[Intel-xe] [Intel-gfx] [PATCH] drm/i915: don't use uncore spinlock to protect critical section in vblank

[Rodrigo Vivi]

On Fri, Nov 17, 2023 at 11:26:44AM +0200, Ville Syrjälä wrote:
> On Fri, Nov 17, 2023 at 10:41:43AM +0200, Ville Syrjälä wrote:
> > On Fri, Nov 17, 2023 at 08:05:21AM +0000, Coelho, Luciano wrote:
> > > Thanks for your comments, Ville!
> > > 
> > > On Fri, 2023-11-17 at 09:19 +0200, Ville Syrjälä wrote:
> > > > On Thu, Nov 16, 2023 at 01:27:00PM +0200, Luca Coelho wrote:
> > > > > Since we're abstracting the display code from the underlying driver
> > > > > (i.e. i915 vs xe), we can't use the uncore's spinlock to protect
> > > > > critical sections of our code.
> > > > > 
> > > > > After further inspection, it seems that the spinlock is not needed at
> > > > > all and this can be handled by disabling preemption and interrupts
> > > > > instead.
> > > > 
> > > > uncore.lock has multiple purposes:
> > > > 1. serialize all register accesses to the same cacheline as on
> > > >    certain platforms that can hang the machine
> > > 
> > > Okay, do you remember which platforms?
> > 
> > HSW is the one I remember for sure being affected.
> > Althoguh I don't recall if I ever managed to hang it
> > using display registers specifically. intel_gpu_top
> > certainly was very good at reproducing the problem.
> > 
> > > I couldn't find any reference to
> > > this reason. 
> > 
> > If all else fails git log is your friend.
> 
> It seems to be documented in intel_uncore.h. Though that one
> mentions IVB instead of HSW for some reason. I don't recall
> seeing it on IVB myself, but I suppose it might have been an
> issue there as well. How long the problem remained after HSW
> I have no idea.

Paulo very recently told me that he could easily reproduce the issue
on IVB, simply by running 2 glxgears at the same time.

> 
> > 
> > > Also, the only place where where we take the uncore.lock
> > > is in this vblank code I changed, where the only explanation I found
> > > was about timing, specifically when using RT-kernels and in very old
> > > and slow platforms... (this was added 10 years ago).
> > > 
> > > 
> > > > 2. protect the forcewake/etc. state
> > > > 
> > > > 1 is relevant here, 2 is not.
> > > 
> > > Okay, good that we have only one known problem. :)

and good it is an old one! :)

> > > 
> > > --
> > > Cheers,
> > > Luca.
> > 
> > -- 
> > Ville Syrjälä
> > Intel
> 
> -- 
> Ville Syrjälä
> Intel