[Intel-gfx] [PATCH 24/46] drm/i915: Do a synchronous switch-to-kernel-context on idling

[Chris Wilson]

Quoting Daniele Ceraolo Spurio (2019-02-21 22:53:41)
> 
> 
> On 2/21/19 1:42 PM, Chris Wilson wrote:
> > Quoting Daniele Ceraolo Spurio (2019-02-21 21:31:45)
> >>
> >>
> >> On 2/21/19 1:17 PM, Chris Wilson wrote:
> >>> Quoting Daniele Ceraolo Spurio (2019-02-21 19:48:01)
> >>>>
> >>>> <snip>
> >>>>
> >>>>> @@ -4481,19 +4471,7 @@ int i915_gem_suspend(struct drm_i915_private *i915)
> >>>>>          * state. Fortunately, the kernel_context is disposable and we do
> >>>>>          * not rely on its state.
> >>>>>          */
> >>>>> -     if (!i915_terminally_wedged(&i915->gpu_error)) {
> >>>>> -             ret = i915_gem_switch_to_kernel_context(i915);
> >>>>> -             if (ret)
> >>>>> -                     goto err_unlock;
> >>>>> -
> >>>>> -             ret = i915_gem_wait_for_idle(i915,
> >>>>> -                                          I915_WAIT_INTERRUPTIBLE |
> >>>>> -                                          I915_WAIT_LOCKED |
> >>>>> -                                          I915_WAIT_FOR_IDLE_BOOST,
> >>>>> -                                          HZ / 5);
> >>>>> -             if (ret == -EINTR)
> >>>>> -                     goto err_unlock;
> >>>>> -
> >>>>> +     if (!switch_to_kernel_context_sync(i915)) { >                   /* Forcibly cancel outstanding work and leave the gpu quiet. */
> >>>>>                 i915_gem_set_wedged(i915);
> >>>>>         }
> >>>>
> >>>> GuC-related question: what's your expectation here in regards to GuC
> >>>> status? The current i915 flow expect either uc_reset_prepare() or
> >>>> uc_suspend() to be called to clean up the guc status, but we're calling
> >>>> neither of them here if the switch is successful. Do you expect the
> >>>> resume code to always blank out the GuC status before a reload?
> >>>
> >>> (A few patches later on I propose that we always just do a reset+wedge
> >>> on suspend in lieu of hangcheck.)
> >>>
> >>> On resume, we have to bring the HW up from scratch and do another reset
> >>> in the process. Some platforms have been known to survive the trips to
> >>> PCI_D3 (someone is lying!) and so we _have_ to do a reset to be sure we
> >>> clear the HW state. I expect we would need to force a reset on resume
> >>> even for the guc, to be sure we cover all cases such as kexec.
> >>> -Chris
> >>>
> >> More than about the HW state, my question here was about the SW
> >> tracking. At which point do we go and stop guc communication and mark
> >> guc as not loaded/accessible? e.g. we need to disable and re-enable CT
> >> buffers before GuC is reset/suspended to make sure the shared memory
> >> area is cleaned correctly (we currently avoid memsetting all of it on
> >> reload since it is quite big). Also, communication with GuC is going to
> >> increase going forward, so we'll need to make sure we accurately track
> >> its state and do all the relevant cleanups.
> > 
> > Across suspend/resume, we issue a couple of resets and scrub/sanitize our
> > state tracking. By the time we load the fw again, both the fw and our
> > state should be starting from scratch.
> > 
> > That all seems unavoidable, so I am not understanding the essence of
> > your question.
> > -Chris
> > 
> 
> We're not doing the state scrubbing for guc in all paths at the moment. 
> There is logic in gem_suspend_late(), but that doesn't seem to be called 
> on all paths; e.g. it isn't when we run 
> igt at gem_exec_suspend@basic-s4-devices

Yup, the dummy hibernate code throws a few surprises, and why
i915_gem_sanitize is so fiddly to get right between that and
gem_eio/suspend.

> and that's why Suja's patch moved 
> the disabling of communication from uc_sanitize to uc_suspend.

That should also help as previously it tried to talk to the guc after we
reset it.

> The guc 
> resume code also doesn't currently clean everything as some of the 
> structures (including stuff we allocate for guc usage) are carried over. 
> We can either add something more in the cleanup path or go and rework 
> the resume to blank everything (which would be time consuming since 
> there is tens of MBs involved), but before putting down any code one way 
> or another I wanted to understand what the expectation is.

I may be naive, but my expectations is that we just have to reset the
comm ringbuffer pointers. We shouldn't need to hand the guc pristine
pages, it will zero on allocate when its needs to, surely? We do have to
rebuild the set of clients everytime we load the guc, so that can't be
the issue (as that has to be done on resume, device reset etc today),
although that should only have to be the pinned clients?

We have to restart the comm channels on loading the guc, so what's
changing?

On suspend, hit the device reset & kill guc. On resume, load the guc fw,
restart comm. After fiddling about making sure we are in the right
callpaths, the intent is that resume just looks like a fresh module
load (so we only have to reason about init sequence [nearly] once).
-Chris