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>]

Adding Tvrtko, for some reason he didn't get CCed before.


On Fri, 2023-11-17 at 11:26 +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.

Oh, somehow I missed that.  Thanks.

So, it seems that the locking is indeed needed.  I think there are two
options, then:

1. Go back to my previous version of the patch, where I had the wrapper
that didn't lock anything on Xe and implement the display part when we
get a similar implementation of the uncore.lock on Xe (if ever needed).

2. Implement a display-local lock for this, as suggested at some point,
including the other intel_de*() accesses.  But can we be sure that it's
enough to protect only the registers accessed by the display? I.e.
won't accessing display registers non-serially in relation to non-
display registers?


Preferences?

--
Cheers,
Luca.