Re: [PATCH] drm/i915/backlight: fix CPU mode backlight takeover on LPT

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

On Fri, 08 Jan 2021, Lyude Paul <lyude@redhat.com> wrote:
> Reviewed-by: Lyude Paul <lyude@redhat.com>
>
> Let me know when you've pushed this upstream and I'll go ahead and send out a
> rebased version of my backlight series.

Pushed, thanks for the review.

I'm hoping to do more review of the series today, so please hold off on
actually sending the rebased version for a bit longer.

BR,
Jani.


>
> On Fri, 2021-01-08 at 17:28 +0200, Jani Nikula wrote:
>> The pch_get_backlight(), lpt_get_backlight(), and lpt_set_backlight()
>> functions operate directly on the hardware registers. If inverting the
>> value is needed, using intel_panel_compute_brightness(), it should only
>> be done in the interface between hardware registers and
>> panel->backlight.level.
>> 
>> The CPU mode takeover code added in commit 5b1ec9ac7ab5
>> ("drm/i915/backlight: Fix backlight takeover on LPT, v3.") reads the
>> hardware register and converts to panel->backlight.level correctly,
>> however the value written back should remain in the hardware register
>> "domain".
>> 
>> This hasn't been an issue, because GM45 machines are the only known
>> users of i915.invert_brightness and the brightness invert quirk, and
>> without one of them no conversion is made. It's likely nobody's ever hit
>> the problem.
>> 
>> Fixes: 5b1ec9ac7ab5 ("drm/i915/backlight: Fix backlight takeover on LPT, v3.")
>> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
>> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
>> Cc: Lyude Paul <lyude@redhat.com>
>> Cc: <stable@vger.kernel.org> # v5.1+
>> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
>> ---
>>  drivers/gpu/drm/i915/display/intel_panel.c | 9 +++++----
>>  1 file changed, 5 insertions(+), 4 deletions(-)
>> 
>> diff --git a/drivers/gpu/drm/i915/display/intel_panel.c
>> b/drivers/gpu/drm/i915/display/intel_panel.c
>> index 67f81ae995c4..7a4239d1c241 100644
>> --- a/drivers/gpu/drm/i915/display/intel_panel.c
>> +++ b/drivers/gpu/drm/i915/display/intel_panel.c
>> @@ -1649,16 +1649,13 @@ static int lpt_setup_backlight(struct intel_connector
>> *connector, enum pipe unus
>>                 val = pch_get_backlight(connector);
>>         else
>>                 val = lpt_get_backlight(connector);
>> -       val = intel_panel_compute_brightness(connector, val);
>> -       panel->backlight.level = clamp(val, panel->backlight.min,
>> -                                      panel->backlight.max);
>>  
>>         if (cpu_mode) {
>>                 drm_dbg_kms(&dev_priv->drm,
>>                             "CPU backlight register was enabled, switching to
>> PCH override\n");
>>  
>>                 /* Write converted CPU PWM value to PCH override register */
>> -               lpt_set_backlight(connector->base.state, panel-
>> >backlight.level);
>> +               lpt_set_backlight(connector->base.state, val);
>>                 intel_de_write(dev_priv, BLC_PWM_PCH_CTL1,
>>                                pch_ctl1 | BLM_PCH_OVERRIDE_ENABLE);
>>  
>> @@ -1666,6 +1663,10 @@ static int lpt_setup_backlight(struct intel_connector
>> *connector, enum pipe unus
>>                                cpu_ctl2 & ~BLM_PWM_ENABLE);
>>         }
>>  
>> +       val = intel_panel_compute_brightness(connector, val);
>> +       panel->backlight.level = clamp(val, panel->backlight.min,
>> +                                      panel->backlight.max);
>> +
>>         return 0;
>>  }
>>  

-- 
Jani Nikula, Intel Open Source Graphics Center