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Date:   Tue, 28 Jan 2020 16:34:55 +0900
From:   Sergey Senozhatsky <senozhatsky@chromium.org>
To:     Tomasz Figa <tfiga@chromium.org>
Cc:     Sergey Senozhatsky <senozhatsky@chromium.org>,
        Hans Verkuil <hverkuil@xs4all.nl>,
        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
Message-ID: <20200128073455.GC115889@google.com>
References: <20191217032034.54897-1-senozhatsky@chromium.org>
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On (20/01/28 16:22), Tomasz Figa wrote:
[..]
> > > > 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.
> > >
> > > We alter default cache sync behaviour based both on queue ->memory
> > > type and queue ->dma_dir. Shall both of those cases depend on
> > > ->allow_cache_hints, or q->memory can be independent?
> > >
> > > static void set_buffer_cache_hints(struct vb2_queue *q,
> > >                                  struct vb2_buffer *vb,
> > >                                  struct v4l2_buffer *b)
> > > {
> > >       if (!q->allow_cache_hints)
> > >               return;
> > >
> > >       /*
> > >        * DMA exporter should take care of cache syncs, so we can avoid
> > >        * explicit ->prepare()/->finish() syncs. For other ->memory types
> > >        * we always need ->prepare() or/and ->finish() cache sync.
> > >        */
> > >       if (q->memory == VB2_MEMORY_DMABUF) {
> > >               vb->need_cache_sync_on_finish = 0;
> > >               vb->need_cache_sync_on_prepare = 0;
> > >               return;
> > >       }
> >
> > I personally would prefer this to be above ->allow_cache_hints check,
> > IOW to be independent. Because we need to skip flush/invalidation for
> > DMABUF anyway, that's what allocators do in ->prepare() and ->finish()
> > currently.
>
> I think it's even more than personal preference. We shouldn't even
> attempt to sync imported DMA-bufs, so the code above may result in
> incorrect behavior.

Sure. Allocators test buf->db_attach in prepare()/finish(). This patchset
removes that logic, because we move it to the core code. But if the
conclusion will be to clear ->need_cache_sync_on_finish/prepare only
when ->allow_cache_hints was set, then buf->db_attach bail out must stay
in allocators. And this is where I have preferences :)

	-ss