Re: [PATCH 2/2] drm: Move crtc->{x, y, mode, enabled} to legacy sub-structure

[Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>]

On Thu, Oct 03, 2024 at 05:07:35PM +0200, Louis Chauvet wrote:
> 
> > > > diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c 
> > > > b/drivers/gpu/drm/vkms/vkms_crtc.c
> > > > index a40295c18b48..780681ea77e4 100644
> > > > --- a/drivers/gpu/drm/vkms/vkms_crtc.c
> > > > +++ b/drivers/gpu/drm/vkms/vkms_crtc.c
> > > > @@ -64,7 +64,7 @@ static int vkms_enable_vblank(struct drm_crtc *crtc)
> > > >         struct drm_vblank_crtc *vblank = drm_crtc_vblank_crtc(crtc);
> > > >         struct vkms_output *out = drm_crtc_to_vkms_output(crtc);
> > > >  
> > > > -       drm_calc_timestamping_constants(crtc, &crtc->mode);
> > > > +       drm_calc_timestamping_constants(crtc, &crtc->legacy.mode);
> > > 
> > >   drm_calc_timestamping_constants(crtc, &crtc->state->mode);
> > 
> > This one doesn't look safe. You want to call that during your atomic
> > commit already.
> > 
> 
> This was already not safe with the previous implementation? Or it is only 
> unsafe because now I use state->mode instead of legacy.mode?

Yeah, if you want to look at obj->state then you need the corresponding
lock.

obj->state is also not necessarily the correct state you want because
a parallel commit could have already swapped in a new state but the
hardware is still on the old state.

Basically 99.9% of code should never even look at obj->state, and
instead should always use the for_each_new_<obj>_in_state()
and drm_atomic_get_new_<obj>_state() stuff. Currently that is a
pipe dream though because a lot of drivers haven't been fixed to
do things properly. If we ever manage to fix everything then we
could remove the stall hacks from drm_atomic_helper_swap_state()
and allow a commit pipeline of arbitrary length.

> 
> After inspecting the code, I think I don't need to call it as:
> 
> In `vkms_atomic_commit_tail` (used in 
> `@vkms_mode_config_helpers.atomic_commit_tail`), we call 
> `drm_atomic_helper_commit_modeset_disables`, which call 
> `drm_atomic_helper_calc_timestamping_constants` which call 
> `drm_calc_timestamping_constants` for every CRTC.

Slightly odd place for it, but I think that's just because it was
originally part of drm_atomic_helper_update_legacy_modeset_state()
and I didn't bother looking for a better home for it when I split
it out. But seems like it should work fine as is.

> 
> I tested kms_vblank, all of them are SUCCESS/SKIP, do you know other tests 
> that can trigger bugs?

You would explicitly have to race commits against vblank_enable.
Could of course sprinkle sleep()s around to widen the race window
if you're really keen to hit it.

-- 
Ville Syrjälä
Intel