On Mon, Dec 05, 2011 at 05:18:48PM +0000, Arnd Bergmann wrote: > On Friday 02 December 2011, Sumit Semwal wrote: > > + /* allow allocator to take care of cache ops */ > > + void (*sync_sg_for_cpu) (struct dma_buf *, struct device *); > > + void (*sync_sg_for_device)(struct dma_buf *, struct device *); > > I don't see how this works with multiple consumers: For the streaming > DMA mapping, there must be exactly one owner, either the device or > the CPU. Obviously, this rule needs to be extended when you get to > multiple devices and multiple device drivers, plus possibly user > mappings. Simply assigning the buffer to "the device" from one > driver does not block other drivers from touching the buffer, and > assigning it to "the cpu" does not stop other hardware that the > code calling sync_sg_for_cpu is not aware of. > > The only way to solve this that I can think of right now is to > mandate that the mappings are all coherent (i.e. noncachable > on noncoherent architectures like ARM). If you do that, you no > longer need the sync_sg_for_* calls. Woops, totally missed the addition of these. Can somebody explain to used to rather coherent x86 what we need these for and the code-flow would look like in a typical example. I was kinda assuming that devices would bracket their use of a buffer with the attachment_map/unmap calls and any cache coherency magic that might be needed would be somewhat transparent to users of the interface? The map call gets the dma_data_direction parameter, so it should be able to do the right thing. And because we keep the attachement around, any caching of mappings should be possible, too. Yours, Daniel PS: Slightly related, because it will make the coherency nightmare worse, afaict: Can we kill mmap support? -- Daniel Vetter Mail: daniel@xxxxxxxx Mobile: +41 (0)79 365 57 48 _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
