From: Hans Verkuil <hverkuil@xs4all.nl>
To: Tomasz Figa <tfiga@chromium.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>,
Hans Verkuil <hans.verkuil@cisco.com>,
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 Mailing List <linux-media@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC][PATCH 13/15] videobuf2: do not sync buffers for DMABUF queues
Date: Tue, 28 Jan 2020 09:47:43 +0100 [thread overview]
Message-ID: <84606676-9b24-5feb-0451-7db0527da3f0@xs4all.nl> (raw)
In-Reply-To: <CAAFQd5Dbw=B7kb-p8nkPN3GxwL0O-5q=y1HRAVUVOwv4eEAv-Q@mail.gmail.com>
On 1/28/20 8:19 AM, Tomasz Figa wrote:
> On Thu, Jan 23, 2020 at 8:35 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:
>>
>> On 1/22/20 6:05 AM, Sergey Senozhatsky wrote:
>>> On (20/01/10 11:30), Hans Verkuil wrote:
>>> [..]
>>>>> diff --git a/drivers/media/common/videobuf2/videobuf2-v4l2.c b/drivers/media/common/videobuf2/videobuf2-v4l2.c
>>>>> index 1762849288ae..2b9d3318e6fb 100644
>>>>> --- a/drivers/media/common/videobuf2/videobuf2-v4l2.c
>>>>> +++ b/drivers/media/common/videobuf2/videobuf2-v4l2.c
>>>>> @@ -341,8 +341,22 @@ static void set_buffer_cache_hints(struct vb2_queue *q,
>>>>> struct vb2_buffer *vb,
>>>>> struct v4l2_buffer *b)
>>>>> {
>>>>> - vb->need_cache_sync_on_prepare = 1;
>>>>> + /*
>>>>> + * DMA exporter should take care of cache syncs, so we can avoid
>>>>> + * explicit ->prepare()/->finish() syncs.
>>>>> + */
>>>>> + if (q->memory == VB2_MEMORY_DMABUF) {
>>>>> + vb->need_cache_sync_on_finish = 0;
>>>>> + vb->need_cache_sync_on_prepare = 0;
>>>>> + return;
>>>>> + }
>>>>>
>>>>> + /*
>>>>> + * For other ->memory types we always need ->prepare() cache
>>>>> + * sync. ->finish() cache sync, however, can be avoided when queue
>>>>> + * direction is TO_DEVICE.
>>>>> + */
>>>>> + vb->need_cache_sync_on_prepare = 1;
>>>>
>>>> I'm trying to remember: what needs to be done in prepare()
>>>> for a capture buffer? I thought that for capture you only
>>>> needed to invalidate the cache in finish(), but nothing needs
>>>> to be done in the prepare().
>>>
>>> Hmm. Not sure. A precaution in case if user-space wrote to that buffer?
>>
>> But whatever was written in the buffer is going to be overwritten anyway.
>>
>> Unless I am mistaken the current situation is that the cache syncs are done
>> in both prepare and finish, regardless of the DMA direction.
>>
>> I would keep that behavior to avoid introducing any unexpected regressions.
>>
>
> It wouldn't be surprising if the buffer was first filled with default
> values (e.g. all zeroes) on the CPU. That would make the cache lines
> dirty and they could overwrite what the device writes. So we need to
> flush (aka clean) the write-back caches on prepare for CAPTURE
> buffers.
Very true, I'd forgotten about that. This should be documented in the userspace
documentation. And possible in this function as well.
I think these issues are complex enough that there is no such things as too
much documentation :-)
>
>> Then, if q->allow_cache_hint is set, then default to a cache sync (cache clean)
>> in the prepare for OUTPUT buffers and a cache sync (cache invalidate) in the
>> finish for CAPTURE buffers.
>
> I'd still default to the existing behavior even if allow_cache_hint is
> set, because of what I wrote above. Then if the userspace doesn't ever
> write to the buffers, it can request no flush/clean by setting the
> V4L2_BUF_FLAG_NO_CACHE_CLEAN flag when queuing the buffer.
>
>>
>> This also means that any drivers that want to access a buffer in between the
>> prepare...finish calls will need to do a begin/end_cpu_access. But that's a
>> separate matter.
>
> AFAIR with current design of the series, the drivers can opt-in for
> userspace cache sync hints, so by default even if the userspace
> requests sync to be skipped, it wouldn't have any effect unless the
> driver allows so. Then I'd imagine migrating all the drivers to
> request clean/invalidate explicitly. Note that the DMA-buf
> begin/end_cpu_access isn't enough here. We'd need something like
> vb2_begin/end_cpu_access() which also takes care of syncing
> inconsistent MMAP and USERPTR buffers.
I did just that in my old git branch for this:
https://git.linuxtv.org/hverkuil/media_tree.git/log/?h=vb2-cpu-access
Regards,
Hans
>
>>
>> Regards,
>>
>> Hans
>>
>>>
>>> + if (q->dma_dir == DMA_FROM_DEVICE)
>>> + q->need_cache_sync_on_prepare = 0;
>>>
>>> ?
