Re: [PATCH v6 1/4] drm/i915: Keep track of pwm-related backlight hooks separately

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

On Thu, 14 Jan 2021, Jani Nikula <jani.nikula@intel.com> wrote:
> On Wed, 13 Jan 2021, Lyude Paul <lyude@redhat.com> wrote:
>> Currently, every different type of backlight hook that i915 supports is
>> pretty straight forward - you have a backlight, probably through PWM
>> (but maybe DPCD), with a single set of platform-specific hooks that are
>> used for controlling it.
>>
>> HDR backlights, in particular VESA and Intel's HDR backlight
>> implementations, can end up being more complicated. With Intel's
>> proprietary interface, HDR backlight controls always run through the
>> DPCD. When the backlight is in SDR backlight mode however, the driver
>> may need to bypass the TCON and control the backlight directly through
>> PWM.
>>
>> So, in order to support this we'll need to split our backlight callbacks
>> into two groups: a set of high-level backlight control callbacks in
>> intel_panel, and an additional set of pwm-specific backlight control
>> callbacks. This also implies a functional changes for how these
>> callbacks are used:
>>
>> * We now keep track of two separate backlight level ranges, one for the
>>   high-level backlight, and one for the pwm backlight range
>> * We also keep track of backlight enablement and PWM backlight
>>   enablement separately
>> * Since the currently set backlight level might not be the same as the
>>   currently programmed PWM backlight level, we stop setting
>>   panel->backlight.level with the currently programmed PWM backlight
>>   level in panel->backlight.pwm_funcs->setup(). Instead, we rely
>>   on the higher level backlight control functions to retrieve the
>>   current PWM backlight level (in this case, intel_pwm_get_backlight()).
>>   Note that there are still a few PWM backlight setup callbacks that
>>   do actually need to retrieve the current PWM backlight level, although
>>   we no longer save this value in panel->backlight.level like before.
>>
>> Additionally, we drop the call to lpt_get_backlight() in
>> lpt_setup_backlight(), and avoid unconditionally writing the PWM value that
>> we get from it and only write it back if we're in CPU mode, and switching
>> to PCH mode. The reason for this is because in the original codepath for
>> this, it was expected that the intel_panel_bl_funcs->setup() hook would be
>> responsible for fetching the initial backlight level. On lpt systems, the
>> only time we could ever be in PCH backlight mode is during the initial
>> driver load - meaning that outside of the setup() hook, lpt_get_backlight()
>> will always be the callback used for retrieving the current backlight
>> level. After this patch we still need to fetch and write-back the PCH
>> backlight value if we're switching from CPU mode to PCH, but because
>> intel_pwm_setup_backlight() will retrieve the backlight level after setup()
>> using the get() hook, which always ends up being lpt_get_backlight(). Thus
>> - an additional call to lpt_get_backlight() in lpt_setup_backlight() is
>> made redundant.
>>
>> v7:
>> * Use panel->backlight.pwm_funcs->get() to get the backlight level in
>>   intel_pwm_setup_backlight(), lest we upset lockdep
>
> I think this change is wrong, as it now bypasses
> intel_panel_invert_pwm_level(). Please explain. I don't see anything in
> there that could trigger a lockdep warning.
>
> Perhaps it's the below you're referring to, but I think the root cause
> is different?
>
>> @@ -1788,22 +1780,17 @@ static int vlv_setup_backlight(struct intel_connector *connector, enum pipe pipe
>>  	panel->backlight.active_low_pwm = ctl2 & BLM_POLARITY_I965;
>>  
>>  	ctl = intel_de_read(dev_priv, VLV_BLC_PWM_CTL(pipe));
>> -	panel->backlight.max = ctl >> 16;
>> +	panel->backlight.pwm_level_max = ctl >> 16;
>>  
>> -	if (!panel->backlight.max)
>> -		panel->backlight.max = get_backlight_max_vbt(connector);
>> +	if (!panel->backlight.pwm_level_max)
>> +		panel->backlight.pwm_level_max = get_backlight_max_vbt(connector);
>>  
>> -	if (!panel->backlight.max)
>> +	if (!panel->backlight.pwm_level_max)
>>  		return -ENODEV;
>>  
>> -	panel->backlight.min = get_backlight_min_vbt(connector);
>> +	panel->backlight.pwm_level_min = get_backlight_min_vbt(connector);
>>  
>> -	val = _vlv_get_backlight(dev_priv, pipe);
>
> Turns out this is a meaningful change, as the higher level
> vlv_get_backlight() function that will be called instead hits:
>
> <4>[   12.870202] i915 0000:00:02.0: drm_WARN_ON(!drm_modeset_is_locked(&dev->mode_config.connection_mutex))
>
> in intel_connector_get_pipe(connector).
>
> It's a real problem. See this, it's obvious (in retrospect):
>
> https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_19348/fi-bsw-kefka/igt@runner@aborted.html
> https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_19348/fi-bsw-kefka/boot0.txt
>
> I don't have a quick answer how this could be handled neatly. Perhaps
> the ->get call (or rather, intel_pwm_get_backlight) to set
> panel->backlight.level needs to be spread out to the end of each
> pwm_funcs->setup function after all? Though it's at the wrong
> abstraction level wrt level being a higher level, uh, level.
>
> I don't think it's enough to just grab connection_mutex around setup
> (and even checking if we can do that is a bunch of digging) - I think
> it's likely intel_connector_get_pipe() returns INVALID_PIPE at that
> point.
>
> Okay, here's a clumsy suggestion that I think works around this and
> unblocks the series until we figure out a better way:
>
> 1. At the end of vlv_setup_backlight():
>
> 	/* add fixme comment about how wrong this is */
> 	panel->backlight.level = intel_panel_invert_pwm_level(connector, _vlv_get_backlight());
> 	
>
> 2. In intel_pwm_setup_backlight() only set level if ->setup didn't:
>
> 	if (!panel->backlight.level)
>         	panel->backlight.level = intel_pwm_get_backlight(connector);

Of course, if ->setup ends up setting the level to 0, it hits the same
issue. :(

>
> What do you think?
>
> BR,
> Jani.
>
>> -	val = intel_panel_compute_brightness(connector, val);
>> -	panel->backlight.level = clamp(val, panel->backlight.min,
>> -				       panel->backlight.max);
>> -
>> -	panel->backlight.enabled = ctl2 & BLM_PWM_ENABLE;
>> +	panel->backlight.pwm_enabled = ctl2 & BLM_PWM_ENABLE;
>>  
>>  	return 0;
>>  }
>> @@ -1828,24 +1815,18 @@ bxt_setup_backlight(struct intel_connector *connector, enum pipe unused)

-- 
Jani Nikula, Intel Open Source Graphics Center