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

From: [email-masked] (Mark yao)
Date: Mon, 18 Jan 2016 09:40:32 +0800

On 2016?01?15? 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 <[email-masked]>
>>> ---
>>>   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. :-)

-- 
?ark Yao