[Intel-gfx] [PATCH] drm/i915/dp: Backlight PWM enable before BL Enable assert

[Clint Taylor]

On 08/20/2014 04:23 AM, Ville Syrjälä wrote:
> On Mon, Aug 18, 2014 at 01:48:35PM -0700, clinton.a.taylor at intel.com wrote:
>> From: Clint Taylor <clinton.a.taylor at intel.com>
>>
>> Backlight on delay uses PWM enable time to seperate PWM to
>> backlight enable assert. Previous time difference used timing
>> from VDD enable which occur several seconds before resulting
>> in PWM starting 5ms after backlight enable. Changes to backlight
>>   duty cycle take affect at the end of the current PWM cycle.
>> Measured time for the PWM cycle is 5ms. 5 additional ms must be
>> added to the backlight_on_delay to get correct VBT timing of
>> PWM to backlight enable assert.
>>
>> V2: Rebase to Jani Nikula backlight power patch 1/4
>>
>> Change-Id: I2982f9785d92e8d64c04ca25c8bd4c5d1dc8067c
>> Signed-off-by: Clint Taylor <clinton.a.taylor at intel.com>
>> ---
>>   drivers/gpu/drm/i915/intel_dp.c  | 6 ++++--
>>   drivers/gpu/drm/i915/intel_drv.h | 1 +
>>   2 files changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
>> index d8baf60..aed923b 100644
>> --- a/drivers/gpu/drm/i915/intel_dp.c
>> +++ b/drivers/gpu/drm/i915/intel_dp.c
>> @@ -1141,7 +1141,7 @@ static void wait_panel_power_cycle(struct intel_dp *intel_dp)
>>
>>   static void wait_backlight_on(struct intel_dp *intel_dp)
>>   {
>> -	wait_remaining_ms_from_jiffies(intel_dp->last_power_on,
>> +	wait_remaining_ms_from_jiffies(intel_dp->last_backlight_on,
>>   				       intel_dp->backlight_on_delay);
>>   }
>>
>> @@ -1418,6 +1418,7 @@ void intel_edp_backlight_on(struct intel_dp *intel_dp)
>>   	DRM_DEBUG_KMS("\n");
>>
>>   	intel_panel_enable_backlight(intel_dp->attached_connector);
>> +	intel_dp->last_backlight_on = jiffies;
>
> Seems to me we should just have a msleep(PWM_CYCLE_DELAY) here since
> (assuming I understood correctly) only the PWM cycle time is important
> between these two steps, and the "t8" (vbt spec says t7 actually) is
> relevant only from end of link training to backlight on.
>

The driver has overloaded both T8 and T9 VBT timing entries. The eDP 
specification really doesn't have timing data defined for this 
requirement, but the panel manufacturers have it in their panel 
specification and they use various names like (Te, t17, etc...) for this 
delay.

> Hmm, no actually your idea might be better since on VLV/CHV we turn the
> pipe on after link training, and I assume t7/t8 should really start
> counting from the pipe on on those platforms. So I guess the most
> appropriate solution might be somehting like
>
>   {
>     intel_dp->last_backlight_on = jiffies;
>     intel_panel_enable_backlight(intel_dp->attached_connector);
>     msleep(POWER_CYCLE_DELAY);
>     _intel_edp_backlight_on(intel_dp);
>   }
>
> But I'm not sure the distinction makes any difference since
> intel_panel_enable_backlight() shouldn't take a significant amount of
> time, and msleep() is itself rather inaccurate.
>

There is also a need to add this delay when turning off the PWM enable 
bit through intel_panel_disable_backlight(). If the PWM enable bit is 
turned off while the PWM signal is high then the signal remains high. If 
the bit is turned off when the signal is low the PWM will remain low. 
Since we don't know the current state of the PWM signal we must set the 
duty_cycle to 0, wait PWM_CYCLE_DELAY, and then turn off the enable bit.

Actually it might be better if we never turn off the PWM enable bit in 
intel_panel_disable_backlight() and we just use the duty cycle register 
to control the PWM.

> So I guess your approach is the simplest option here.
>
>>   	_intel_edp_backlight_on(intel_dp);
>>   }
>>
>> @@ -4256,9 +4257,10 @@ intel_dp_init_panel_power_sequencer(struct drm_device *dev,
>>   	assign_final(t11_t12);
>>   #undef assign_final
>>
>> +#define PWM_CYCLE_DELAY 5
>
> Shoduln't this be calculated from the PWM frequqncy? Not sure what a PWM
> cycle here is exactly. Just one full period of the signal?
>

The PWM cycle in this case turns out to be 1 full cycle of the PWM 
waveform though it depends on which display core clock (128 or 
25Mhz(S0ix) is being used. Since we only care about the minimum elapsed 
time to meet the panel specification a value of 5ms will work even 
though more or less PWM cycles would take place before the BL_ENABLE is 
asserted. I would prefer not to add complexity to switch between panel 
timings for normal and S0ix display clock modes. How often does the 
backlight get enabled while in S0ix?

> Also would be nice to have a comment in the code explaining what this is
> and why we need to add it to the delay.

Sure, As you may have noticed in other patches I really like comments.

>
>>   #define get_delay(field)	(DIV_ROUND_UP(final.field, 10))
>>   	intel_dp->panel_power_up_delay = get_delay(t1_t3);
>> -	intel_dp->backlight_on_delay = get_delay(t8);
>> +	intel_dp->backlight_on_delay = get_delay(t8) + PWM_CYCLE_DELAY;
>>   	intel_dp->backlight_off_delay = get_delay(t9);
>>   	intel_dp->panel_power_down_delay = get_delay(t10);
>>   	intel_dp->panel_power_cycle_delay = get_delay(t11_t12);
>> diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
>> index 3abc915..ad6fcc1 100644
>> --- a/drivers/gpu/drm/i915/intel_drv.h
>> +++ b/drivers/gpu/drm/i915/intel_drv.h
>> @@ -556,6 +556,7 @@ struct intel_dp {
>>   	bool want_panel_vdd;
>>   	unsigned long last_power_cycle;
>>   	unsigned long last_power_on;
>> +	unsigned long last_backlight_on;
>>   	unsigned long last_backlight_off;
>>
>>   	struct notifier_block edp_notifier;
>> --
>> 1.8.3.2
>>
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>