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

Tvrtko Ursulin
2016-06-28T11:05:44Z

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.

So I think Imre's patches are good in principle, should go in, and
probably afterwards we can talk about improving wait_for_us for timeouts
under 10us and potentially the timeout precision as well.

Regards,

Tvrtko