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

["Coelho, Luciano" <luciano.coelho@intel.com>]

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? I couldn't find any reference to
this reason.  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.