[PATCH v7 31/36] staging: tegra-vde: fix common struct sg_table related issues

Marek Szyprowski m.szyprowski at samsung.com
Tue Jun 30 06:07:26 EDT 2020

On 21.06.2020 06:00, Dmitry Osipenko wrote:
> В Fri, 19 Jun 2020 12:36:31 +0200
> Marek Szyprowski <m.szyprowski at samsung.com> пишет:
>> 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.
>> To avoid such issues, lets use a common dma-mapping wrappers operating
>> directly on the struct sg_table objects and use scatterlist page
>> iterators where possible. This, almost always, hides references to the
>> nents and orig_nents entries, making the code robust, easier to follow
>> and copy/paste safe.
>> Signed-off-by: Marek Szyprowski <m.szyprowski at samsung.com>
>> Reviewed-by: Dmitry Osipenko <digetx at gmail.com>
>> ---
>>   drivers/staging/media/tegra-vde/iommu.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>> diff --git a/drivers/staging/media/tegra-vde/iommu.c
>> b/drivers/staging/media/tegra-vde/iommu.c index
>> 6af863d92123..adf8dc7ee25c 100644 ---
>> a/drivers/staging/media/tegra-vde/iommu.c +++
>> b/drivers/staging/media/tegra-vde/iommu.c @@ -36,8 +36,8 @@ int
>> tegra_vde_iommu_map(struct tegra_vde *vde,
>>   	addr = iova_dma_addr(&vde->iova, iova);
>> -	size = iommu_map_sg(vde->domain, addr, sgt->sgl, sgt->nents,
>> +	size = iommu_map_sgtable(vde->domain, addr, sgt,
>>   	if (!size) {
>>   		__free_iova(&vde->iova, iova);
>>   		return -ENXIO;
> Ahh, I saw the build failure report. You're changing the DMA API in
> this series, while DMA API isn't used by this driver, it uses IOMMU
> API. Hence there is no need to touch this code. Similar problem in the
> host1x driver patch.

The issue is caused by the lack of iommu_map_sgtable() stub when no 
IOMMU support is configured. I've posted a patch for this:


The patch for this driver is fine, we have to wait until the above fix 
gets merged and then it can be applied during the next release cycle.

Best regards
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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