Re: [PATCH 3/3] drm/rockchip: explain why we can't wait_for_vblanks

[Mark yao]

On 2016å01æ15æ 00:26, John Keeping wrote:
> On Thu, 14 Jan 2016 15:57:05 +0100, Thierry Reding wrote:
>
> > On Thu, Jan 14, 2016 at 02:39:42PM +0000, John Keeping wrote:
> > > Signed-off-by: John Keeping <john@xxxxxxxxxxxx>
> > > ---
> > >   drivers/gpu/drm/rockchip/rockchip_drm_fb.c | 6 ++++++
> > >   1 file changed, 6 insertions(+)
> > >
> > > diff --git a/drivers/gpu/drm/rockchip/rockchip_drm_fb.c b/drivers/gpu/drm/rockchip/rockchip_drm_fb.c
> > > index 679d23a..b267ce4 100644
> > > --- a/drivers/gpu/drm/rockchip/rockchip_drm_fb.c
> > > +++ b/drivers/gpu/drm/rockchip/rockchip_drm_fb.c
> > > @@ -177,6 +177,12 @@ static void rockchip_crtc_wait_for_update(struct drm_crtc *crtc)
> > >   		crtc_funcs->wait_for_update(crtc);
> > >   }
> > >   
> > > +/*
> > > + * We can't use drm_atomic_helper_wait_for_vblanks() because rk3288 and rk3066
> > > + * have hardware counters for neither vblanks nor scanlines.  This function is
> > > + * equivalent but uses rockchip_crtc_wait_for_update() instead of waiting for
> > > + * vblank_count to change.
> > > + */
> > This is kind of misleading. From reading earlier parts of the thread the
> > reason why drm_atomic_helper_wait_for_vblanks() won't work is because it
> > has a potential race condition that can't be detected unless you also
> > have a vblank counter. However, the above comment makes it work like
> > drm_atomic_helper_wait_for_vblanks() doesn't work in the absence of a
> > vblank counter, which isn't quite true.
> How about something like this (using the sequence from Mark's message):
>
> /*
>   * We can't use drm_atomic_helper_wait_for_vblanks() because rk3288 and rk3066
>   * have hardware counters for neither vblanks nor scanlines, which results in
>   * a race where:
>   *				| <-- HW vsync irq and reg take effect
>   *	       plane_commit --> |
>   *	get_vblank and wait --> |
>   *				| <-- handle_vblank, vblank->count + 1
>   *		 cleanup_fb --> |
>   *		iommu crash --> |
>   *				| <-- HW vsync irq and reg take effect
>   *
>   * This function is equivalent but uses rockchip_crtc_wait_for_update() instead
>   * of waiting for vblank_count to change.
>   */

Looks good for me, but maybe Thierry has some more advices. :-)

--
ïark Yao