Quoting Mika Kuoppala (2018-08-14 14:42:41)
> Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes:
>
> > Before a reset, we set the STOP_RING bit of RING_MI_MODE to freeze the
> > engine. However, sometimes we observe that upon restart, the engine
> > freezes again with the STOP_RING bit still asserted. By inspection, we
> > know that the register itself is cleared by the GPU reset, so that bit
> > must be preserved inside the context image and reloaded from there. A
> > simple fix (as the RING_MI_MODE is at a fixed offset in a valid context)
> > is to clobber the STOP_RING bit inside the image before replaying the
> > request.
> >
> > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> > Cc: Michel Thierry <michel.thierry@xxxxxxxxx>
> > Cc: Michał Winiarski <michal.winiarski@xxxxxxxxx>
> > ---
> > drivers/gpu/drm/i915/intel_lrc.c | 17 +++++++++++++++--
> > drivers/gpu/drm/i915/intel_lrc_reg.h | 2 ++
> > 2 files changed, 17 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> > index 3f90c74038ef..37fe842de639 100644
> > --- a/drivers/gpu/drm/i915/intel_lrc.c
> > +++ b/drivers/gpu/drm/i915/intel_lrc.c
> > @@ -1918,6 +1918,20 @@ static void execlists_reset(struct intel_engine_cs *engine,
> >
> > spin_unlock_irqrestore(&engine->timeline.lock, flags);
> >
> > + if (!request)
> > + return;
> > +
> > + regs = request->hw_context->lrc_reg_state;
> > +
> > + /*
> > + * After a reset, the context may have preserved the STOP bit
> > + * of RING_MI_MODE we used to freeze the active engine before
> > + * the reset. If that bit is restored the ring stops instead
> > + * of being executed.
> > + */
> > + regs[CTX_MI_MODE + 1] |= STOP_RING << 16;
> > + regs[CTX_MI_MODE + 1] &= ~STOP_RING;
>
> Ok, I did go and pondered if this is truely the simplest
> way. Forcing the start outside a context would not
> be simpler and would be slower.
>
> Nice find and it will has potential to explain our some troubles
> of re-enabling engines.
>
> Did you find out this by looking at the error states or what?
We're dumping sufficient info between the set-wedged and GEM_TRACE to
track the MI_MODE bits; e.g.
commit 9a4dc80399b1630cea0f1ad8ef0417436cbb95d0
Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Date: Fri May 18 11:09:33 2018 +0100
drm/i915: Flush the ring stop bit after clearing RING_HEAD in reset
Inside the live_hangcheck (reset) selftests, we occasionally see
failures like
<7>[ 239.094840] i915_gem_set_wedged rcs0
<7>[ 239.094843] i915_gem_set_wedged current seqno 19a98, last 19a9a, hangcheck 0 [5158 ms]
<7>[ 239.094846] i915_gem_set_wedged Reset count: 6239 (global 1)
<7>[ 239.094848] i915_gem_set_wedged Requests:
<7>[ 239.095052] i915_gem_set_wedged first 19a99 [e8c:5f] prio=1024 @ 5159ms: (null)
<7>[ 239.095056] i915_gem_set_wedged last 19a9a [e81:1a] prio=139 @ 5159ms: igt/rcs0[5977]/1
<7>[ 239.095059] i915_gem_set_wedged active 19a99 [e8c:5f] prio=1024 @ 5159ms: (null)
<7>[ 239.095062] i915_gem_set_wedged [head 0220, postfix 0280, tail 02a8, batch 0xffffffff_ffffffff]
<7>[ 239.100050] i915_gem_set_wedged ring->start: 0x00283000
<7>[ 239.100053] i915_gem_set_wedged ring->head: 0x000001f8
<7>[ 239.100055] i915_gem_set_wedged ring->tail: 0x000002a8
<7>[ 239.100057] i915_gem_set_wedged ring->emit: 0x000002a8
<7>[ 239.100059] i915_gem_set_wedged ring->space: 0x00000f10
<7>[ 239.100085] i915_gem_set_wedged RING_START: 0x00283000
<7>[ 239.100088] i915_gem_set_wedged RING_HEAD: 0x00000260
<7>[ 239.100091] i915_gem_set_wedged RING_TAIL: 0x000002a8
<7>[ 239.100094] i915_gem_set_wedged RING_CTL: 0x00000001
<7>[ 239.100097] i915_gem_set_wedged RING_MODE: 0x00000300 [idle]
<7>[ 239.100100] i915_gem_set_wedged RING_IMR: fffffefe
<7>[ 239.100104] i915_gem_set_wedged ACTHD: 0x00000000_0000609c
<7>[ 239.100108] i915_gem_set_wedged BBADDR: 0x00000000_0000609d
<7>[ 239.100111] i915_gem_set_wedged DMA_FADDR: 0x00000000_00283260
<7>[ 239.100114] i915_gem_set_wedged IPEIR: 0x00000000
<7>[ 239.100117] i915_gem_set_wedged IPEHR: 0x02800000
<7>[ 239.100120] i915_gem_set_wedged Execlist status: 0x00044052 00000002
<7>[ 239.100124] i915_gem_set_wedged Execlist CSB read 5 [5 cached], write 5 [5 from hws], interrupt posted? no, tasklet queued? no (enabled)
<7>[ 239.100128] i915_gem_set_wedged ELSP[0] count=1, ring->start=00283000, rq: 19a99 [e8c:5f] prio=1024 @ 5164ms: (null)
<7>[ 239.100132] i915_gem_set_wedged ELSP[1] count=1, ring->start=00257000, rq: 19a9a [e81:1a] prio=139 @ 5164ms: igt/rcs0[5977]/1
<7>[ 239.100135] i915_gem_set_wedged HW active? 0x5
<7>[ 239.100250] i915_gem_set_wedged E 19a99 [e8c:5f] prio=1024 @ 5164ms: (null)
<7>[ 239.100338] i915_gem_set_wedged E 19a9a [e81:1a] prio=139 @ 5164ms: igt/rcs0[5977]/1
<7>[ 239.100340] i915_gem_set_wedged Queue priority: 139
<7>[ 239.100343] i915_gem_set_wedged Q 0 [e98:19] prio=132 @ 5164ms: igt/rcs0[5977]/8
<7>[ 239.100346] i915_gem_set_wedged Q 0 [e84:19] prio=121 @ 5165ms: igt/rcs0[5977]/2
<7>[ 239.100349] i915_gem_set_wedged Q 0 [e87:19] prio=82 @ 5165ms: igt/rcs0[5977]/3
<7>[ 239.100352] i915_gem_set_wedged Q 0 [e84:1a] prio=44 @ 5164ms: igt/rcs0[5977]/2
<7>[ 239.100356] i915_gem_set_wedged Q 0 [e8b:19] prio=20 @ 5165ms: igt/rcs0[5977]/4
<7>[ 239.100362] i915_gem_set_wedged drv_selftest [5894] waiting for 19a99
where the GPU saw an arbitration point and idles; AND HAS NOT BEEN RESET!
The RING_MODE indicates that is idle and has the STOP_RING bit set, so
try clearing it.
v2: Only clear the bit on restarting the ring, as we want to be sure the
STOP_RING bit is kept if reset fails on wedging.
v3: Spot when the ring state doesn't make sense when re-initialising the
engine and dump it to the logs so that we don't have to wait for an
error later and try to guess what happened earlier.
v4: Prepare to print all the unexpected state, not just the first.
and that patch proves that the register state across reset is fine; ergo
context image it is.
-Chris
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