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Date:   Wed, 19 Aug 2020 14:49:01 +0200
From:   Tomasz Figa <tfiga@...omium.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>, alsa-devel@...a-project.org,
        linux-ia64@...r.kernel.org,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        nouveau@...ts.freedesktop.org, linux-nvme@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        linux-mm@...ck.org, Marek Szyprowski <m.szyprowski@...sung.com>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        Joonyoung Shim <jy0922.shim@...sung.com>,
        linux-scsi@...r.kernel.org,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Matt Porter <mporter@...nel.crashing.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Pawel Osciak <pawel@...iak.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Joerg Roedel <joro@...tes.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <linux-arm-kernel@...ts.infradead.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-parisc@...r.kernel.org, netdev@...r.kernel.org,
        Seung-Woo Kim <sw0312.kim@...sung.com>,
        linux-mips@...r.kernel.org
Subject: Re: [PATCH 05/28] media/v4l2: remove V4L2-FLAG-MEMORY-NON-CONSISTENT

On Wed, Aug 19, 2020 at 1:51 PM Robin Murphy <robin.murphy@....com> wrote:
>
> Hi Tomasz,
>
> On 2020-08-19 12:16, Tomasz Figa wrote:
> > Hi Christoph,
> >
> > On Wed, Aug 19, 2020 at 8:56 AM Christoph Hellwig <hch@....de> wrote:
> >>
> >> The V4L2-FLAG-MEMORY-NON-CONSISTENT flag is entirely unused,
> >
> > Could you explain what makes you think it's unused? It's a feature of
> > the UAPI generally supported by the videobuf2 framework and relied on
> > by Chromium OS to get any kind of reasonable performance when
> > accessing V4L2 buffers in the userspace.
> >
> >> and causes
> >> weird gymanstics with the DMA_ATTR_NON_CONSISTENT flag, which is
> >> unimplemented except on PARISC and some MIPS configs, and about to be
> >> removed.
> >
> > It is implemented by the generic DMA mapping layer [1], which is used
> > by a number of architectures including ARM64 and supposed to be used
> > by new architectures going forward.
>
> AFAICS all that V4L2_FLAG_MEMORY_NON_CONSISTENT does is end up
> controling whether DMA_ATTR_NON_CONSISTENT is added to vb2_queue::dma_attrs.
>
> Please can you point to where DMA_ATTR_NON_CONSISTENT does anything at
> all on arm64?
>

With the default config it doesn't, but with
CONFIG_DMA_NONCOHERENT_CACHE_SYNC enabled it makes dma_pgprot() keep
the pgprot value as is, without enforcing coherence attributes.


> Also, I posit that videobuf2 is not actually relying on
> DMA_ATTR_NON_CONSISTENT anyway, since it's clearly not using it properly:
>
> "By using this API, you are guaranteeing to the platform
> that you have all the correct and necessary sync points for this memory
> in the driver should it choose to return non-consistent memory."
>
> $ git grep dma_cache_sync drivers/media
> $

AFAIK dma_cache_sync() isn't the only way to perform the cache
synchronization. The earlier patch series that I reviewed relied on
dma_get_sgtable() and then dma_sync_sg_*() (which existed in the
vb2-dc since forever [1]). However, it looks like with the final code
the sgtable isn't acquired and the synchronization isn't happening, so
you have a point.

FWIW, I asked back in time what the plan is for non-coherent
allocations and it seemed like DMA_ATTR_NON_CONSISTENT and
dma_sync_*() was supposed to be the right thing to go with. [2] The
same thread also explains why dma_alloc_pages() isn't suitable for the
users of dma_alloc_attrs() and DMA_ATTR_NON_CONSISTENT.

I think we could make a deal here. We could revert back the parts
using DMA_ATTR_NON_CONSISTENT, keeping the UAPI intact, but just
rendering it no-op, since it's just a hint after all. Then, you would
propose a proper, functionally equivalent and working for ARM64,
replacement for dma_alloc_attrs(..., DMA_ATTR_NON_CONSISTENT), which
we could then use to enable the functionality expected by this UAPI.
Does it sound like something that could work as a way forward here?

