Hi Laurent, On Tue, Aug 04, 2015 at 09:54:27PM +0100, Laurent Pinchart wrote: > On Tuesday 04 August 2015 15:56:42 Russell King - ARM Linux wrote: > > On Tue, Aug 04, 2015 at 03:47:13PM +0100, Robin Murphy wrote: > > > On 04/08/15 14:16, Laurent Pinchart wrote: > > >> This is what I believe to be an API abuse. The > > >> dma_sync_single_for_device() > > >> API is meant to pass ownership of a buffer to the device. Unless I'm > > >> mistaken, once that's done the CPU isn't allowed to touch the buffer > > >> anymore until dma_sync_single_for_cpu() is called to get ownership of > > >> the buffer back. > > > > That's what I thought up until recently, but it's not strictly true - see > > Documentation/DMA-API.txt which Robin quoted. > > I find the documentation slightly unclear on this topic. I have nothing > against updating it to state that the sync functions ensure visibility of the > data to the device or CPU instead of transferring ownership if that's the > general understanding of how the API work (or should work). This would of > course need to be carefully reviewed to ensure that the current > implementation > really works that way across all architectures, or at least that it can be > made to work that way.
I was hoping to send this lot off to Joerg tomorrow morning, since it's removing a hack from the io-pgtable users before it has chance to spread further. This use of the API is certainly ok on arm and arm64 (where these drivers currently run) and I don't see why it wouldn't work for any IOMMU that treats the page tables as read-only. Once we start getting into hardware access/dirty bits, then the streaming-DMA API is a lost cause but I would expect those systems to be using SVM with the CPU page tables anyway and therefore be cache-coherent. I agree that the Documentation isn't brilliant, but that could be addressed in a separate patch (especially since I don't think it would be merged via the iommu tree). Is that alright? Will _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
