Re: [PATCH 1/4] drm/i915/bxt: Avoid early timeout during PLL enable

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 28/06/16 12:16, Imre Deak wrote:
> On ti, 2016-06-28 at 12:05 +0100, Tvrtko Ursulin wrote:
>> On 28/06/16 11:48, Chris Wilson wrote:
>>> On Tue, Jun 28, 2016 at 01:37:30PM +0300, Imre Deak wrote:
>>>> Since wait_for_atomic doesn't re-check the wait-for condition after
>>>> expiry of the timeout it can fail when called from non-atomic context
>>>> even if the condition is set correctly before the expiry. Fix this by
>>>> using the non-atomic wait_for instead.
>>>
>>> wait_for_atomic is indeed only safe to be called from atomic context.
>>> Likewise, wait_for is only safe to called from !atomic context.
>>>
>>>> I noticed this via the PLL locking timing out incorrectly, with this fix
>>>> I couldn't reproduce the problem.
>>>>
>>>> Fixes: 0351b93992aa ("drm/i915: Do not lie about atomic timeout granularity")
>>>
>>> The bug would be using wait_for_atomic from non-atomic context, and so
>>> older.
>>>
>>>
>>>> CC: Chris Wilson <chris@chris-wilson.co.uk>
>>>> CC: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>> Signed-off-by: Imre Deak <imre.deak@intel.com>
>>>> ---
>>>>    drivers/gpu/drm/i915/intel_dpll_mgr.c | 4 ++--
>>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/i915/intel_dpll_mgr.c b/drivers/gpu/drm/i915/intel_dpll_mgr.c
>>>> index c0eff15..e130c3e 100644
>>>> --- a/drivers/gpu/drm/i915/intel_dpll_mgr.c
>>>> +++ b/drivers/gpu/drm/i915/intel_dpll_mgr.c
>>>> @@ -1374,8 +1374,8 @@ static void bxt_ddi_pll_enable(struct drm_i915_private *dev_priv,
>>>>    	I915_WRITE(BXT_PORT_PLL_ENABLE(port), temp);
>>>>    	POSTING_READ(BXT_PORT_PLL_ENABLE(port));
>>>>
>>>> -	if (wait_for_atomic_us((I915_READ(BXT_PORT_PLL_ENABLE(port)) &
>>>> -			PORT_PLL_LOCK), 200))
>>>> +	if (wait_for_us((I915_READ(BXT_PORT_PLL_ENABLE(port)) & PORT_PLL_LOCK),
>>>> +			200))
>>>
>>> Does this work with CONFIG_I915_DEBUG and CONFIG_DEBUG_ATOMIC_SLEEP ?
>>
>> CONFIG_PREEMPT_COUNT is also required.
>>
>> There were a bunch of these WARNs triggering in various places. I think
>> I had patches to fix them but at the same time Mika had a more
>> comprehensive work in progress for the whole area. I suppose that just
>> got delayed to much.
>>
>> AFAIR the meat of the discussion was what is more important - sleep
>> granularity or timeout accuracy. I preferred the former to avoid waiting
>> for too long for operations which are normally much quicker than a
>> jiffie and normally succeed.
>>
>> Another issue if wait_for_us for sleeps < 10us is not the most efficient
>> implementation. So another idea I had is to implement those via the
>> wait_for_atomic but without the in_atomic WARN. And obviously now after
>> Imre found this with the extra cond check as well.
>
> For that kind of optimization, the comment at cpu_clock() could be
> interesting when comparing cpu_clock(i) wrt. cpu_clock(j) and i!=j. I
> couldn't see any backward jumps between such timestamps, but I'm not
> sure if that comment can be disregarded. Maybe on Intel/TSC it can.

You are right, to bad. Perhaps disabling preemption would do the trick 
in that case.

Regards,

Tvrtko

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