linux-kernel - Re: [PATCH v6 00/14] Implement V4L2_BUF_FLAG_NO_CACHE

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Date:   Mon, 18 May 2020 17:18:34 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Hans Verkuil <hans.verkuil@...co.com>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Tomasz Figa <tfiga@...omium.org>, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 00/14] Implement V4L2_BUF_FLAG_NO_CACHE_* flags

Hi Sergey,

On 14/05/2020 18:01, Sergey Senozhatsky wrote:
> Hello
> 
> v6 changes:
> The design has been slightly reworked. The cache-hints capability has
> been renamed to SUPPORTS_MMAP_CACHE_HINTS and is reported for all queues
> that support MMAP and allow cache hints. However, the actual hints and
> memory consistency are ignored unless the queue is used for the MMAP
> streaming I/O. Plus some cleanups, documentation updates, and so on.

This looks good. If there are no new comments then I plan to make a PR for 5.9 in
two weeks.

Thank you for all your work on this!

Regards,

	Hans

> 
> Previous versions:
> v5 link: https://lore.kernel.org/lkml/20200424092920.4801-1-sergey.senozhatsky@gmail.com
> v4 link: https://lore.kernel.org/lkml/20200302041213.27662-1-senozhatsky@chromium.org/
> v3 link: https://lore.kernel.org/lkml/20200226111529.180197-1-senozhatsky@chromium.org
> v2 link: https://lore.kernel.org/lkml/20200204025641.218376-1-senozhatsky@chromium.org/
> v1 link: https://lore.kernel.org/lkml/20191217032034.54897-1-senozhatsky@chromium.org/
> 
> Series Intro
> ========================================================================
> 
>         This is a reworked version of the vb2 cache hints
> (V4L2_BUF_FLAG_NO_CACHE_INVALIDATE / V4L2_BUF_FLAG_NO_CACHE_CLEAN)
> support patch series which previsouly was developed by Sakari and
> Laurent [0].
> 
> The patch set attempts to preserve the existing behvaiour - cache
> sync is performed in ->prepare() and ->finish() (unless the buffer
> is DMA exported). User space can request “default behavior” override
> with cache management hints, which are handled on a per-buffer basis
> and should be supplied with v4l2_buffer ->flags during buffer
> preparation. There are two possible hints:
> 
> - V4L2_BUF_FLAG_NO_CACHE_INVALIDATE
>         No cache sync on ->finish()
> 
> - V4L2_BUF_FLAG_NO_CACHE_CLEAN
>         No cache sync on ->prepare()
> 
> In order to keep things on the safe side, we also require driver
> to explicitly state which of its queues (if any) support user space
> cache management hints (such queues should have ->allow_cache_hints
> bit set).
> 
> The patch set also (to some extent) simplifies allocators' ->prepare()
> and ->finish() callbacks. Namely, we move cache management decision
> making to the upper - core - layer. For example, if, previously, we
> would have something like this
> 
>         vb2_buffer_done()
>           vb2_dc_finish()
>             if (buf->db_attach)
>               return;
> 
> where each allocators' ->finish() callback would either bail
> out (DMA exported buffer, for instance) or sync, now that "bail
> out or sync" decision is made before we call into the allocator.
> 
> Along with cache management hints, user space is also able to
> adjust queue's memory consistency attributes. Memory consistency
> attribute (dma_attrs) is per-queue, yet it plays its role on the
> allocator level, when we allocate buffers’ private memory (planes).
> For the time being, only one consistency attribute is supported:
> DMA_ATTR_NON_CONSISTENT.
> 
> [0] https://www.mail-archive.com/linux-media@vger.kernel.org/msg112459.html
> 
> Sergey Senozhatsky (14):
>   videobuf2: use explicit unsigned int in vb2_queue
>   videobuf2: add cache management members
>   videobuf2: handle V4L2 buffer cache flags
>   videobuf2: add V4L2_FLAG_MEMORY_NON_CONSISTENT flag
>   videobuf2: add queue memory consistency parameter
>   videobuf2: handle V4L2_FLAG_MEMORY_NON_CONSISTENT flag
>   videobuf2: factor out planes prepare/finish functions
>   videobuf2: do not sync caches when we are allowed not to
>   videobuf2: check ->synced flag in prepare() and finish()
>   videobuf2: add begin/end cpu_access callbacks to dma-contig
>   videobuf2: add begin/end cpu_access callbacks to dma-sg
>   videobuf2: don't test db_attach in dma-contig prepare and finish
>   videobuf2: remove redundant if-statement
>   media: vivid: add cache_hints module param
> 
>  Documentation/admin-guide/media/vivid.rst     |   9 ++
>  .../userspace-api/media/v4l/buffer.rst        |  40 +++++-
>  .../media/v4l/vidioc-create-bufs.rst          |   7 +-
>  .../media/v4l/vidioc-reqbufs.rst              |  21 ++-
>  .../media/common/videobuf2/videobuf2-core.c   | 121 +++++++++++++-----
>  .../common/videobuf2/videobuf2-dma-contig.c   |  44 ++++++-
>  .../media/common/videobuf2/videobuf2-dma-sg.c |  38 ++++--
>  .../media/common/videobuf2/videobuf2-v4l2.c   |  72 ++++++++++-
>  drivers/media/dvb-core/dvb_vb2.c              |   2 +-
>  drivers/media/test-drivers/vivid/vivid-core.c |   9 ++
>  drivers/media/v4l2-core/v4l2-compat-ioctl32.c |  10 +-
>  drivers/media/v4l2-core/v4l2-ioctl.c          |   5 +-
>  include/media/videobuf2-core.h                |  47 +++++--
>  include/uapi/linux/videodev2.h                |  14 +-
>  14 files changed, 366 insertions(+), 73 deletions(-)
>