On Mon, Oct 24, 2016 at 08:04:47AM -0400, Alexander Duyck wrote: > The use of DMA_ATTR_SKIP_CPU_SYNC was not consistent across all of the DMA > APIs in the arch/arm folder. This change is meant to correct that so that > we get consistent behavior. I'm really not convinced that this is anywhere close to correct behaviour. If we're DMA-ing to a buffer, and we unmap it or sync_for_cpu, then we will want to access the DMA'd data - especially in the sync_for_cpu case, it's pointless to call sync_for_cpu if we're not going to access the data. So the idea of skipping the CPU copy when DMA_ATTR_SKIP_CPU_SYNC is set seems to be completely wrong - it means we end up reading the stale data that was in the buffer, completely ignoring whatever was DMA'd to it. What's the use case for DMA_ATTR_SKIP_CPU_SYNC ? -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>