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Date:   Fri, 08 Jan 2021 17:05:16 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Lyude Paul <lyude@...hat.com>, intel-gfx@...ts.freedesktop.org
Cc:     thaytan@...aisin.net, Vasily Khoruzhick <anarsoul@...il.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        Imre Deak <imre.deak@...el.com>,
        Manasi Navare <manasi.d.navare@...el.com>,
        Dave Airlie <airlied@...hat.com>,
        Sean Paul <seanpaul@...omium.org>,
        Lucas De Marchi <lucas.demarchi@...el.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Hans de Goede <hdegoede@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        "open list\:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 1/4] drm/i915: Keep track of pwm-related backlight hooks separately

On Thu, 07 Jan 2021, Lyude Paul <lyude@...hat.com> wrote:
> @@ -1628,37 +1633,32 @@ static int lpt_setup_backlight(struct intel_connector *connector, enum pipe unus
>  	panel->backlight.active_low_pwm = pch_ctl1 & BLM_PCH_POLARITY;
>  
>  	pch_ctl2 = intel_de_read(dev_priv, BLC_PWM_PCH_CTL2);
> -	panel->backlight.max = pch_ctl2 >> 16;
> +	panel->backlight.pwm_level_max = pch_ctl2 >> 16;
>  
>  	cpu_ctl2 = intel_de_read(dev_priv, BLC_PWM_CPU_CTL2);
>  
> -	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);
>  
> -	panel->backlight.enabled = pch_ctl1 & BLM_PCH_PWM_ENABLE;
> +	panel->backlight.pwm_enabled = pch_ctl1 & BLM_PCH_PWM_ENABLE;
>  
> -	cpu_mode = panel->backlight.enabled && HAS_PCH_LPT(dev_priv) &&
> +	cpu_mode = panel->backlight.pwm_enabled && HAS_PCH_LPT(dev_priv) &&
>  		   !(pch_ctl1 & BLM_PCH_OVERRIDE_ENABLE) &&
>  		   (cpu_ctl2 & BLM_PWM_ENABLE);
> -	if (cpu_mode)
> -		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) {
> +		val = intel_panel_sanitize_pwm_level(connector, pch_get_backlight(connector));
> +

(This really is a PITA to review, not because of how you do it but
because of the hardware and the code itself. I'm just pointing out one
thing here, but I'm not finished yet.)

I think this sanitize call is wrong here. It should be called only when
converting to and from the hw register. Here, we read directly from one
hw register and write back to another hw register.

Now, looking at the history, I think it's been wrong all the way since
commit 5b1ec9ac7ab5 ("drm/i915/backlight: Fix backlight takeover on LPT,
v3."). Probably nobody noticed, because AFAIK inverted brightness
control has only ever been an issue on some gen4 platforms...

*facepalm*

BR,
Jani.

>  		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);
>  

-- 
Jani Nikula, Intel Open Source Graphics Center

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