[PATCH v2 1/3] drm/fb-helper: Synchronize dirty worker with vblank

[Thomas Zimmermann]

Hi

Am 17.09.19 um 15:06 schrieb Daniel Vetter:
> On Thu, Sep 12, 2019 at 08:42:28AM +0200, Thomas Zimmermann wrote:
>> Before updating the display from the console's shadow buffer, the dirty
>> worker now waits for vblank. This allows several screen updates to pile
>> up and acts as a rate limiter.
>>
>> v2:
>> 	* don't hold helper->lock while waiting for vblank
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
>> ---
>>   drivers/gpu/drm/drm_fb_helper.c | 10 ++++++++++
>>   1 file changed, 10 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
>> index a7ba5b4902d6..d0cb1fa4f909 100644
>> --- a/drivers/gpu/drm/drm_fb_helper.c
>> +++ b/drivers/gpu/drm/drm_fb_helper.c
>> @@ -402,8 +402,18 @@ static void drm_fb_helper_dirty_work(struct work_struct *work)
>>   						    dirty_work);
>>   	struct drm_clip_rect *clip = &helper->dirty_clip;
>>   	struct drm_clip_rect clip_copy;
>> +	struct drm_crtc *crtc;
>>   	unsigned long flags;
>>   	void *vaddr;
>> +	int ret;
>> +
>> +	/* rate-limit update frequency */
>> +	crtc = helper->client.modesets[0].crtc;
>> +	ret = drm_crtc_vblank_get(crtc);
>> +	if (!ret) {
>> +		drm_crtc_wait_one_vblank(crtc);
> 
> Without the locking (specifically, preventing other masters) this can go
> boom since it again calls drm_vblank_get. If someone turned of the display
> meanwhile that will fail, and result in an unsightly WARN backtrace.
> 
> I think we need a __drm_crtc_wait_one_vblank(crtc); which requires that
> callers hold a full vblank reference already. The other issue is that we
> might race with the disabling and hit the timeout, which again gives us an
> unsightly WARNING backtrace. Both can happen without locks (that's why the
> ioctl path needs them), but we need to avoid.

Here's a status update: I've been working on this patch for a while, but 
the worker still cannot wait reliable for vblanks. When the worker runs, 
the display could be off and hence no vblank events occur. That's 
especially a problem during boot. The worker warns about missed vblanks 
because the console's video mode is still being programmed.

Best regards
Thomas

> -Daniel
>> +		drm_crtc_vblank_put(crtc);
>> +	}
>>   
>>   	spin_lock_irqsave(&helper->dirty_lock, flags);
>>   	clip_copy = *clip;
>> -- 
>> 2.23.0
>>
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Linux GmbH, Maxfeldstrasse 5, 90409 Nuernberg, Germany
GF: Felix Imendörffer, Mary Higgins, Sri Rasiah
HRB 21284 (AG Nürnberg)