linux-kernel - Re: [PATCH v7 31/36] staging: tegra-vde: fix common struct sg

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Message-ID: <559970b6-e80f-90ec-7fb0-1fab742d99de@samsung.com>
Date:   Tue, 30 Jun 2020 12:07:26 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     dri-devel@...ts.freedesktop.org, iommu@...ts.linux-foundation.org,
        linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        linux-arm-kernel@...ts.infradead.org,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        linux-media@...r.kernel.org, linux-tegra@...r.kernel.org,
        devel@...verdev.osuosl.org
Subject: Re: [PATCH v7 31/36] staging: tegra-vde: fix common struct sg_table
 related issues

On 21.06.2020 06:00, Dmitry Osipenko wrote:
> В Fri, 19 Jun 2020 12:36:31 +0200
> Marek Szyprowski <m.szyprowski@...sung.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@...sung.com>
>> Reviewed-by: Dmitry Osipenko <digetx@...il.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,
>> -			    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@samsung.com/

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