Re: [Intel-gfx] [PATCH v9 08/32] drm: i915: fix common struct sg_table related issues

[Robin Murphy]

On 2020-09-01 20:38, Ruhl, Michael J wrote:
> > -----Original Message-----
> > From: Intel-gfx <intel-gfx-bounces@xxxxxxxxxxxxxxxxxxxxx> On Behalf Of
> > Marek Szyprowski
> > Sent: Wednesday, August 26, 2020 2:33 AM
> > To: dri-devel@xxxxxxxxxxxxxxxxxxxxx; iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx;
> > linaro-mm-sig@xxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> > Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>; David Airlie
> > <airlied@xxxxxxxx>; intel-gfx@xxxxxxxxxxxxxxxxxxxxx; Robin Murphy
> > <robin.murphy@xxxxxxx>; Christoph Hellwig <hch@xxxxxx>; linux-arm-
> > kernel@xxxxxxxxxxxxxxxxxxx; Marek Szyprowski
> > <m.szyprowski@xxxxxxxxxxx>
> > Subject: [Intel-gfx] [PATCH v9 08/32] drm: i915: fix common struct sg_table
> > related issues
> >
> > The Documentation/DMA-API-HOWTO.txt states that the dma_map_sg()
> > function
> > returns the number of the created entries in the DMA address space.
> > However the subsequent calls to the dma_sync_sg_for_{device,cpu}() and
> > dma_unmap_sg must be called with the original number of the entries
> > passed to the dma_map_sg().
> >
> > struct sg_table is a common structure used for describing a non-contiguous
> > memory buffer, used commonly in the DRM and graphics subsystems. It
> > consists of a scatterlist with memory pages and DMA addresses (sgl entry),
> > as well as the number of scatterlist entries: CPU pages (orig_nents entry)
> > and DMA mapped pages (nents entry).
> >
> > It turned out that it was a common mistake to misuse nents and orig_nents
> > entries, calling DMA-mapping functions with a wrong number of entries or
> > ignoring the number of mapped entries returned by the dma_map_sg()
> > function.
> >
> > This driver creatively uses sg_table->orig_nents to store the size of the
> > allocated scatterlist and ignores the number of the entries returned by
> > dma_map_sg function. The sg_table->orig_nents is (mis)used to properly
> > free the (over)allocated scatterlist.
> >
> > This patch only introduces the common DMA-mapping wrappers operating
> > directly on the struct sg_table objects to the dmabuf related functions,
> > so the other drivers, which might share buffers with i915 could rely on
> > the properly set nents and orig_nents values.
> >
> > Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> > ---
> > drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c       | 11 +++--------
> > drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c |  7 +++----
> > 2 files changed, 6 insertions(+), 12 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> > b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> > index 2679380159fc..8a988592715b 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> > @@ -48,12 +48,9 @@ static struct sg_table *i915_gem_map_dma_buf(struct
> > dma_buf_attachment *attachme
> > 		src = sg_next(src);
> > 	}
> >
> > -	if (!dma_map_sg_attrs(attachment->dev,
> > -			      st->sgl, st->nents, dir,
> > -			      DMA_ATTR_SKIP_CPU_SYNC)) {
> > -		ret = -ENOMEM;
>
> You have dropped this error value.
>
> Do you now if this is a benign loss?

True, dma_map_sgtable() will return -EINVAL rather than -ENOMEM for 
failure. A quick look through other .map_dma_buf callbacks suggests 
they're returning a motley mix of error values and NULL for failure 
cases, so I'd imagine that importers shouldn't be too sensitive to the 
exact value.

Robin.

> M
>
> > +	ret = dma_map_sgtable(attachment->dev, st, dir,
> > DMA_ATTR_SKIP_CPU_SYNC);
> > +	if (ret)
> > 		goto err_free_sg;
> > -	}
> >
> > 	return st;
> >
> > @@ -73,9 +70,7 @@ static void i915_gem_unmap_dma_buf(struct
> > dma_buf_attachment *attachment,
> > {
> > 	struct drm_i915_gem_object *obj = dma_buf_to_obj(attachment-
> > > dmabuf);
> >
> > -	dma_unmap_sg_attrs(attachment->dev,
> > -			   sg->sgl, sg->nents, dir,
> > -			   DMA_ATTR_SKIP_CPU_SYNC);
> > +	dma_unmap_sgtable(attachment->dev, sg, dir,
> > DMA_ATTR_SKIP_CPU_SYNC);
> > 	sg_free_table(sg);
> > 	kfree(sg);
> >
> > diff --git a/drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c
> > b/drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c
> > index debaf7b18ab5..be30b27e2926 100644
> > --- a/drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c
> > +++ b/drivers/gpu/drm/i915/gem/selftests/mock_dmabuf.c
> > @@ -28,10 +28,9 @@ static struct sg_table *mock_map_dma_buf(struct
> > dma_buf_attachment *attachment,
> > 		sg = sg_next(sg);
> > 	}
> >
> > -	if (!dma_map_sg(attachment->dev, st->sgl, st->nents, dir)) {
> > -		err = -ENOMEM;
> > +	err = dma_map_sgtable(attachment->dev, st, dir, 0);
> > +	if (err)
> > 		goto err_st;
> > -	}
> >
> > 	return st;
> >
> > @@ -46,7 +45,7 @@ static void mock_unmap_dma_buf(struct
> > dma_buf_attachment *attachment,
> > 			       struct sg_table *st,
> > 			       enum dma_data_direction dir)
> > {
> > -	dma_unmap_sg(attachment->dev, st->sgl, st->nents, dir);
> > +	dma_unmap_sgtable(attachment->dev, st, dir, 0);
> > 	sg_free_table(st);
> > 	kfree(st);
> > }
> > --
> > 2.17.1
> >
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