Re: [PATCH] media: venus: use contig vb2 ops

From: Tomasz Figa
Date: Tue Dec 15 2020 - 22:16:29 EST

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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?

>
> regards,
> Nicolas
>
> >
> > > buffer. This seems to be the requirement of the hardware/firmware
> > > handled by the venus driver. If the device is behind an IOMMU, which
> > > is the case for the SoCs in question, the underlying DMA ops will
> > > actually allocate a discontiguous set of pages, so it has nothing to
> > > do to system memory amount or fragmentation. If for some reason the
> > > IOMMU can't be used, there is no way around, the memory needs to be
> > > contiguous because of the hardware/firmware/driver expectation.
> > >
> > > On the other hand, the vb2-dma-sg allocator doesn't have any
> > > continuity guarantees for the DMA, or any other, address space. The
> > > current code works fine, because it calls dma_map_sg() on the whole
> > > set of pages and that ends up mapping it contiguously in the IOVA
> > > space, but that's just an implementation detail, not an API guarantee.
> >
> > It was good to know. Thanks for the explanation.
> >
> > >
> > > Best regards,
> > > Tomasz
> > >
> > > >
> > > > [1]
> > > > https://elixir.bootlin.com/linux/v5.10.1/source/drivers/iommu/dma-iommu.c#L782
> > > >
> > > > > is always enforced. Since the device is supposed to be behind an IOMMU
> > > > > this should have little to none practical consequences beyond making the
> > > > > driver not rely on a particular behavior of the SG implementation.
> > > > >
> > > > > Reported-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> > > > > Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxxxx>
> > > > > ---
> > > > > Hi everyone,
> > > > >
> > > > > It probably doesn't hurt to fix this issue before some actual issue
> > > > > happens.
> > > > > I have tested this patch on Chrome OS and playback was just as fine as
> > > > > with
> > > > > the SG ops.
> > > > >
> > > > > drivers/media/platform/Kconfig | 2 +-
> > > > > drivers/media/platform/qcom/venus/helpers.c | 9 ++-------
> > > > > drivers/media/platform/qcom/venus/vdec.c | 6 +++---
> > > > > drivers/media/platform/qcom/venus/venc.c | 6 +++---
> > > > > 4 files changed, 9 insertions(+), 14 deletions(-)
> > > > >
> > > > > diff --git a/drivers/media/platform/Kconfig
> > > > > b/drivers/media/platform/Kconfig
> > > > > index 35a18d388f3f..d9d7954111f2 100644
> > > > > --- a/drivers/media/platform/Kconfig
> > > > > +++ b/drivers/media/platform/Kconfig
> > > > > @@ -533,7 +533,7 @@ config VIDEO_QCOM_VENUS
> > > > > depends on INTERCONNECT || !INTERCONNECT
> > > > > select QCOM_MDT_LOADER if ARCH_QCOM
> > > > > select QCOM_SCM if ARCH_QCOM
> > > > > - select VIDEOBUF2_DMA_SG
> > > > > + select VIDEOBUF2_DMA_CONTIG
> > > > > select V4L2_MEM2MEM_DEV
> > > > > help
> > > > > This is a V4L2 driver for Qualcomm Venus video accelerator
> > > > > diff --git a/drivers/media/platform/qcom/venus/helpers.c
> > > > > b/drivers/media/platform/qcom/venus/helpers.c
> > > > > index 50439eb1ffea..859d260f002b 100644
> > > > > --- a/drivers/media/platform/qcom/venus/helpers.c
> > > > > +++ b/drivers/media/platform/qcom/venus/helpers.c
> > > > > @@ -7,7 +7,7 @@
> > > > > #include <linux/mutex.h>
> > > > > #include <linux/slab.h>
> > > > > #include <linux/kernel.h>
> > > > > -#include <media/videobuf2-dma-sg.h>
> > > > > +#include <media/videobuf2-dma-contig.h>
> > > > > #include <media/v4l2-mem2mem.h>
> > > > > #include <asm/div64.h>
> > > > >
> > > > > @@ -1284,14 +1284,9 @@ int venus_helper_vb2_buf_init(struct vb2_buffer
> > > > > *vb)
> > > > > struct venus_inst *inst = vb2_get_drv_priv(vb->vb2_queue);
> > > > > struct vb2_v4l2_buffer *vbuf = to_vb2_v4l2_buffer(vb);
> > > > > struct venus_buffer *buf = to_venus_buffer(vbuf);
> > > > > - struct sg_table *sgt;
> > > > > -
> > > > > - sgt = vb2_dma_sg_plane_desc(vb, 0);
> > > > > - if (!sgt)
> > > > > - return -EFAULT;
> > > > >
> > > > > buf->size = vb2_plane_size(vb, 0);
> > > > > - buf->dma_addr = sg_dma_address(sgt->sgl);
> > > >
> > > > Can we do it:
> > > >
