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>]

On Fri, 2023-11-17 at 12:46 +0000, Tvrtko Ursulin wrote:
> On 17/11/2023 12:21, Coelho, Luciano wrote:
> > 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?
> 
> AFAIR my initial complaint was the naming which was along the lines of 
> intel_spin_lock(uncore). How to come up with a clean and logical name is 
> the question...

You're right, that was your first comment, and now we're back to it. :)


> On Xe you don't need a lock since HSW/IVB/cacheline angle does not exist 
> but you need a name which does not clash with either.
> 
> Assuming you still need the preempt off (or irq off) on Xe for better 
> timings, maybe along the lines of intel_vblank_section_enter/exit(i915)?

I like this name, because it's indeed this vblank section we're trying
to protect here, and this is not used anywhere else.

 
> Although I am not up to speed with what object instead of i915 would you 
> be passing in from Xe ie. how exactly polymorphism is implemented in 
> shared code.

AFAICT we are still using the i915 structure for most things inside the
display code, so I guess we should use that for now.

I'll send a new version of my original patch in a bit.

--
Cheers,
Luca.