Re: [Intel-gfx] [RESEND PATCHv3] drm/i915/dp: Change aux_ctl reg read to polling read

[Jani Nikula <jani.nikula@intel.com>]

On Tue, 13 Dec 2022, "Murthy, Arun R" <arun.r.murthy@intel.com> wrote:
>> -----Original Message-----
>> From: Nikula, Jani <jani.nikula@intel.com>
>> Sent: Friday, December 9, 2022 4:16 PM
>> To: Murthy, Arun R <arun.r.murthy@intel.com>; intel-
>> gfx@lists.freedesktop.org; ville.syrjala@linux.intel.com; Deak, Imre
>> <imre.deak@intel.com>
>> Cc: Murthy, Arun R <arun.r.murthy@intel.com>
>> Subject: Re: [RESEND PATCHv3] drm/i915/dp: Change aux_ctl reg read to
>> polling read
>>
>> On Fri, 09 Dec 2022, Arun R Murthy <arun.r.murthy@intel.com> wrote:
>> > The busy timeout logic checks for the AUX BUSY, then waits for the
>> > timeout period and then after timeout reads the register for BUSY or
>> > Success.
>> > Instead replace interrupt with polling so as to read the AUX CTL
>> > register often before the timeout period. Looks like there might be
>> > some issue with interrupt-on-read. Hence changing the logic to polling
>> read.
>> >
>> > v2: replace interrupt with polling read
>> > v3: use usleep_rang instead of msleep, updated commit msg
>> >
>> > Signed-off-by: Arun R Murthy <arun.r.murthy@intel.com>
>> > ---
>> >  drivers/gpu/drm/i915/display/intel_dp_aux.c | 24
>> > ++++++++++++---------
>> >  1 file changed, 14 insertions(+), 10 deletions(-)
>> >
>> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux.c
>> > b/drivers/gpu/drm/i915/display/intel_dp_aux.c
>> > index 91c93c93e5fc..230f27d75846 100644
>> > --- a/drivers/gpu/drm/i915/display/intel_dp_aux.c
>> > +++ b/drivers/gpu/drm/i915/display/intel_dp_aux.c
>> > @@ -41,21 +41,25 @@ intel_dp_aux_wait_done(struct intel_dp *intel_dp)
>> >     i915_reg_t ch_ctl = intel_dp->aux_ch_ctl_reg(intel_dp);
>> >     const unsigned int timeout_ms = 10;
>> >     u32 status;
>> > -   bool done;
>> > +   int try;
>> >
>> > -#define C (((status = intel_de_read_notrace(i915, ch_ctl)) &
>> DP_AUX_CH_CTL_SEND_BUSY) == 0)
>> > -   done = wait_event_timeout(i915->display.gmbus.wait_queue, C,
>> > -                             msecs_to_jiffies_timeout(timeout_ms));
>> > +   for (try = 0; try < 10; try++) {
>> > +           status = intel_uncore_read_notrace(&i915->uncore, ch_ctl);
>> > +           if ((status & DP_AUX_CH_CTL_SEND_BUSY) == 0)
>> > +                   break;
>> > +           usleep_range(400, 500);
>> > +   }
>> > +   if (try == 3) {
>> > +           status = intel_uncore_read_notrace(&i915->uncore, ch_ctl);
>> > +           if ((status & DP_AUX_CH_CTL_SEND_BUSY) != 0)
>> > +                   drm_err(&i915->drm,
>> > +                           "%s: did not complete or timeout within
>> %ums (status 0x%08x)\n",
>> > +                           intel_dp->aux.name, timeout_ms, status);
>> > +   }
>> >
>> >     /* just trace the final value */
>> >     trace_i915_reg_rw(false, ch_ctl, status, sizeof(status), true);
>>
>> Okay, so there are still a bunch of issues above. For example, try < 10 vs. try
>> == 3, reverting back to intel_uncore_* functions after conflict resolution,
>> having duplicated reads and conditions.
> The logic tries to read in a loop for 10 times with a sleep of 500usec between each reads.
> Finally after the 10th iteration for the last time the register is read again and the status is returned.

Yeah, but the condition is (try == 3).

>
>>
>> Now, I should've taken a step back earlier and realized you should use the
>> helper we already have for this: intel_de_wait_for_register().
>
> I checked this earlier. The reason for not opting this is this function is its
> interrupt-on-read, but we need a polling read with timeout.

It *is* a polling read with a timeout, with a bunch of smarts.

BR,
Jani.

>
> Thanks and Regards,
> Arun R Murthy
> --------------------
>>
>> All of the above shrinks to just a few lines:
>>
>>       ret = intel_de_wait_for_register(i915, ch_ctl,
>> DP_AUX_CH_CTL_SEND_BUSY, 0, timeout_ms);
>>       if (ret)
>>               drm_err(...);
>>
>> Sorry for missing this earlier.
>>
>> BR,
>> Jani.
>>
>> >
>> > -   if (!done)
>> > -           drm_err(&i915->drm,
>> > -                   "%s: did not complete or timeout within %ums
>> (status 0x%08x)\n",
>> > -                   intel_dp->aux.name, timeout_ms, status);
>> > -#undef C
>> > -
>> >     return status;
>> >  }
>>
>> --
>> Jani Nikula, Intel Open Source Graphics Center

-- 
Jani Nikula, Intel Open Source Graphics Center