By the way, as a videobuf2 reviewer, I'd appreciate being CC'd on any
series related to the subsystem-facing DMA API changes, since
videobuf2 is one of the biggest users of it.

[1] https://elixir.bootlin.com/linux/v5.9-rc1/source/drivers/media/common/videobuf2/videobuf2-dma-contig.c#L98
[2] https://patchwork.kernel.org/comment/23312203/

Best regards,
Tomasz


>
> Robin.
>
> > [1] https://elixir.bootlin.com/linux/v5.9-rc1/source/kernel/dma/mapping.c#L341
> >
> > When removing features from generic kernel code, I'd suggest first
> > providing viable alternatives for its users, rather than killing the
> > users altogether.
> >
> > Given the above, I'm afraid I have to NAK this.
> >
> > Best regards,
> > Tomasz
> >
> >>
> >> Signed-off-by: Christoph Hellwig <hch@....de>
> >> ---
> >>   .../userspace-api/media/v4l/buffer.rst        | 17 ---------
> >>   .../media/v4l/vidioc-reqbufs.rst              |  1 -
> >>   .../media/common/videobuf2/videobuf2-core.c   | 36 +------------------
> >>   .../common/videobuf2/videobuf2-dma-contig.c   | 19 ----------
> >>   .../media/common/videobuf2/videobuf2-dma-sg.c |  3 +-
> >>   .../media/common/videobuf2/videobuf2-v4l2.c   | 12 -------
> >>   include/media/videobuf2-core.h                |  3 +-
> >>   include/uapi/linux/videodev2.h                |  2 --
> >>   8 files changed, 3 insertions(+), 90 deletions(-)
> >>
> >> diff --git a/Documentation/userspace-api/media/v4l/buffer.rst b/Documentation/userspace-api/media/v4l/buffer.rst
> >> index 57e752aaf414a7..2044ed13cd9d7d 100644
> >> --- a/Documentation/userspace-api/media/v4l/buffer.rst
> >> +++ b/Documentation/userspace-api/media/v4l/buffer.rst
> >> @@ -701,23 +701,6 @@ Memory Consistency Flags
> >>       :stub-columns: 0
> >>       :widths:       3 1 4
> >>
> >> -    * .. _`V4L2-FLAG-MEMORY-NON-CONSISTENT`:
> >> -
> >> -      - ``V4L2_FLAG_MEMORY_NON_CONSISTENT``
> >> -      - 0x00000001
> >> -      - A buffer is allocated either in consistent (it will be automatically
> >> -       coherent between the CPU and the bus) or non-consistent memory. The
> >> -       latter can provide performance gains, for instance the CPU cache
> >> -       sync/flush operations can be avoided if the buffer is accessed by the
> >> -       corresponding device only and the CPU does not read/write to/from that
> >> -       buffer. However, this requires extra care from the driver -- it must
> >> -       guarantee memory consistency by issuing a cache flush/sync when
> >> -       consistency is needed. If this flag is set V4L2 will attempt to
> >> -       allocate the buffer in non-consistent memory. The flag takes effect
> >> -       only if the buffer is used for :ref:`memory mapping <mmap>` I/O and the
> >> -       queue reports the :ref:`V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS
> >> -       <V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS>` capability.
> >> -
> >>   .. c:type:: v4l2_memory
> >>
> >>   enum v4l2_memory
> >> diff --git a/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst b/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst
> >> index 75d894d9c36c42..3180c111d368ee 100644
> >> --- a/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst
> >> +++ b/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst
> >> @@ -169,7 +169,6 @@ aborting or finishing any DMA in progress, an implicit
> >>         - This capability is set by the driver to indicate that the queue supports
> >>           cache and memory management hints. However, it's only valid when the
> >>           queue is used for :ref:`memory mapping <mmap>` streaming I/O. See
> >> -        :ref:`V4L2_FLAG_MEMORY_NON_CONSISTENT <V4L2-FLAG-MEMORY-NON-CONSISTENT>`,
