Hi Robin,
Thank you for your comments!
> From: Robin Murphy, Sent: Wednesday, June 5, 2019 9:22 PM
<snip>
> > @@ -902,8 +914,18 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
> > if (iommu_map_sg(domain, iova, sg, nents, prot) < iova_len)
> > goto out_free_iova;
> >
> > - return __finalise_sg(dev, sg, nents, iova);
> > + ret = __finalise_sg(dev, sg, nents, iova);
> > + /*
> > + * Check whether the sg entry is single if a device requires and
> > + * the IOMMU driver has the capable.
> > + */
> > + if (iova_contiguous && ret != 1)
> > + goto out_unmap_sg;
>
> I don't see that just failing really gives this option any value.
> Clearly the MMC driver has to do *something* to handle the failure (plus
> presumably the case of not having IOMMU DMA ops at all), which begs the
> question of why it couldn't just do whatever that is anyway, without all
> this infrastructure. For starters, it would be a far simpler and less
> invasive patch:
>
> if (dma_map_sg(...) > 1) {
> dma_unmap_sg(...);
> /* split into multiple requests and try again */
> }
I understood it.
> But then it would make even more sense to just have the driver be
> proactive about its special requirement in the first place, and simply
> validate the list before it even tries to map it:
>
> for_each_sg(sgl, sg, n, i)
> if ((i > 0 && sg->offset % PAGE_SIZE) ||
> (i < n - 1 && sg->length % PAGE_SIZE))
> /* segment will not be mergeable */
In previous version, I made such a code [1].
But, I think I misunderstood Christoph's comments [2] [3].
[1]
https://patchwork.kernel.org/patch/10970047/
[2]
https://marc.info/?l=linux-renesas-soc&m=155956751811689&w=2
[3]
https://marc.info/?l=linux-renesas-soc&m=155852814607202&w=2
> For reference, I think v4l2 and possibly some areas of DRM already do
> something vaguely similar to judge whether they get contiguous buffers
> or not.
I see. I'll check these areas later.
> > +
> > + return ret;
> >
> > +out_unmap_sg:
> > + iommu_dma_unmap_sg(dev, sg, nents, dir, attrs);
> > out_free_iova:
> > iommu_dma_free_iova(cookie, iova, iova_len);
> > out_restore_sg:
> > diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> > index 91af22a..f971dd3 100644
> > --- a/include/linux/iommu.h
> > +++ b/include/linux/iommu.h
> > @@ -104,6 +104,7 @@ enum iommu_cap {
> > transactions */
> > IOMMU_CAP_INTR_REMAP, /* IOMMU supports interrupt isolation */
> > IOMMU_CAP_NOEXEC, /* IOMMU_NOEXEC flag */
> > + IOMMU_CAP_MERGING, /* IOMMU supports segments merging */
>
> This isn't a 'capability' of the IOMMU - "segment merging" equates to
> just remapping pages, and there's already a fundamental assumption that
> IOMMUs are capable of that. Plus it's very much a DMA API concept, so
> hardly belongs in the IOMMU API anyway.
I got it.
> All in all, I'm struggling to see the point of this. Although it's not a
> DMA API guarantee, iommu-dma already merges scatterlists as aggressively
> as it is allowed to, and will continue to do so for the foreseeable
> future (since it avoids considerable complication in the IOVA
> allocation), so if you want to make sure iommu_dma_map_sg() merges an
> entire list, just don't give it a non-mergeable list.
Thank you for the explanation. I didn't know that a driver should not
give it a non-mergeable list.
> And if you still
> really really want dma_map_sg() to have a behaviour of "merge to a
> single segment or fail", then that should at least be a DMA API
> attribute, which could in principle be honoured by bounce-buffering
> implementations as well.
I got it. For this patch series, it seems I have to modify a block layer
so that such a new DMA API is not needed though.
> And if the problem is really that you're not getting merging because of
> exposing the wrong parameters to the DMA API and/or the block layer, or
> that you just can't quite express your requirement to the block layer in
> the first place, then that should really be tackled at the source rather
> than worked around further down in the stack.
I'll reply on Christoph email about this topic later.
Best regards,
Yoshihiro Shimoda
> Robin.
