[igt-dev] [PATCH i-g-t] i915/i915_pm_rpm: Only check for suspend failures after each debugfs entry

[Chris Wilson]

Quoting Martin Peres (2020-02-21 07:33:59)
> On 2020-02-20 19:41, Chris Wilson wrote:
> > Since we check before and then after each debugfs entry, we do not need
> > to check before each time as well. We will error out as soon as it does
> > fail, at all other times we know the system to be idle.
> > 
> > No impact on runtime for glk (which apparently is one of the better
> > behaving systems).
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Martin Peres <martin.peres at linux.intel.com>
> 
> I don't like this patch because the first read might not have the gpu
> suspended, and there shouldn't be much overhead in checking twice rather
> than once.
> 
> What's your rationale here?

We always do a check before after each file. We start in a known state,
and expect to be able to return to that suspended state, and the _real_
guts of the test is that any device access is accounted for.

assert(suspended) would be a better check for non-interference.
 
> To me, the issue is that some platforms suspend in milliseconds while
> some take seconds, and that might be indicative a real bug in the driver.

Exactly.
-Chris