> >>           :ref:`V4L2_BUF_FLAG_NO_CACHE_INVALIDATE <V4L2-BUF-FLAG-NO-CACHE-INVALIDATE>` and
> >>           :ref:`V4L2_BUF_FLAG_NO_CACHE_CLEAN <V4L2-BUF-FLAG-NO-CACHE-CLEAN>`.
> >>
> >> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> >> index f544d3393e9d6b..66a41cef33c1b1 100644
> >> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> >> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> >> @@ -721,39 +721,14 @@ int vb2_verify_memory_type(struct vb2_queue *q,
> >>   }
> >>   EXPORT_SYMBOL(vb2_verify_memory_type);
> >>
> >> -static void set_queue_consistency(struct vb2_queue *q, bool consistent_mem)
> >> -{
> >> -       q->dma_attrs &= ~DMA_ATTR_NON_CONSISTENT;
> >> -
> >> -       if (!vb2_queue_allows_cache_hints(q))
> >> -               return;
> >> -       if (!consistent_mem)
> >> -               q->dma_attrs |= DMA_ATTR_NON_CONSISTENT;
> >> -}
> >> -
> >> -static bool verify_consistency_attr(struct vb2_queue *q, bool consistent_mem)
> >> -{
> >> -       bool queue_is_consistent = !(q->dma_attrs & DMA_ATTR_NON_CONSISTENT);
> >> -
> >> -       if (consistent_mem != queue_is_consistent) {
> >> -               dprintk(q, 1, "memory consistency model mismatch\n");
> >> -               return false;
> >> -       }
> >> -       return true;
> >> -}
> >> -
> >>   int vb2_core_reqbufs(struct vb2_queue *q, enum vb2_memory memory,
> >>                       unsigned int flags, unsigned int *count)
> >>   {
> >>          unsigned int num_buffers, allocated_buffers, num_planes = 0;
> >>          unsigned plane_sizes[VB2_MAX_PLANES] = { };
> >> -       bool consistent_mem = true;
> >>          unsigned int i;
> >>          int ret;
> >>
> >> -       if (flags & V4L2_FLAG_MEMORY_NON_CONSISTENT)
> >> -               consistent_mem = false;
> >> -
> >>          if (q->streaming) {
> >>                  dprintk(q, 1, "streaming active\n");
> >>                  return -EBUSY;
> >> @@ -765,8 +740,7 @@ int vb2_core_reqbufs(struct vb2_queue *q, enum vb2_memory memory,
> >>          }
> >>
> >>          if (*count == 0 || q->num_buffers != 0 ||
> >> -           (q->memory != VB2_MEMORY_UNKNOWN && q->memory != memory) ||
> >> -           !verify_consistency_attr(q, consistent_mem)) {
> >> +           (q->memory != VB2_MEMORY_UNKNOWN && q->memory != memory)) {
> >>                  /*
> >>                   * We already have buffers allocated, so first check if they
> >>                   * are not in use and can be freed.
> >> @@ -803,7 +777,6 @@ int vb2_core_reqbufs(struct vb2_queue *q, enum vb2_memory memory,
> >>          num_buffers = min_t(unsigned int, num_buffers, VB2_MAX_FRAME);
> >>          memset(q->alloc_devs, 0, sizeof(q->alloc_devs));
> >>          q->memory = memory;
> >> -       set_queue_consistency(q, consistent_mem);
> >>
> >>          /*
> >>           * Ask the driver how many buffers and planes per buffer it requires.
> >> @@ -894,12 +867,8 @@ int vb2_core_create_bufs(struct vb2_queue *q, enum vb2_memory memory,
> >>   {
> >>          unsigned int num_planes = 0, num_buffers, allocated_buffers;
> >>          unsigned plane_sizes[VB2_MAX_PLANES] = { };
> >> -       bool consistent_mem = true;
> >>          int ret;
> >>
> >> -       if (flags & V4L2_FLAG_MEMORY_NON_CONSISTENT)
> >> -               consistent_mem = false;
> >> -
> >>          if (q->num_buffers == VB2_MAX_FRAME) {
> >>                  dprintk(q, 1, "maximum number of buffers already allocated\n");
> >>                  return -ENOBUFS;
> >> @@ -912,15 +881,12 @@ int vb2_core_create_bufs(struct vb2_queue *q, enum vb2_memory memory,
> >>                  }
> >>                  memset(q->alloc_devs, 0, sizeof(q->alloc_devs));
> >>                  q->memory = memory;
