Re: [PATCH] drm/vblank: Restart enabling vblank timer on possible deadlock

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Nov 03, 2026 at 04:55:34PM +0100, Thomas Zimmermann wrote:
> Hi
> 
> Am 02.11.26 um 18:15 schrieb Ville Syrjälä:
> [...]
> > But what I'm thinking is:
> 
> I've ran a lengthy session to explore various solutions.
> 
> > - stop with the explicit cancel entirely and just rely on the
> >    timer cancelling itself
> > - try to start the timer in vblank_enable(), and if that can somehow
> >    fail because the timer is still running it should be fine because
> >    the self cancellation should anyway have been aborted
> 
> Having an old vblank timer still running while enable_vblank sets up a 
> new one can mess up the hrtimer's internal ordering. It's full of subtle 
> bugs, according to my LLM.

The hrtimer code looks like it's designed to handle this. It
first removes the old queued timer and then inserts it back
with the new expire time. And there's also locks in there.

> 
> 
> > - explicitly wait for the timer to have stoppped in drm_vblank_off()
> >    to make sure the wrong interval can't leak from one mode to
> >    another
> 
> I'll soon send out a series implementing this idea.
> 
> Best regards
> Thomas
> 
> >
> 
> -- 
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
> GF: Stefan Gaiser, Jochen Jaser, Abhinav Puri, (HRB 36809, AG Nürnberg)
> 

-- 
Ville Syrjälä
Intel