[Intel-gfx] [PATCH 00/12] drm: Put drm_display_mode on diet

[Ville Syrjälä]

On Fri, Feb 21, 2020 at 06:16:57PM +0100, Daniel Vetter wrote:
> On Fri, Feb 21, 2020 at 06:09:04PM +0200, Ville Syrjälä wrote:
> > On Fri, Feb 21, 2020 at 05:40:31PM +0200, Ville Syrjälä wrote:
> > > On Fri, Feb 21, 2020 at 03:42:56PM +0100, Daniel Vetter wrote:
> > > > On Fri, Feb 21, 2020 at 12:43 PM Ville Syrjälä
> > > > <ville.syrjala at linux.intel.com> wrote:
> > > > >
> > > > > On Fri, Feb 21, 2020 at 01:32:29PM +0200, Jani Nikula wrote:
> > > > > > On Thu, 20 Feb 2020, Ville Syrjälä <ville.syrjala at linux.intel.com> wrote:
> > > > > > > Looks like getting rid of private_flags is going to be pretty
> > > > > > > straightforward. I'll post patches for that once this first series
> > > > > > > lands.
> > > > > >
> > > > > > Going all in on crtc state? I suppose migrating away from private_flags
> > > > > > could easily start in i915 before that. Seems rather independent.
> > > > > >
> > > > > > I guess it's __intel_get_crtc_scanline() and:
> > > > > >
> > > > > >       vblank = &crtc->base.dev->vblank[drm_crtc_index(&crtc->base)];
> > > > > >       mode = &vblank->hwmode;
> > > > > >
> > > > > >       if (mode->private_flags & I915_MODE_FLAG_GET_SCANLINE_FROM_TIMESTAMP)
> > > > > >
> > > > > > that gives me the creeps in reviewing all that.
> > > > > >
> > > > > > There's also [1] adding new uses for private_flags; I think there were
> > > > > > issues in getting at the right crtc state on some of those paths, but I
> > > > > > forget the exact details. Ideas?
> > > > >
> > > > > I'm just going to move them to the crtc_state and put a copy into the
> > > > > crtc itself for the vblank code. Pretty much a 1:1 replacement.
> > > > > Saves me from having to think ;)
> > > > 
> > > > I've looked through the patches, and didn't spot any place where we
> > > > couldn't just get at the full crtc state. Might need some crtc->state
> > > > dereferencing and upcasting and making sure stuff is ordered correctly
> > > > with enable/disable paths of crtc, but nothing to jump over.
> > > 
> > > swap_state() could easily race with the irq handler. I guess
> > > practically unlikely the old crtc state would disappear before
> > > the irq handler is done, but still seems somewhat dubious.
> > 
> > And I guess the bigger problem is that swap_state() happens way too
> > early. So crtc->state would be pointing to bogus stuff while we're
> > disabling the crtc.
> 
> Uh, so we're essentially piggy-packing some random i915 state on top of
> the hw timing stuff the vblank handler does, and hope that this is
> race-free enough to not matter?
> 
> I think the right solution there would be to have a proper
> spinlock_irqsafe for this stuff that the dsi TE handler needs, and through
> that make sure that we're actually not going boom. At least it looked like
> there's also irq handling bits outside of the vblank code, so the vblank
> locking is not going to safe the day.

I haven't actually looked at the DSI TE stuff so far, so no
idea what's going on there.

-- 
Ville Syrjälä
Intel