> >> -               set_queue_consistency(q, consistent_mem);
> >>                  q->waiting_for_buffers = !q->is_output;
> >>          } else {
> >>                  if (q->memory != memory) {
> >>                          dprintk(q, 1, "memory model mismatch\n");
> >>                          return -EINVAL;
> >>                  }
> >> -               if (!verify_consistency_attr(q, consistent_mem))
> >> -                       return -EINVAL;
> >>          }
> >>
> >>          num_buffers = min(*count, VB2_MAX_FRAME - q->num_buffers);
> >> diff --git a/drivers/media/common/videobuf2/videobuf2-dma-contig.c b/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> >> index ec3446cc45b8da..7b1b86ec942d7d 100644
> >> --- a/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> >> +++ b/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> >> @@ -42,11 +42,6 @@ struct vb2_dc_buf {
> >>          struct dma_buf_attachment       *db_attach;
> >>   };
> >>
> >> -static inline bool vb2_dc_buffer_consistent(unsigned long attr)
> >> -{
> >> -       return !(attr & DMA_ATTR_NON_CONSISTENT);
> >> -}
> >> -
> >>   /*********************************************/
> >>   /*        scatterlist table functions        */
> >>   /*********************************************/
> >> @@ -341,13 +336,6 @@ static int
> >>   vb2_dc_dmabuf_ops_begin_cpu_access(struct dma_buf *dbuf,
> >>                                     enum dma_data_direction direction)
> >>   {
> >> -       struct vb2_dc_buf *buf = dbuf->priv;
> >> -       struct sg_table *sgt = buf->dma_sgt;
> >> -
> >> -       if (vb2_dc_buffer_consistent(buf->attrs))
> >> -               return 0;
> >> -
> >> -       dma_sync_sg_for_cpu(buf->dev, sgt->sgl, sgt->nents, buf->dma_dir);
> >>          return 0;
> >>   }
> >>
> >> @@ -355,13 +343,6 @@ static int
> >>   vb2_dc_dmabuf_ops_end_cpu_access(struct dma_buf *dbuf,
> >>                                   enum dma_data_direction direction)
> >>   {
> >> -       struct vb2_dc_buf *buf = dbuf->priv;
> >> -       struct sg_table *sgt = buf->dma_sgt;
> >> -
> >> -       if (vb2_dc_buffer_consistent(buf->attrs))
> >> -               return 0;
> >> -
> >> -       dma_sync_sg_for_device(buf->dev, sgt->sgl, sgt->nents, buf->dma_dir);
> >>          return 0;
> >>   }
> >>
> >> diff --git a/drivers/media/common/videobuf2/videobuf2-dma-sg.c b/drivers/media/common/videobuf2/videobuf2-dma-sg.c
> >> index 0a40e00f0d7e5c..a86fce5d8ea8bf 100644
> >> --- a/drivers/media/common/videobuf2/videobuf2-dma-sg.c
> >> +++ b/drivers/media/common/videobuf2/videobuf2-dma-sg.c
> >> @@ -123,8 +123,7 @@ static void *vb2_dma_sg_alloc(struct device *dev, unsigned long dma_attrs,
> >>          /*
> >>           * NOTE: dma-sg allocates memory using the page allocator directly, so
> >>           * there is no memory consistency guarantee, hence dma-sg ignores DMA
> >> -        * attributes passed from the upper layer. That means that
> >> -        * V4L2_FLAG_MEMORY_NON_CONSISTENT has no effect on dma-sg buffers.
> >> +        * attributes passed from the upper layer.
> >>           */
> >>          buf->pages = kvmalloc_array(buf->num_pages, sizeof(struct page *),
> >>                                      GFP_KERNEL | __GFP_ZERO);
> >> diff --git a/drivers/media/common/videobuf2/videobuf2-v4l2.c b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> >> index 30caad27281e1a..de83ad48783821 100644
> >> --- a/drivers/media/common/videobuf2/videobuf2-v4l2.c
> >> +++ b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> >> @@ -722,20 +722,11 @@ static void fill_buf_caps(struct vb2_queue *q, u32 *caps)
> >>   #endif
> >>   }
> >>
> >> -static void clear_consistency_attr(struct vb2_queue *q,
