On Mon, Mar 1, 2021 at 7:22 PM Stanimir Varbanov <stanimir.varbanov@xxxxxxxxxx> wrote: > > > > 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@xxxxxxxxxxxx> wrote: > >> > >> On Wed, Dec 16, 2020 at 4:21 AM Nicolas Dufresne <nicolas@xxxxxxxxxxxx> 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@xxxxxxxxxx> 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. Great, thanks!
