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

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 10.09.19 um 15:34 schrieb Noralf Trønnes:
> 
> 
> Den 10.09.2019 14.48, skrev 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@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.
>>
> 
> The client->modesets array and the crtc struct member are invariant over
> the lifetime of client (drm_client_modeset_create()). No need to take a
> lock for access. See drm_client_modeset_release() for the things that
> _can_ change and needs protection (protected by client->modeset_mutex).

Thanks for the reply. So we don't need the lock? But why does
drm_fb_helper_ioctl() take it? ioctl exclusiveness?

> I don't see a problem with sleeping in the worker though, but I might
> miss out on something. AFAICS changes will just pile up in >dirty_clip
> and the worker will be scheduled for a new run to happen when it's done
> with the current update.

Yes, that's the intention of the patch. We hope to reduce the number of
memcpys by handling several of them at once.

Best regards
Thomas

> 
> Noralf.
> 
>> 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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