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