Date:   Tue, 15 Dec 2020 14:21:48 -0500
From:   Nicolas Dufresne <nicolas@...fresne.ca>
To:     Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Tomasz Figa <tfiga@...omium.org>
Cc:     Alexandre Courbot <acourbot@...omium.org>,
        Fritz Koenig <frkoenig@...omium.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        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

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.
 */

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@...omium.org>
> > > > Signed-off-by: Alexandre Courbot <acourbot@...omium.org>
> > > > ---
> > > > 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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