Re: [PATCH v3 1/2] drm: introduce KMS recovery mechanism

Hamza Mahfooz
2026-02-18T00:46:11Z

On Mon, Feb 16, 2026 at 10:28:13AM +0100, Michel Dänzer wrote:
> On 2/14/26 23:16, Hamza Mahfooz wrote:
> > On Sat, Feb 14, 2026 at 03:02:49PM +0100, Michel Dänzer wrote:
> >> Why couldn't a full modeset?
> >
> > As far as I see it the only reasons why we should be timing out is
> > either an interrupt went missing (perhaps due to a race condition in
> > driver code) or hung hardware. In either case, the interrupt associated
> > with the page flip for the current commit is long gone.
>
> That's a matter of bookkeeping, the interrupt isn't required to keep track of the commit and complete it.

Oh, if you're talking about sending out a "fake"
drm_crtc_send_vblank_event(), I had considered that. Though,
drivers that have hardware vblank counters take ownership of the
relevant `struct drm_pending_vblank_event` [1] pretty early on.
So, there wouldn't be a way to ensure that they send that out
and that would mean we never get a chance to force a full modeset.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/drm/drm_crtc.h?h=v6.19#n386

>
>
> >> In principle it's possible to do (the equivalent of) a modeset with the current state for all CRTCs, no need to do it separately per CRTC.
> >
> > AFAIK that is what the uevent is already doing (unless I'm mistaken).
>
> This is about just doing a full modeset, which isn't something user space can do in response to a wedged event.

I was referring to what compositors are doing in response to
`drm_kms_helper_hotplug_event()`. Perhaps, the enum constants should be
renamed, since the forced modeset is actually sent when the current
reset phase is DRM_KMS_RESET_NONE (the phase is updated before sending
out the event though).