Re: [PATCH] drm/i915: Allow user to override PWM backlight frequency and duty cycle

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

On Tue, 16 Jan 2018, Alex Ivanov <gnidorah@ya.ru> wrote:
> Since vendor may set bad defaults or user just generally may prefer her health over
> display life.
> 1. Allow overriding of PWM frequency otherwise people use solutions like
> http://devbraindom.blogspot.ru/2013/03/eliminate-led-screen-flicker-with-intel.html
> which are bad because
> I. frequency convertation logic varies from chip to chip, so people set totally wrong
> frequences in most cases
> II. writing to backlight control registers directly goes into conflict with
> i915 driver doing the same
> 2. Allow to override minimal brightness. Even if vendor limit was totally correct,
> it may be too bright to use in night, plus the default will become most likely wrong
> now, since frequency was overriden

I'm divided. Clearly, the patch at hand is a technically better solution
than what the blog post has.

But I also think the folks over at the blog know they're directly
hacking on graphics registers, and whatever it is they're doing is not
supported. There's also warranted speculation this might be harmful to
their displays.

The modulation frequency and the minimum duty cycle have been chosen by
the OEM to work with their board design and display specs. We don't have
enough data to validate the values given by the user are within spec. If
we add this, it'll get used, it'll be expected to be supported, the
"unsafe" there will go unnoticed by folks copy-pasting the parameters
from blogs and forums, and if it breaks displays for folks, they'll be
angry at us, not at some random blog poster.

I guess damned if you do, damned if you don't.

BR,
Jani.

>
> ---
>  drivers/gpu/drm/i915/i915_params.c |  6 ++++++
>  drivers/gpu/drm/i915/i915_params.h |  2 ++
>  drivers/gpu/drm/i915/intel_panel.c | 17 +++++++++++++++++
>  3 files changed, 25 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/i915_params.c b/drivers/gpu/drm/i915/i915_params.c
> index b4faeb6aa2bd..ba37257846b0 100644
> --- a/drivers/gpu/drm/i915/i915_params.c
> +++ b/drivers/gpu/drm/i915/i915_params.c
> @@ -190,3 +190,9 @@ i915_param_named(enable_dpcd_backlight, bool, 0600,
>  
>  i915_param_named(enable_gvt, bool, 0400,
>  	"Enable support for Intel GVT-g graphics virtualization host support(default:false)");
> +
> +i915_param_named_unsafe(backlight_freq, uint, 0400,
> +	"Override PWM backlight frequency (set in Hz, 0=ignore [default])");
> +
> +i915_param_named_unsafe(backlight_min_level, uint, 0400,
> +	"Override PWM backlight minimum level (duty cycle, 0=ignore [default])");
> diff --git a/drivers/gpu/drm/i915/i915_params.h b/drivers/gpu/drm/i915/i915_params.h
> index c7292268ed43..7aba85db3da2 100644
> --- a/drivers/gpu/drm/i915/i915_params.h
> +++ b/drivers/gpu/drm/i915/i915_params.h
> @@ -53,6 +53,8 @@
>  	param(int, edp_vswing, 0) \
>  	param(int, reset, 2) \
>  	param(unsigned int, inject_load_failure, 0) \
> +	param(unsigned int, backlight_freq, 0) \
> +	param(unsigned int, backlight_min_level, 0) \
>  	/* leave bools at the end to not create holes */ \
>  	param(bool, alpha_support, IS_ENABLED(CONFIG_DRM_I915_ALPHA_SUPPORT)) \
>  	param(bool, enable_cmd_parser, true) \
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index adc51e452e3e..f188d7c1d2de 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -1479,6 +1479,12 @@ static u32 get_backlight_min_vbt(struct intel_connector *connector)
>  
>  	WARN_ON(panel->backlight.max == 0);
>  
> +	if (i915_modparams.backlight_min_level) {
> +		DRM_DEBUG_KMS("Override backlight duty cycle %u\n",
> +						i915_modparams.backlight_min_level);
> +		return i915_modparams.backlight_min_level;
> +	}
> +
>  	/*
>  	 * XXX: If the vbt value is 255, it makes min equal to max, which leads
>  	 * to problems. There are such machines out there. Either our
> @@ -1795,6 +1801,7 @@ int intel_panel_setup_backlight(struct drm_connector *connector, enum pipe pipe)
>  	struct intel_connector *intel_connector = to_intel_connector(connector);
>  	struct intel_panel *panel = &intel_connector->panel;
>  	int ret;
> +	u32 pwm;
>  
>  	if (!dev_priv->vbt.backlight.present) {
>  		if (dev_priv->quirks & QUIRK_BACKLIGHT_PRESENT) {
> @@ -1812,6 +1819,16 @@ int intel_panel_setup_backlight(struct drm_connector *connector, enum pipe pipe)
>  	/* set level and max in panel struct */
>  	mutex_lock(&dev_priv->backlight_lock);
>  	ret = panel->backlight.setup(intel_connector, pipe);
> +	if (i915_modparams.backlight_freq) {
> +		if (panel->backlight.hz_to_pwm &&
> +				(pwm = panel->backlight.hz_to_pwm(intel_connector, i915_modparams.backlight_freq))) {
> +			panel->backlight.max = pwm;
> +			DRM_DEBUG_KMS("Override backlight frequency %u Hz\n",
> +							i915_modparams.backlight_freq);
> +		} else {
> +			DRM_DEBUG_KMS("failed to override backlight frequency\n");
> +		}
> +	}
>  	mutex_unlock(&dev_priv->backlight_lock);
>  
>  	if (ret) {

-- 
Jani Nikula, Intel Open Source Technology Center
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