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

[Ruhl, Michael J]

>-----Original Message-----
>From: Robin Murphy <robin.murphy@xxxxxxx>
>Sent: Tuesday, September 1, 2020 3:54 PM
>To: Ruhl, Michael J <michael.j.ruhl@xxxxxxxxx>; Marek Szyprowski
><m.szyprowski@xxxxxxxxxxx>; 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; Christoph Hellwig
><hch@xxxxxx>; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>Subject: Re: [Intel-gfx] [PATCH v9 08/32] drm: i915: fix common struct
>sg_table related issues
>
>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.

I followed some of our code through to see if anyone is checking for -ENOMEM...

I have found in some test paths... However, it is not clear to me if we can get
to those paths from here.

Anyways,

Reviewed-by: Michael J. Ruhl <michael.j.ruhl@xxxxxxxxx>

Mike

>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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