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

Imre Deak
2016-06-28T11:00:28Z

On ti, 2016-06-28 at 11:48 +0100, 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.

I agree that calling wait_for_atomic() wasn't correct even before, but
only because of busy waiting when we could just sleep. The condition
was rechecked after expiry so the function didn't fail in the above
case.

>
> > 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 ?

Yes, I have CONFIG_DEBUG_ATOMIC_SLEEP enabled and AFAICS
CONFIG_I915_DEBUG shouldn't matter for the changed code. I'll enable
now also the latter although it'll trigger for the GuC path at least.

--Imre