> >> -                                  int memory,
> >> -                                  unsigned int *flags)
> >> -{
> >> -       if (!q->allow_cache_hints || memory != V4L2_MEMORY_MMAP)
> >> -               *flags &= ~V4L2_FLAG_MEMORY_NON_CONSISTENT;
> >> -}
> >> -
> >>   int vb2_reqbufs(struct vb2_queue *q, struct v4l2_requestbuffers *req)
> >>   {
> >>          int ret = vb2_verify_memory_type(q, req->memory, req->type);
> >>
> >>          fill_buf_caps(q, &req->capabilities);
> >> -       clear_consistency_attr(q, req->memory, &req->flags);
> >>          return ret ? ret : vb2_core_reqbufs(q, req->memory,
> >>                                              req->flags, &req->count);
> >>   }
> >> @@ -769,7 +760,6 @@ int vb2_create_bufs(struct vb2_queue *q, struct v4l2_create_buffers *create)
> >>          unsigned i;
> >>
> >>          fill_buf_caps(q, &create->capabilities);
> >> -       clear_consistency_attr(q, create->memory, &create->flags);
> >>          create->index = q->num_buffers;
> >>          if (create->count == 0)
> >>                  return ret != -EBUSY ? ret : 0;
> >> @@ -998,7 +988,6 @@ int vb2_ioctl_reqbufs(struct file *file, void *priv,
> >>          int res = vb2_verify_memory_type(vdev->queue, p->memory, p->type);
> >>
> >>          fill_buf_caps(vdev->queue, &p->capabilities);
> >> -       clear_consistency_attr(vdev->queue, p->memory, &p->flags);
> >>          if (res)
> >>                  return res;
> >>          if (vb2_queue_is_busy(vdev, file))
> >> @@ -1021,7 +1010,6 @@ int vb2_ioctl_create_bufs(struct file *file, void *priv,
> >>
> >>          p->index = vdev->queue->num_buffers;
> >>          fill_buf_caps(vdev->queue, &p->capabilities);
> >> -       clear_consistency_attr(vdev->queue, p->memory, &p->flags);
> >>          /*
> >>           * If count == 0, then just check if memory and type are valid.
> >>           * Any -EBUSY result from vb2_verify_memory_type can be mapped to 0.
> >> diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
> >> index 52ef92049073e3..4c7f25b07e9375 100644
> >> --- a/include/media/videobuf2-core.h
> >> +++ b/include/media/videobuf2-core.h
> >> @@ -744,8 +744,7 @@ void vb2_core_querybuf(struct vb2_queue *q, unsigned int index, void *pb);
> >>    * vb2_core_reqbufs() - Initiate streaming.
> >>    * @q:         pointer to &struct vb2_queue with videobuf2 queue.
> >>    * @memory:    memory type, as defined by &enum vb2_memory.
> >> - * @flags:     auxiliary queue/buffer management flags. Currently, the only
> >> - *             used flag is %V4L2_FLAG_MEMORY_NON_CONSISTENT.
> >> + * @flags:     auxiliary queue/buffer management flags.
> >>    * @count:     requested buffer count.
> >>    *
> >>    * Videobuf2 core helper to implement VIDIOC_REQBUF() operation. It is called
> >> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> >> index c7b70ff53bc1dd..5c00f63d9c1b58 100644
> >> --- a/include/uapi/linux/videodev2.h
> >> +++ b/include/uapi/linux/videodev2.h
> >> @@ -191,8 +191,6 @@ enum v4l2_memory {
> >>          V4L2_MEMORY_DMABUF           = 4,
> >>   };
> >>
> >> -#define V4L2_FLAG_MEMORY_NON_CONSISTENT                (1 << 0)
> >> -
> >>   /* see also http://vektor.theorem.ca/graphics/ycbcr/ */
> >>   enum v4l2_colorspace {
> >>          /*
> >> --
> >> 2.28.0
> >>
> >> _______________________________________________
> >> iommu mailing list
> >> iommu@...ts.linux-foundation.org
> >> https://lists.linuxfoundation.org/mailman/listinfo/iommu
> >
> > _______________________________________________
> > linux-arm-kernel mailing list
> > linux-arm-kernel@...ts.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> >

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