linux-kernel - Re: [PATCH 1/2] drm: introduce page_flip

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Message-ID: <2349754.vFx2qVVIhK@timur-hyperion>
Date: Fri, 23 Jan 2026 15:44:50 +0100
From: Timur Kristóf <timur.kristof@...il.com>
To: Hamza Mahfooz <someguy@...ective-light.com>,
 dri-devel@...ts.freedesktop.org,
 Christian König <christian.koenig@....com>
Cc: Alex Deucher <alexander.deucher@....com>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Harry Wentland <harry.wentland@....com>, Leo Li <sunpeng.li@....com>,
 Rodrigo Siqueira <siqueira@...lia.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 Sunil Khatri <sunil.khatri@....com>, Ce Sun <cesun102@....com>,
 Lijo Lazar <lijo.lazar@....com>, Kenneth Feng <kenneth.feng@....com>,
 Ivan Lipski <ivan.lipski@....com>, Alex Hung <alex.hung@....com>,
 Tom Chung <chiahsuan.chung@....com>, Melissa Wen <mwen@...lia.com>,
 Michel Dänzer <mdaenzer@...hat.com>,
 Fangzhi Zuo <Jerry.Zuo@....com>, amd-gfx@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, Harry Wentland <harry.wentland@....com>,
 Mario Limonciello <mario.limonciello@....com>
Subject: Re: [PATCH 1/2] drm: introduce page_flip_timeout()

On Friday, January 23, 2026 2:52:44 PM Central European Standard Time 
Christian König wrote:
> On 1/23/26 01:05, Hamza Mahfooz wrote:
> > There should be a mechanism for drivers to respond to flip_done
> > time outs.
> 

I am adding Harry and Mario to this email as they are more familiar with this.

> I can only see two reasons why you could run into a timeout:
> 
> 1. A dma_fence never signals.
> 	How that should be handled is already well documented and doesn't 
require
> any of this.

Page flip timeouts have nothing to do with fence timeouts.
A page flip timeout can occur even when all fences of all job submissions 
complete correctly and on time.

> 
> 2. A coding error in the vblank or page flip handler leading to waiting
> forever. In that case calling back into the driver doesn't help either.

At the moment, a page flip timeout will leave the whole system in a hung state 
and the driver does not even attempt to recover it in any way, it just stops 
doing anything, which is unacceptable and I'm pretty surprised that it was 
left like that for so long.

Note that we have approximately a hundred bug reports open on the drm/amd bug 
tracker about "random" page flip timeouts. It affects a lot of users.

> 
> So as far as I can see the whole approach doesn't make any sense at all.

Actually this approach was proposed as a solution at XDC 2025 in Harry's 
presentation, "DRM calls driver callback to attempt recovery", see page 9 in 
this slide deck:

https://indico.freedesktop.org/event/10/contributions/431/attachments/
267/355/2025%20XDC%20Hackfest%20Update%20v1.2.pdf

If you disagree with Harry, please make a counter-proposal.

Thanks,
Timur



> 
> > Since, as it stands it is possible for the display
> > to stall indefinitely, necessitating a hard reset. So, introduce
> > a new crtc callback that is called by
> > drm_atomic_helper_wait_for_flip_done() to give drivers a shot
> > at recovering from page flip timeouts.
> > 
> > Signed-off-by: Hamza Mahfooz <someguy@...ective-light.com>
> > ---
> > 
> >  drivers/gpu/drm/drm_atomic_helper.c | 6 +++++-
> >  include/drm/drm_crtc.h              | 9 +++++++++
> >  2 files changed, 14 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_atomic_helper.c
> > b/drivers/gpu/drm/drm_atomic_helper.c index 5840e9cc6f66..3a144c324b19
> > 100644
> > --- a/drivers/gpu/drm/drm_atomic_helper.c
> > +++ b/drivers/gpu/drm/drm_atomic_helper.c
> > @@ -1881,9 +1881,13 @@ void drm_atomic_helper_wait_for_flip_done(struct
> > drm_device *dev,> 
> >  			continue;
> >  		
> >  		ret = wait_for_completion_timeout(&commit->flip_done, 10 
* HZ);
> > 
> > -		if (ret == 0)
> > +		if (!ret) {
> > 
> >  			drm_err(dev, "[CRTC:%d:%s] flip_done timed 
out\n",
> >  			
> >  				crtc->base.id, crtc->name);
> > 
> > +
> > +			if (crtc->funcs->page_flip_timeout)
> > +				crtc->funcs-
>page_flip_timeout(crtc);
> > +		}
> > 
> >  	}
> >  	
> >  	if (state->fake_commit)
> > 
> > diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
> > index 66278ffeebd6..45dc5a76e915 100644
> > --- a/include/drm/drm_crtc.h
> > +++ b/include/drm/drm_crtc.h
> > @@ -609,6 +609,15 @@ struct drm_crtc_funcs {
> > 
> >  				uint32_t flags, uint32_t target,
> >  				struct drm_modeset_acquire_ctx 
*ctx);
> > 
> > +	/**
> > +	 * @page_flip_timeout:
> > +	 *
> > +	 * This optional hook is called if &drm_crtc_commit.flip_done times 
out,
> > +	 * and can be used by drivers to attempt to recover from a page 
flip
> > +	 * timeout.
> > +	 */
> > +	void (*page_flip_timeout)(struct drm_crtc *crtc);
> > +
> > 
> >  	/**
> >  	
> >  	 * @set_property:
> >  	 *