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From:   Tomasz Figa <tfiga@chromium.org>
Date:   Tue, 28 Jan 2020 16:22:16 +0900
Message-ID: <CAAFQd5Dx50nt=yOCj-ufDDey=Zur_F5yE34a9OQP630A1opBSQ@mail.gmail.com>
Subject: Re: [RFC][PATCH 13/15] videobuf2: do not sync buffers for DMABUF queues
To:     Sergey Senozhatsky <senozhatsky@chromium.org>
Cc:     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>
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On Fri, Jan 24, 2020 at 4:32 PM Sergey Senozhatsky
<senozhatsky@chromium.org> wrote:
>
> On (20/01/24 11:25), Sergey Senozhatsky 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.