From: Sergey Senozhatsky <senozhatsky@chromium.org>
To: Hans Verkuil <hverkuil@xs4all.nl>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>,
Hans Verkuil <hans.verkuil@cisco.com>,
Tomasz Figa <tfiga@chromium.org>,
Mauro Carvalho Chehab <mchehab@kernel.org>,
Kyungmin Park <kyungmin.park@samsung.com>,
Marek Szyprowski <m.szyprowski@samsung.com>,
Sakari Ailus <sakari.ailus@iki.fi>,
Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
Pawel Osciak <posciak@chromium.org>,
linux-media@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCHv2 03/12] videobuf2: add V4L2_FLAG_MEMORY_NON_CONSISTENT flag
Date: Tue, 25 Feb 2020 16:45:12 +0900 [thread overview]
Message-ID: <20200225074512.GG122464@google.com> (raw)
In-Reply-To: <17e99adc-d86d-ebd3-ab52-d6c8e3ac14f3@xs4all.nl>
On (20/02/19 09:56), Hans Verkuil wrote:
> > +Memory Consistency Flags
> > +========================
> > +
> > +.. tabularcolumns:: |p{7.0cm}|p{2.2cm}|p{8.3cm}|
> > +
> > +.. cssclass:: longtable
> > +
> > +.. flat-table::
> > + :header-rows: 0
> > + :stub-columns: 0
> > + :widths: 3 1 4
> > +
> > + * .. _`V4L2_FLAG_MEMORY_NON_CONSISTENT`:
> > +
> > + - ``V4L2_FLAG_MEMORY_NON_CONSISTENT``
> > + - 0x00000001
> > + - vb2 buffer is allocated either in consistent (it will be automatically
> > + coherent between CPU and bus) or non-consistent memory. The latter
> > + can provide performance gains, for instance CPU cache sync/flush
> > + operations can be avoided if the buffer is accesed by the corresponding
> > + device only and CPU does not read/write to/from that buffer. However,
> > + this requires extra care from the driver -- it must guarantee memory
> > + consistency by issuing cache flush/sync when consistency is needed.
> > + If this flag is set V4L2 will attempt to allocate vb2 buffer in
> > + non-consistent memory. This flag is ignored if queue does not report
> > + :ret:`V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS` capability.
>
> This flag only makes sense for the MMAP memory model, right? That should be
> documented and checked in the code.
Not all buffer allocators respect DMA attrs (V4L2_FLAG_MEMORY_NON_CONSISTENT
is a DMA attribute) even if we use MMAP memory model. Right? E.g. dma-cont
does, dma-sg - does not.
So the list is:
a) buffer allocated for MMAP I/O
b) buffer allocator which does not allocate pages from kernel page
allocator
c) queue that supports user space cache hints
If the driver does not set vb2_dma_contig_memops as q->mem_ops then
that queue should not have ->allow_cache_hints set. But even if it does,
the flag is ignored by the allocator.
So maybe the text can be:
+ ................... The flag takes effect only if the buffer is
+ used for :ref:`memory mapping <mmap>` I/O and the queue reports
+ :ref:`V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS` capability.
-ss
next prev parent reply other threads:[~2020-02-25 7:45 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-04 2:56 [RFC][PATCHv2 00/12] Implement V4L2_BUF_FLAG_NO_CACHE_* flags Sergey Senozhatsky
2020-02-04 2:56 ` [RFC][PATCHv2 01/12] videobuf2: add cache management members Sergey Senozhatsky
2020-02-19 8:05 ` Hans Verkuil
2020-02-19 8:16 ` Sergey Senozhatsky
2020-02-19 8:27 ` Hans Verkuil
2020-02-04 2:56 ` [RFC][PATCHv2 02/12] videobuf2: handle V4L2 buffer cache flags Sergey Senozhatsky
2020-02-05 8:13 ` Sergey Senozhatsky
2020-02-19 8:07 ` Hans Verkuil
2020-02-19 8:13 ` Sergey Senozhatsky
2020-02-19 8:25 ` Hans Verkuil
2020-02-19 8:46 ` Hans Verkuil
2020-02-04 2:56 ` [RFC][PATCHv2 03/12] videobuf2: add V4L2_FLAG_MEMORY_NON_CONSISTENT flag Sergey Senozhatsky
2020-02-13 7:08 ` Tomasz Figa
2020-02-19 8:19 ` Hans Verkuil
2020-02-19 8:56 ` Hans Verkuil
2020-02-25 7:45 ` Sergey Senozhatsky [this message]
2020-02-04 2:56 ` [RFC][PATCHv2 04/12] videobuf2: add queue memory consistency parameter Sergey Senozhatsky
2020-02-04 2:56 ` [RFC][PATCHv2 05/12] videobuf2: handle V4L2_FLAG_MEMORY_NON_CONSISTENT flag Sergey Senozhatsky
2020-02-19 8:25 ` Hans Verkuil
2020-02-19 8:59 ` Sergey Senozhatsky
2020-02-19 9:04 ` Hans Verkuil
2020-02-19 8:48 ` Hans Verkuil
2020-02-19 9:05 ` Sergey Senozhatsky
2020-02-19 9:11 ` Hans Verkuil
2020-02-04 2:56 ` [RFC][PATCHv2 06/12] videobuf2: factor out planes prepare/finish functions Sergey Senozhatsky
2020-02-04 2:56 ` [RFC][PATCHv2 07/12] videobuf2: do not sync caches when we are allowed not to Sergey Senozhatsky
2020-02-04 2:56 ` [RFC][PATCHv2 08/12] videobuf2: check ->synced flag in prepare() and finish() Sergey Senozhatsky
2020-02-04 2:56 ` [RFC][PATCHv2 09/12] videobuf2: let user-space know if driver supports cache hints Sergey Senozhatsky
2020-02-19 8:33 ` Hans Verkuil
2020-02-19 8:45 ` Sergey Senozhatsky
2020-02-19 8:56 ` Hans Verkuil
2020-02-04 2:56 ` [RFC][PATCHv2 10/12] videobuf2: add begin/end cpu_access callbacks to dma-contig Sergey Senozhatsky
2020-02-04 2:56 ` [RFC][PATCHv2 11/12] videobuf2: add begin/end cpu_access callbacks to dma-sg Sergey Senozhatsky
2020-02-19 8:35 ` Hans Verkuil
2020-02-19 8:42 ` Sergey Senozhatsky
2020-02-04 2:56 ` [RFC][PATCHv2 12/12] videobuf2: don't test db_attach in dma-contig prepare and finish Sergey Senozhatsky
2020-02-19 8:53 ` [RFC][PATCHv2 00/12] Implement V4L2_BUF_FLAG_NO_CACHE_* flags Hans Verkuil
2020-02-26 11:20 ` Sergey Senozhatsky
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