[Bug 111095] Backlight flicker after suspend on certain Intel CherryTrail tablets

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=111095

--- Comment #21 from Hans de Goede <jwrdegoede at fedoraproject.org> ---
(In reply to Jani Nikula from comment #13)
> Hmm, I think the VBT says it uses the CPU PWM.

I have access to both a Miix320 and an Irbis TW90 for testing, both my models
use a DSI panel. AFAIK DSI panels always either the Crystal Cove PMIC PWM, or
the LPSS PWM (which is part of the SoC).

I guess you are basing this "I think the VBT says it uses the CPU PWM" on the
dev_priv->vbt.dsi.config->pwm_blc bit ?  When that is PPS_BLC_SOC then the LPSS
PWM is used, not the PWM which is part of the display controller. Also see:
drivers/gpu/drm/i915/display/intel_panel.c which has:

        } else if (IS_VALLEYVIEW(dev_priv) || IS_CHERRYVIEW(dev_priv)) {
                if (connector->base.connector_type == DRM_MODE_CONNECTOR_DSI) {
                        panel->backlight.setup = pwm_setup_backlight;
                        panel->backlight.enable = pwm_enable_backlight;
                        panel->backlight.disable = pwm_disable_backlight;
                        panel->backlight.set = pwm_set_backlight;
                        panel->backlight.get = pwm_get_backlight;
                } else {
                        ...

And pwm_setup_backlight() does:

        /* Get the PWM chip for backlight control */
        panel->backlight.pwm = pwm_get(dev->dev, "pwm_backlight");

Which means it uses the pwm subsystem to talk to either the CRC or LPSS pwm
controller.

So AFAICT dumping the registers of the display-block PWM controller does not
provide any useful info here. I actually wonder if the displayblock on CHT
devices has the PWM controller at all.

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