Joerg Roedel <joro@xxxxxxxxxx> 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 <punit.agrawal@xxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
