On Thu, Nov 17, 2022 at 7:38 AM Nicolas Dufresne <nicolas@xxxxxxxxxxxx> 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 BR, -R
