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

[Ville Syrjälä]

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.

> 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