On Tue, Oct 15, 2024 at 04:10:44AM +0300, Mina Almasry wrote: > On Tue, Oct 15, 2024 at 3:16 AM Jakub Kicinski <kuba@xxxxxxxxxx> wrote: > > > > On Tue, 15 Oct 2024 01:38:20 +0300 Mina Almasry wrote: > > > Thanks Jason. In that case I agree with Jakub we should take in his change here: > > > > > > https://lore.kernel.org/netdev/20241009170102.1980ed1d@xxxxxxxxxx/ > > > > > > With this change the driver would delegate dma_sync_for_device to the > > > page_pool, and the page_pool will skip it altogether for the dma-buf > > > memory provider. > > > > And we need a wrapper for a sync for CPU which will skip if the page > > comes from an unreadable pool? > > This is where it gets a bit tricky, no? > > Our production code does a dma_sync_for_cpu but no > dma_sync_for_device. That has been working reliably for us with GPU Those functions are all NOP on systems you are testing on. The question is what is correct to do on systems where it is not a NOP, and none of this is really right, as I explained.. > But if you or Jason think that enforcing the 'no dma_buf_sync_for_cpu' > now is critical, no problem. We can also provide this patch, and seek > to revert it or fix it up properly later in the event it turns out it > causes issues. What is important is you organize things going forward to be able to do this properly, which means the required sync type is dependent on the actual page being synced and you will eventually somehow learn which is required from the dmabuf. Most likely nobody will ever run this code on system where dma_sync is not a NOP, but we should still use the DMA API properly and things should make architectural sense. Jason
