Re: [PATCH] drm/client: check whether CRTC is active before waiting for vblank

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

Am 22.05.26 um 15:13 schrieb Ville Syrjälä:
> On Fri, May 22, 2026 at 01:55:59PM +0200, Thomas Zimmermann wrote:
>> Hi
>>
>> Am 19.05.26 um 11:24 schrieb Icenowy Zheng:
>>> Currently the implementaion of drm_client_modeset_wait_for_vblank()
>>> assumes drm_vblank_get() will fail when the CRTC isn't active. However
>>> it seems that this is not true, and running fbcon on a device with the
>>> first CRTC inactive will lead to kernel warning in some cases (which
>>> could be reproduced with the loongson driver).
>>>
>>> Change the implementation to add a check for the active state (atomic) /
>>> enabled state (non-atomic) before calling drm_vblank_get(). As the
>>> assumption of drm_vblank_get() failing for inactive CRTC isn't met, the
>>> error status of drm_vblank_get() can now be exported too.
>>>
>>> Cc: stable@vger.kernel.org
>>> Fixes: d8c4bddcd8bc ("drm/fb-helper: Synchronize dirty worker with vblank")
>>> Signed-off-by: Icenowy Zheng <zhengxingda@iscas.ac.cn>
>>> ---
>>>    drivers/gpu/drm/drm_client_modeset.c | 13 +++++++++++--
>>>    1 file changed, 11 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_client_modeset.c b/drivers/gpu/drm/drm_client_modeset.c
>>> index bb49b8361271a..1b03bf351256e 100644
>>> --- a/drivers/gpu/drm/drm_client_modeset.c
>>> +++ b/drivers/gpu/drm/drm_client_modeset.c
>>> @@ -1310,7 +1310,7 @@ int drm_client_modeset_wait_for_vblank(struct drm_client_dev *client, unsigned i
>>>    {
>>>    	struct drm_device *dev = client->dev;
>>>    	struct drm_crtc *crtc;
>>> -	int ret;
>>> +	int ret = 0;
>>>    
>>>    	/*
>>>    	 * Rate-limit update frequency to vblank. If there's a DRM master
>>> @@ -1326,15 +1326,24 @@ int drm_client_modeset_wait_for_vblank(struct drm_client_dev *client, unsigned i
>>>    	 * Only wait for a vblank event if the CRTC is enabled, otherwise
>>>    	 * just don't do anything, not even report an error.
>>>    	 */
>>> +	if (drm_drv_uses_atomic_modeset(dev)) {
>>> +		if (!crtc->state || !crtc->state->active)
>>> +			goto out;
>>> +	} else {
>>> +		if (!crtc->enabled)
>>> +			goto out;
>>> +	}
>>> +
>> This part is good.
> Locking is missing.

Ok

>
>>>    	ret = drm_crtc_vblank_get(crtc);
>>>    	if (!ret) {
>>>    		drm_crtc_wait_one_vblank(crtc);
>>>    		drm_crtc_vblank_put(crtc);
>>>    	}
>>>    
>>> +out:
>>>    	drm_master_internal_release(dev);
>>>    
>>> -	return 0;
>>> +	return ret;
>> But this isn't. There can be CRTCs without any vblank at all. We still
>> want to fail silently for them. So we still have to return 0 here.
>>
>> Having set this, fixing this helper is only partially what you want.
>> Since your device has vblanking, the emulation should check on the
>> correct CRTC. IOW you need to pass the right CRTC index at
>>
>> https://elixir.bootlin.com/linux/v7.1-rc1/source/drivers/gpu/drm/drm_fb_helper.c#L237
>> https://elixir.bootlin.com/linux/v7.1-rc1/source/drivers/gpu/drm/drm_fb_helper.c#L920
>>
>> I'm not quite sure how to support this. The CRTC is under
>> fb_helper->client.modesets.crtc. You'd have to figure out which is the
>> relevant one and use that. But that's also not so great, as fbdev ioctls
>> only support CRTC 0. Doing internal re-mappings only complicates matters.
>>
>> But why does your HW use CRTC 1 in the first place.
> Could be eg. the enabled outputs can't be driven with CRTC 0.
>
> I guess what you want to do is pick the first crtc from modesets[]
> which is enabled. Or perhaps even "pick the Nth enabled crtc from
> modesets[] based on the ioctl argument".

The enable-status of each CRTC could change later on, which might lead 
to problems.  Picking the one CRTC/output with the lowest spec and 
mirroring it to the others might work. This CRTC would then be the one 
to wait for.

Best regards
Thomas

>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)