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

[Dmitry Osipenko]

30.06.2020 13:07, Marek Szyprowski ÐÐÑÐÑ:
> On 21.06.2020 06:00, Dmitry Osipenko wrote:
>> Ð Fri, 19 Jun 2020 12:36:31 +0200
>> Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> ÐÐÑÐÑ:
>>
>>> 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@xxxxxxxxxxx>
>>> Reviewed-by: Dmitry Osipenko <digetx@xxxxxxxxx>
>>> ---
>>>   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,
>>> -			    IOMMU_READ | IOMMU_WRITE);
>>> +	size = iommu_map_sgtable(vde->domain, addr, sgt,
>>> +				 IOMMU_READ | IOMMU_WRITE);
>>>   	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:
> 
> https://lore.kernel.org/lkml/20200630081756.18526-1-m.szyprowski@xxxxxxxxxxx/
> 
> 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.

Thank you for the clarification!