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

[thierry.reding@gmail.com (Thierry Reding)]

On Thu, Jan 14, 2016 at 02:39:42PM +0000, John Keeping wrote:
> Signed-off-by: John Keeping <john@metanate.com>
> ---
>  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.

Perhaps also the drm_atomic_helper_wait_for_vblanks() kerneldoc needs to
be updated with these restrictions on its use?

Thierry
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