Re: [PATCH] drm: make drm_wait_vblank return immediately for very old sequence values

[Michel D. <mi...@da...>]

On Tue, 2009-02-17 at 09:27 -0800, Jesse Barnes wrote:
> On Tuesday, February 17, 2009 9:04 am Michel Dänzer wrote:
> > On Mon, 2009-02-16 at 10:42 -0800, Jesse Barnes wrote:
> > > On Sunday, February 15, 2009 11:33 pm Michel Dänzer wrote:
> > > > On Fri, 2009-02-13 at 10:27 -0800, Jesse Barnes wrote:
> > > > > Recall our last discussion where I outlined the cases we'd have to
> > > > > deal with in the modeset ioctl if we didn't use get/put to just keep
> > > > > interrupts on around the calls:
> > > >
> > > > But we are intending to keep them on around the calls. So the problem
> > > > is that you are disabling the IRQ in between? Maybe a solution could be
> > > > not to mess with the counter when disabling/enabling the IRQ. It needs
> > > > to be guarded by the modeset ioctl anyway, so shouldn't that work?
> > >
> > > The current problem isn't a disable between modeset ioctls, it's a
> > > disable followed by a counter reset of any kind (modeset ioctls or not),
> > > since the "last" count we track is just done in the disable function, not
> > > in the modeset ioctl.  Doing in in the modeset ioctl instead may be
> > > possible, but as I said there are lots of cases to deal with.
> >
> > Isn't the actual problem that drm_irq_uninstall() updates last_vblank?
> > If it didn't (and the modeset ioctl is properly called around the
> > disabling and re-enabling of the IRQ), wouldn't it just work?
> 
> No, this happens w/o an uninstall, it's just due to wraparound being added 
> between the disable timer & the next drm_vblank_get.

I still don't understand the problem then; this can only happen if there
actually was a wraparound, as the drm_vblank_get must happen (via the
modeset ioctl) before the hardware frame counter resets for any other
reason.


> > > But apps can't assume that; frames *will* be lost when the monitor goes
> > > off, for as long as the monitor is off.  So if an app isn't actively
> > > using the framecount for something it can't count on a given value. 
> > > Right?
> >
> > Again, I disagree with the notion that because something may not work
> > under some (in this case exceptional) circumstances, we shouldn't try to
> > make it work whenever possible.
> 
> So DPMS is exceptional?  I think it's normal.

Surely it's exceptional while a 'normal' OpenGL application is running.


-- 
Earthling Michel Dänzer           |                http://www.vmware.com
Libre software enthusiast         |          Debian, X and DRI developer