[PATCH] drm/amdgpu: fix ib test hang with gfxoff enabled

[Huang Rui]

On Sat, Jun 02, 2018 at 03:01:57AM +0800, Kuehling, Felix wrote:
> On 2018-06-01 06:09 AM, Christian König wrote:
> > Am 01.06.2018 um 11:29 schrieb Huang Rui:
> >> On Fri, Jun 01, 2018 at 05:13:49PM +0800, Christian König wrote:
> >>> Am 01.06.2018 um 08:41 schrieb Huang Rui:
> >>>> After defer the execution of gfx/compute ib tests. However, at that
> >>>> time, the
> >>>> gfx already go into "mid state" of gfxoff.
> >>>>
> >>>> PWR_MISC_CNTL_STATUS: PWR_GFXOFF_STATUS field (2:1 bits)
> >>>> 0 = GFXOFF.
> >>>> 1 = Transition out of GFXOFF state.
> >>>> 2 = Not in GFXOFF.
> >>>> 3 = Transition into GFXOFF.
> >>>>
> >>>> If hit the mid state (1 or 3), the doorbell writing interrupt
> >>>> cannot wake up the
> >>>> gfx back successfully. And the field value is 1 when we issue the
> >>>> ib test at
> >>>> that, so we got the hang. This is the root cause that we
> >>>> encountered the issue.
> >>>>
> >>>> Meanwhile, we cannot set clockgating of GFX after gfx is already in
> >>>> "off" state.
> >>>> So here we should move the gfx powergating and gfxoff enabling
> >>>> behavior at the
> >>>> end of initialization behind ib test and clockgating.
> >>> Mhm, that still looks like a only halve backed solution:
> >>>
> >>> 1. What prevents this bug from happening during "normal" IB submission
> >>> from userspace?
> >>>
> >>> 2. Shouldn't we poll the PWR_MISC_CNTL_STATUS register to make sure we
> >>> are not in any transition phase instead?
> >>>
> >> Yes, right. How about also add polling of PWR_MISC_CNTL_STATUS in
> >> amdgpu_ring_commit() behind set_wptr that confirm the status as "0"
> >> or "2"?
> >
> > You could add an end_use() callback for that, but I think we rather
> > need to do this in gfx_v9_0_ring_set_wptr_gfx() before we write the
> > doorbell.
> Isn't testing the status like this is a potential race condition.
> 
> Having to do this at all is contrary to the documentation that I've
> read. Writing a doorbell should wake up the GFX engine. Are we sure that
> we understand the cause of the problem correctly? Does the IB test use
> any MMIO? Maybe it's doing an HDP flush using MMIO for a ring that
> doesn't support HDP flushing.
> 

Felix, thanks to reminder. I supposed you mentioned MMIO using is to avoid
runtime gfx register access, right? Our IB test uses WRITE_DATA packet to
write specific pattern value into the gart memory. I don't use any gfx
registers. And gfxoff is only supported on raven, we don't emit hdp flush
on apu. Actually, I also doubted whether it is caused race condition. But
the hang happens when only modprobe amdgpu module, and not startx at that
time. It won't have any other commands from user space.

+ Morris, who works for raven SMC firmware.
After discuessed with him, he suggested that we would better to confirm the
GFXOFF_STATUS as 0 or 2, then write the doorbell. Because if GFXOFF_STATUS
is 1 or 3 in mid state (in-progress of translation), SMC will drop the
doorbell interrupt. When GFXOFF status is 0 or 2, already in the target
state, SMC can repond interrupt at once. Morris, please correct me if I was
wrong.

Thanks,
Ray