linux-kernel - Re: [PATCH] media: venus: use contig vb2 ops

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Date:   Mon, 1 Mar 2021 12:22:14 +0200
From:   Stanimir Varbanov <stanimir.varbanov@...aro.org>
To:     Tomasz Figa <tfiga@...omium.org>,
        Alexandre Courbot <acourbot@...omium.org>
Cc:     Fritz Koenig <frkoenig@...omium.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Nicolas Dufresne <nicolas@...fresne.ca>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH] media: venus: use contig vb2 ops



On 3/1/21 11:23 AM, Tomasz Figa wrote:
> Hi Alex, Stanimir,
> 
> On Wed, Dec 16, 2020 at 12:15 PM Tomasz Figa <tfiga@...omium.org> wrote:
>>
>> On Wed, Dec 16, 2020 at 4:21 AM Nicolas Dufresne <nicolas@...fresne.ca> wrote:
>>>
>>> Le mardi 15 décembre 2020 à 15:54 +0200, Stanimir Varbanov a écrit :
>>>> Hi Tomasz,
>>>>
>>>> On 12/15/20 1:47 PM, Tomasz Figa wrote:
>>>>> On Tue, Dec 15, 2020 at 8:16 PM Stanimir Varbanov
>>>>> <stanimir.varbanov@...aro.org> wrote:
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> Cc: Robin
>>>>>>
>>>>>> On 12/14/20 2:57 PM, Alexandre Courbot wrote:
>>>>>>> This driver uses the SG vb2 ops, but effectively only ever accesses the
>>>>>>> first entry of the SG table, indicating that it expects a flat layout.
>>>>>>> Switch it to use the contiguous ops to make sure this expected invariant
>>>>>>
>>>>>> Under what circumstances the sg table will has nents > 1? I came down to
>>>>>> [1] but not sure I got it right.
>>>>>>
>>>>>> I'm afraid that for systems with low amount of system memory and when
>>>>>> the memory become fragmented, the driver will not work. That's why I
>>>>>> started with sg allocator.
>>>>>
>>>>> It is exactly the opposite. The vb2-dma-contig allocator is "contig"
>>>>> in terms of the DMA (aka IOVA) address space. In other words, it
>>>>> guarantees that having one DMA address and length fully describes the
>>>>
>>>> Ahh, I missed that part. Looks like I misunderstood videobu2 contig
>>>> allocator.
>>>
>>> I'm learning everyday too, but I'm surprised I don't see a call to
>>> vb2_dma_contig_set_max_seg_size() in this driver (I could also just have missed
>>> a patch when overlooking this thread) ?
>>>
>>> The reason I'm asking, doc says it should be called by driver supporting IOMMU,
>>> which seems to be the case for such drivers (MFC, exynos4-is, exynos-gsc, mtk-
>>> mdp, s5p-g2d, hantro, rkvdec, zoran, ti-vpe, ..). I posting it, worst case it's
>>> all covered and we are good, otherwise perhaps a downstream patch didn't make it
>>> ?
>>>
>>> /**
>>>  * vb2_dma_contig_set_max_seg_size() - configure DMA max segment size
>>>  * @dev:        device for configuring DMA parameters
>>>  * @size:       size of DMA max segment size to set
>>>  *
>>>  * To allow mapping the scatter-list into a single chunk in the DMA
>>>  * address space, the device is required to have the DMA max segment
>>>  * size parameter set to a value larger than the buffer size. Otherwise,
>>>  * the DMA-mapping subsystem will split the mapping into max segment
>>>  * size chunks. This function sets the DMA max segment size
>>>  * parameter to let DMA-mapping map a buffer as a single chunk in DMA
>>>  * address space.
>>>  * This code assumes that the DMA-mapping subsystem will merge all
>>>  * scatterlist segments if this is really possible (for example when
>>>  * an IOMMU is available and enabled).
>>>  * Ideally, this parameter should be set by the generic bus code, but it
>>>  * is left with the default 64KiB value due to historical litmiations in
>>>  * other subsystems (like limited USB host drivers) and there no good
>>>  * place to set it to the proper value.
>>>  * This function should be called from the drivers, which are known to
>>>  * operate on platforms with IOMMU and provide access to shared buffers
>>>  * (either USERPTR or DMABUF). This should be done before initializing
>>>  * videobuf2 queue.
>>>  */
>>
>> It does call dma_set_max_seg_size() directly:
>> https://elixir.bootlin.com/linux/latest/source/drivers/media/platform/qcom/venus/core.c#L230
>>
>> Actually, why do we even need a vb2 helper for this?
>>
> 
> What's the plan for this patch?

It will be part of v5.12.

-- 
regards,
Stan