Re: [PATCH 2/8] drm/rockchip: Get rid of some unnecessary code

[Mark yao]

On 2016å09æ18æ 12:01, Tomasz Figa wrote:
> Hi Mark,
>
> On Sun, Sep 18, 2016 at 10:50 AM, Mark yao <mark.yao@xxxxxxxxxxxxxx> wrote:
> > On 2016å09æ14æ 20:54, Tomasz Figa wrote:
> > > Current code implements prepare_fb and cleanup_fb callbacks only to
> > > grab/release fb references, which is already done by atomic framework
> > > when creating/destryoing plane state. Let's remove these
> > > unused bits.
> > >
> > > Signed-off-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> > > ---
> > >    drivers/gpu/drm/rockchip/rockchip_drm_vop.c | 18 ------------------
> > >    1 file changed, 18 deletions(-)
> >
> > Hi Tomasz
> >
> > I think we can't get rid of the prepare_fb and cleanup_fb
> I think I have to disagree. Please see below for detailed explanation.
>
> > see the reason:
> > commit 44d0237a26395ac94160cf23f32769013b365590
> > Author: Mark Yao <mark.yao@xxxxxxxxxxxxxx>
> > Date:   Fri Apr 29 11:37:20 2016 +0800
> >
> >      drm/rockchip: vop: fix iommu crash with async atomic
> >
> >      After async atomic_commit callback, drm_atomic_clean_old_fb will
> >      clean all old fb, but because async, the old fb may be also on
> >      the vop hardware, dma will access the old fb buffer, clean old
> >      fb will cause iommu page fault.
> I think the above is not quite right. Atomic plane state holds a
> reference to its fb and old state is not supposed to be destroyed
> until the flip completes.
>
> Indeed current rockchip_atomic_commit implementation has following
> order of calls: rockchip_atomic_wait_for_complete(),
> drm_atomic_helper_cleanup_planes(), drm_atomic_state_free(). This
> means that .cleanup_fb() is called from
> drm_atomic_helper_cleanup_planes() just before drm_atomic_state_free()
> will release references by destroying old plane states. Note that both
> are called already after rockchip_atomic_wait_for_complete(), so it
> should be already safe to free the old fbs.
>
> So the above fix doesn't really do anything, possibly just covers the
> race condition of the original wait for vblank function by delaying
> drm_atomic_state_free() a bit.
>
> Moreover, the whole series has been thoroughly tested in Chrome OS 4.4
> kernel, including async commits. (There is still a possibility some
> newer upstream changes slightly modified the semantics, but I couldn't
> find such difference. Actually one of the advantages of atomic helpers
> was to avoid manually refcounting the fbs from the driver.)
>
> Best regards,
> Tomasz
Hi Tomasz

You are right, plane_duplicate_state/plane_destroy_state already protect 
the old fbs.
we can get rid of prepare_fb and cleanup_fb.

--
ïark Yao