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

[Ville Syrjälä]

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.

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

-- 
Ville Syrjälä
Intel