>>>
>>> -ss
>>>
>>
next prev parent reply other threads:[~2020-01-28 8:47 UTC|newest]
Thread overview: 56+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-17 3:20 [RFC][PATCH 00/15] Implement V4L2_BUF_FLAG_NO_CACHE_* flags Sergey Senozhatsky
2019-12-17 3:20 ` [RFC][PATCH 01/15] videobuf2: add cache management members Sergey Senozhatsky
2019-12-17 3:20 ` [RFC][PATCH 02/15] videobuf2: handle V4L2 buffer cache flags Sergey Senozhatsky
2020-01-10 10:24 ` Hans Verkuil
2020-01-22 1:39 ` Sergey Senozhatsky
2020-01-22 2:53 ` Sergey Senozhatsky
2020-01-28 4:35 ` Tomasz Figa
2019-12-17 3:20 ` [RFC][PATCH 03/15] videobuf2: add V4L2_FLAG_MEMORY_NON_CONSISTENT flag Sergey Senozhatsky
2020-01-10 9:36 ` Hans Verkuil
2020-01-10 9:46 ` Sergey Senozhatsky
2019-12-17 3:20 ` [RFC][PATCH 04/15] videobuf2: add queue memory consistency parameter Sergey Senozhatsky
2020-01-10 9:47 ` Hans Verkuil
2020-01-22 2:05 ` Sergey Senozhatsky
2020-01-23 11:02 ` Hans Verkuil
2020-01-24 2:04 ` Sergey Senozhatsky
2019-12-17 3:20 ` [RFC][PATCH 05/15] videobuf2: handle V4L2_FLAG_MEMORY_NON_CONSISTENT in REQBUFS Sergey Senozhatsky
2020-01-10 9:55 ` Hans Verkuil
2020-01-22 2:18 ` Sergey Senozhatsky
2020-01-22 3:48 ` Sergey Senozhatsky
2020-01-23 11:08 ` Hans Verkuil
2020-01-28 4:45 ` Tomasz Figa
2020-01-28 8:38 ` Hans Verkuil
2019-12-17 3:20 ` [RFC][PATCH 06/15] videobuf2: handle V4L2_FLAG_MEMORY_NON_CONSISTENT in CREATE_BUFS Sergey Senozhatsky
2020-01-10 9:59 ` Hans Verkuil
2020-01-23 3:41 ` Sergey Senozhatsky
2020-01-23 11:41 ` Hans Verkuil
2020-01-24 1:28 ` Sergey Senozhatsky
2019-12-17 3:20 ` [RFC][PATCH 07/15] videobuf2: factor out planes prepare/finish functions Sergey Senozhatsky
2019-12-17 3:20 ` [RFC][PATCH 08/15] videobuf2: do not sync caches when we are allowed not to Sergey Senozhatsky
2019-12-17 3:20 ` [RFC][PATCH 09/15] videobuf2: check ->synced flag in prepare() and finish() Sergey Senozhatsky
2019-12-17 3:20 ` [RFC][PATCH 10/15] videobuf2: let user-space know when driver supports cache hints Sergey Senozhatsky
2019-12-17 3:20 ` [RFC][PATCH 11/15] videobuf2: add begin/end cpu_access callbacks to dma-contig Sergey Senozhatsky
2019-12-17 3:20 ` [RFC][PATCH 12/15] videobuf2: add begin/end cpu_access callbacks to dma-sg Sergey Senozhatsky
2020-01-10 10:13 ` Hans Verkuil
2020-01-22 6:37 ` Sergey Senozhatsky
2020-01-28 4:38 ` Tomasz Figa
2020-01-28 8:36 ` Hans Verkuil
2020-01-30 11:02 ` Tomasz Figa
2020-01-30 12:18 ` Hans Verkuil
2020-02-03 10:04 ` Tomasz Figa
2020-02-04 2:50 ` Sergey Senozhatsky
2020-02-06 8:51 ` Tomasz Figa
2019-12-17 3:20 ` [RFC][PATCH 13/15] videobuf2: do not sync buffers for DMABUF queues Sergey Senozhatsky
2020-01-10 10:30 ` Hans Verkuil
2020-01-22 5:05 ` Sergey Senozhatsky
2020-01-23 11:35 ` Hans Verkuil
2020-01-24 2:25 ` Sergey Senozhatsky
2020-01-24 7:32 ` Sergey Senozhatsky
2020-01-28 7:22 ` Tomasz Figa
2020-01-28 7:34 ` Sergey Senozhatsky
2020-01-28 7:19 ` Tomasz Figa
2020-01-28 8:47 ` Hans Verkuil [this message]
2019-12-17 3:20 ` [RFC][PATCH 14/15] videobuf2: don't test db_attach in dma-contig prepare and finish Sergey Senozhatsky
2020-01-10 10:32 ` Hans Verkuil
2019-12-17 3:20 ` [RFC][PATCH 15/15] videobuf2: don't test db_attach in dma-sg " Sergey Senozhatsky
2020-01-08 2:27 ` [RFC][PATCH 00/15] Implement V4L2_BUF_FLAG_NO_CACHE_* flags Sergey Senozhatsky
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