Hi Will, On Wednesday 05 August 2015 17:24:52 Will Deacon wrote: > 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?
Yes, that's fine with me. The patch set moves usage of the DMA mapping API from drivers to a single place in the core, so no problem is introduced. If we later consider this to be an API abuse we'll only need to fix it in a single place. And if it's a valid use of the API then the documentation fix/improvement doesn't need to be synchronous with this patch set. -- Regards, Laurent Pinchart _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
