From mboxrd@z Thu Jan 1 00:00:00 1970 From: punit.agrawal@arm.com (Punit Agrawal) Date: Mon, 24 Oct 2016 12:37:56 +0100 Subject: [PATCH] Documentation: DMA-API: Clarify semantics of dma_set_mask_and_coherent In-Reply-To: <20161024110819.GO3541@8bytes.org> (Joerg Roedel's message of "Mon, 24 Oct 2016 13:08:19 +0200") References: <20161017152623.7649-1-punit.agrawal@arm.com> <20161021150916.0d274e9f@lwn.net> <20161024110819.GO3541@8bytes.org> Message-ID: <87k2cy6kq3.fsf@e105922-lin.cambridge.arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Joerg Roedel writes: > On Fri, Oct 21, 2016 at 03:09:16PM -0600, Jonathan Corbet wrote: >> On Mon, 17 Oct 2016 16:26:23 +0100 >> Punit Agrawal wrote: >> >> > The dma mapping api howto gives the impression that using the >> > dma_set_mask_and_coherent (and related DMA APIs) will cause the kernel >> > to check all the components in the path from the device to memory for >> > addressing restrictions. In systems with address translations between >> > the device and memory (e.g., when using IOMMU), this implies that a >> > successful call to set set dma mask has checked the addressing >> > constraints of the intermediaries as well. > > This is basically true when you have DMA controllers in the path from > device to memory. But it is not true for IOMMUs, because IOMMU drivers > are consumers of the dma-masks, they don't really restrict them. An > IOMMU driver knows the limitations of IOMMU hardware and counts that in > when allocating an address for a dma-buffer. Yes, that's what I'd found looking at the IOMMU drivers in the tree. > > So long story short: Any IOMMU restrictions in address space size don't > need to be represented in the dma-mask for a device. That was another rabbit hole I'd spend some time in - whether IOMMU restrictions need to be factored into the dma_mask for devices. As size(dma_mask) > size(iommu supported address size) still works, I came to the conclusion that the documentation can maybe help clarify this. This patch is an attempt to update the documentation to inform the user that even if the dma_set_mask call succeeds, the system may still not use the entire device address space. Thanks for clarifying a few of my doubts. Punit > > > > Joerg