Subject: [PATCH] Documentation: DMA-API: Clarify semantics of dma_set_mask_and_coherent

From: [email-masked] (Joerg Roedel)
Date: Mon, 24 Oct 2016 13:08:19 +0200

On Fri, Oct 21, 2016 at 03:09:16PM -0600, Jonathan Corbet wrote:
> On Mon, 17 Oct 2016 16:26:23 +0100
> Punit Agrawal <[email-masked]> 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.

So long story short: Any IOMMU restrictions in address space size don't
need to be represented in the dma-mask for a device.

	Joerg