[PATCH 1/2] drm/fb-helper: Synchronize dirty worker with vblank

[Thomas Zimmermann]

Hi

Am 10.09.19 um 13:52 schrieb Gerd Hoffmann:
> On Mon, Sep 09, 2019 at 04:06:32PM +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.
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
>> ---
>>  drivers/gpu/drm/drm_fb_helper.c | 12 ++++++++++++
>>  1 file changed, 12 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
>> index a7ba5b4902d6..017e2f6bd1b9 100644
>> --- a/drivers/gpu/drm/drm_fb_helper.c
>> +++ b/drivers/gpu/drm/drm_fb_helper.c
>> @@ -402,8 +402,20 @@ 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 */
>> +	mutex_lock(&helper->lock);
>> +	crtc = helper->client.modesets[0].crtc;
>> +	ret = drm_crtc_vblank_get(crtc);
>> +	if (!ret) {
>> +		drm_crtc_wait_one_vblank(crtc);
>> +		drm_crtc_vblank_put(crtc);
>> +	}
>> +	mutex_unlock(&helper->lock);
> 
> Hmm, not sure it is the best plan to sleep for a while in the worker,
> especially while holding the lock.
> 
> What does the lock protect against here?  Accessing

This lock is hold by the fbdev code during mode-setting operations (but
not drawing operations). So *crtc might be gone if we don't hold it here.

> helper->client.modesets?  If so then you can unlock before going to
> sleep in drm_crtc_wait_one_vblank() I think.

I looked, but I cannot find any code that protects crtc while vblank is
active. I'd rather not change the current code until someone can clarify.

Best regards
Thomas

> 
> cheers,
>   Gerd
> 

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

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