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

Robin Murphy
2020-09-01T19:54:19Z

On 2020-09-01 20:38, Ruhl, Michael J wrote:
>> -----Original Message-----
>> From: Intel-gfx <intel-gfx-bounces@lists.freedesktop.org> On Behalf Of
>> Marek Szyprowski
>> Sent: Wednesday, August 26, 2020 2:33 AM
>> To: dri-devel@lists.freedesktop.org; iommu@lists.linux-foundation.org;
>> linaro-mm-sig@lists.linaro.org; linux-kernel@vger.kernel.org
>> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>; David Airlie
>> <airlied@linux.ie>; intel-gfx@lists.freedesktop.org; Robin Murphy
>> <robin.murphy@arm.com>; Christoph Hellwig <hch@lst.de>; linux-arm-
>> kernel@lists.infradead.org; Marek Szyprowski
>> <m.szyprowski@samsung.com>
>> 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@samsung.com>
>> ---
>> 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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