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.