> > > > if (WARN_ON(sgt->nents > 1))
> > > > return -EFAULT;
> > > >
> > > > I understand that logically using dma-sg when the flat layout is
> > > > expected by the hardware is wrong, but I haven't seen issues until now.
> > > >
> > > > > + buf->dma_addr = vb2_dma_contig_plane_dma_addr(vb, 0);
> > > > >
> > > > > if (vb->type == V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE)
> > > > > list_add_tail(&buf->reg_list, &inst->registeredbufs);
> > > > > diff --git a/drivers/media/platform/qcom/venus/vdec.c
> > > > > b/drivers/media/platform/qcom/venus/vdec.c
> > > > > index 8488411204c3..3fb277c81aca 100644
> > > > > --- a/drivers/media/platform/qcom/venus/vdec.c
> > > > > +++ b/drivers/media/platform/qcom/venus/vdec.c
> > > > > @@ -13,7 +13,7 @@
> > > > > #include <media/v4l2-event.h>
> > > > > #include <media/v4l2-ctrls.h>
> > > > > #include <media/v4l2-mem2mem.h>
> > > > > -#include <media/videobuf2-dma-sg.h>
> > > > > +#include <media/videobuf2-dma-contig.h>
> > > > >
> > > > > #include "hfi_venus_io.h"
> > > > > #include "hfi_parser.h"
> > > > > @@ -1461,7 +1461,7 @@ static int m2m_queue_init(void *priv, struct
> > > > > vb2_queue *src_vq,
> > > > > src_vq->io_modes = VB2_MMAP | VB2_DMABUF;
> > > > > src_vq->timestamp_flags = V4L2_BUF_FLAG_TIMESTAMP_COPY;
> > > > > src_vq->ops = &vdec_vb2_ops;
> > > > > - src_vq->mem_ops = &vb2_dma_sg_memops;
> > > > > + src_vq->mem_ops = &vb2_dma_contig_memops;
> > > > > src_vq->drv_priv = inst;
> > > > > src_vq->buf_struct_size = sizeof(struct venus_buffer);
> > > > > src_vq->allow_zero_bytesused = 1;
> > > > > @@ -1475,7 +1475,7 @@ static int m2m_queue_init(void *priv, struct
> > > > > vb2_queue *src_vq,
> > > > > dst_vq->io_modes = VB2_MMAP | VB2_DMABUF;
> > > > > dst_vq->timestamp_flags = V4L2_BUF_FLAG_TIMESTAMP_COPY;
> > > > > dst_vq->ops = &vdec_vb2_ops;
> > > > > - dst_vq->mem_ops = &vb2_dma_sg_memops;
> > > > > + dst_vq->mem_ops = &vb2_dma_contig_memops;
> > > > > dst_vq->drv_priv = inst;
> > > > > dst_vq->buf_struct_size = sizeof(struct venus_buffer);
> > > > > dst_vq->allow_zero_bytesused = 1;
> > > > > diff --git a/drivers/media/platform/qcom/venus/venc.c
> > > > > b/drivers/media/platform/qcom/venus/venc.c
> > > > > index 1c61602c5de1..a09550cd1dba 100644
> > > > > --- a/drivers/media/platform/qcom/venus/venc.c
> > > > > +++ b/drivers/media/platform/qcom/venus/venc.c
> > > > > @@ -10,7 +10,7 @@
> > > > > #include <linux/pm_runtime.h>
> > > > > #include <linux/slab.h>
> > > > > #include <media/v4l2-mem2mem.h>
> > > > > -#include <media/videobuf2-dma-sg.h>
> > > > > +#include <media/videobuf2-dma-contig.h>
> > > > > #include <media/v4l2-ioctl.h>
> > > > > #include <media/v4l2-event.h>
> > > > > #include <media/v4l2-ctrls.h>
> > > > > @@ -1001,7 +1001,7 @@ static int m2m_queue_init(void *priv, struct
> > > > > vb2_queue *src_vq,
> > > > > src_vq->io_modes = VB2_MMAP | VB2_USERPTR | VB2_DMABUF;
> > > > > src_vq->timestamp_flags = V4L2_BUF_FLAG_TIMESTAMP_COPY;
> > > > > src_vq->ops = &venc_vb2_ops;
> > > > > - src_vq->mem_ops = &vb2_dma_sg_memops;
> > > > > + src_vq->mem_ops = &vb2_dma_contig_memops;
> > > > > src_vq->drv_priv = inst;
> > > > > src_vq->buf_struct_size = sizeof(struct venus_buffer);
> > > > > src_vq->allow_zero_bytesused = 1;
> > > > > @@ -1017,7 +1017,7 @@ static int m2m_queue_init(void *priv, struct
> > > > > vb2_queue *src_vq,
> > > > > dst_vq->io_modes = VB2_MMAP | VB2_USERPTR | VB2_DMABUF;
> > > > > dst_vq->timestamp_flags = V4L2_BUF_FLAG_TIMESTAMP_COPY;
> > > > > dst_vq->ops = &venc_vb2_ops;
> > > > > - dst_vq->mem_ops = &vb2_dma_sg_memops;
> > > > > + dst_vq->mem_ops = &vb2_dma_contig_memops;
> > > > > dst_vq->drv_priv = inst;
> > > > > dst_vq->buf_struct_size = sizeof(struct venus_buffer);
> > > > > dst_vq->allow_zero_bytesused = 1;
> > > > >
> > > >
> > > > --
> > > > regards,
> > > > Stan
> >
>
>

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Previous message: Tomasz Figa: "Re: [PATCH v5 2/7] media: v4l2: Add extended buffer operations"
In reply to: Nicolas Dufresne: "Re: [PATCH] media: venus: use contig vb2 ops"
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