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

[Noralf Trønnes]

Den 17.09.2019 14.55, skrev Daniel Vetter:
> On Tue, Sep 10, 2019 at 04:59:57PM +0200, Noralf Trønnes wrote:
>>
>>
>> Den 10.09.2019 15.51, skrev Thomas Zimmermann:
>>> 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 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.
>>>>>
>>>>
>>>> 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?
>>>
>>
>> Because of drm_master_internal_acquire() it's necessary to take the lock
>> first. That's the locking rules of drm_fb_helper. First take the fb
>> helper lock, then the dev->master_mutex. We stay away if there's a
>> userspace master and if there's none, we prevent userspace from becoming
>> master while we do stuff.
>>
>> But looking at drm_fb_helper_ioctl() now, I see that it's not strictly
>> necessary to do this since all this function can do is vblank wait and
>> that's fine even if userspace is master. The locking was necessary
>> before I refactored and moved stuff to drm_client, because at that time
>> the modeset array was deleted and recreated when probing connectors.
>> But it doesn't hurt either in case more functionality is added to the
>> ioctl. One wouldn't think that would ever happen, since fbdev is going
>> away soon, but still we keep polishing it ;)
> 
> fbdev drivers are hopefully disappearing, I don't think fbdev as the uapi
> interface will disappear soon. Hence why it's still somewhat reasonable to
> keep polishing it imo. It should actually help in convincing people to
> move their fbdev driver over to drm, if that gives them a more polished
> fbdev implementation :-)

Oh right, the uapi stays, in that light it makes sense to keep polishing
the emulation and get fbdev drivers ported over.

Noralf.

> -Daniel
> 
>>
>> Noralf.
>>
>>>> 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
>>>>>>
>>>>>
>>>
>