context. The bracketing system (in
> > this case called map() unmap(), with flag stating the usage intention like reads
> > and write) is combined the the refcount. The basic rules are simple:
> 
> This is all CPU oriented, I think Christian is talking about the case
> where ownership transfer happens without CPU involvement, such as via
> GPU waiting on a fence

HW fences and proper ownership isn't incompatible at all. Even if you have no
software involved during the usage, software still need to share the dmabuf (at
least once), and sharing modify the ownership, and can be made explicit.

p.s. I will agree if someone raises that this is totally off topic

Nicolas
> BR,
> -R