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Date: Mon, 28 Mar 2022 08:37:23 +0200 From: Christoph Hellwig <hch@....de> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Robin Murphy <robin.murphy@....com>, Toke Høiland-Jørgensen <toke@...e.dk>, Christoph Hellwig <hch@....de>, Halil Pasic <pasic@...ux.ibm.com>, Oleksandr Natalenko <oleksandr@...alenko.name>, Marek Szyprowski <m.szyprowski@...sung.com>, Kalle Valo <kvalo@...nel.org>, "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Olha Cherevyk <olha.cherevyk@...il.com>, iommu <iommu@...ts.linux-foundation.org>, linux-wireless <linux-wireless@...r.kernel.org>, Netdev <netdev@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, stable <stable@...r.kernel.org> Subject: Re: [REGRESSION] Recent swiotlb DMA_FROM_DEVICE fixes break ath9k-based AP On Fri, Mar 25, 2022 at 11:46:09AM -0700, Linus Torvalds wrote: > I think my list of three different sync cases (not just two! It's not > just about whether to sync for the CPU or the device, it's also about > what direction the data itself is taking) is correct. > > But maybe I'm wrong. At the high level you are correct. It is all about which direction the data is taking. That is the direction argument that all the map/unmap/sync call take. The sync calls then just toggle the ownership. You seem to hate that ownership concept, but I don't see how things could work without that ownership concept as we're going to be in trouble without having that. And yes, a peek operation could work in some cases, but it would have to be at the cache line granularity. arch/arc/mm/dma.c has a really good comment how these transfers relate to actual cache operations btw> * * | map == for_device | unmap == for_cpu * |---------------------------------------------------------------- * TO_DEV | writeback writeback | none none * FROM_DEV | invalidate invalidate | invalidate* invalidate* * BIDIR | writeback+inv writeback+inv | invalidate invalidate * * [*] needed for CPU speculative prefetches
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