Subject: [PATCH v5 1/1] iommu-api: Add map_sg/unmap_sg functions

From: [email-masked] (Joerg Roedel)
Date: Tue, 19 Aug 2014 13:59:54 +0200

On Mon, Aug 18, 2014 at 03:47:56PM -0700, Olav Haugan wrote:
> If the alignment is not correct then iommu_map() will return error. Not
> sure what other option we have here (and why make it different behavior
> than iommu_map which just return error when it is not aligned properly).
> I don't think we want to force any kind of alignment automatically. I
> would rather have the API tell me I am doing something wrong than having
> the function aligning the values and possibly undermap or overmap.

But sg->offset is an offset into the page (at least it is used that way
in the DMA-API and since you do 'page_len = s->offset + s->length' you
use it the same way).
So when you pass iova + offset the result will no longer be
page-aligned. You should force sg->offset == 0 and sg->length to be
page-aligned instead. This makes more sense because the IOMMU-API works
on (io)-page granularity and not on arbitrary phys-addr ranges like the
DMA-API.

> Yes, I am aware of that. However, several people prefer this than
> passing in scatterlist. It is not very convenient to pass a scatterlist
> in some use cases. Someone mentioned a use case where they would have to
> create a dummy sg list and populate it with the iova just to do an
> unmap. I believe we would have to do this also. There is no use for
> sglist when unmapping. However, would like to keep separate API from
> iommu_unmap() to keep the API function names symmetric (map_sg/unmap_sg).

Keeping it symetric is not more complicated, the caller just needs to
keep the sg-list used for mapping around. I prefer the unmap_sg call to
work in sg-lists too.

> I thought that was why we added the default fallback and set all the
> drivers to point to these fallback functions. Several people wanted this
> so that we don't have to have NULL-check in these functions (and have
> the functions be simple inline functions).

Okay, since you add these call-backs to all drivers I think I can live
with not doing a pointer check here.

	Joerg