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. -- regards, Stan
