linux-kernel - RE: [PATCH v3 2/2] backlight: mp3309c: Add support for MPS MP3309C

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Message-ID: <DU2PR01MB803463A09B0213D2072F598DF9C4A@DU2PR01MB8034.eurprd01.prod.exchangelabs.com>
Date:   Tue, 3 Oct 2023 09:45:27 +0000
From:   Flavio Suligoi <f.suligoi@...m.it>
To:     Daniel Thompson <daniel.thompson@...aro.org>
CC:     Lee Jones <lee@...nel.org>, Jingoo Han <jingoohan1@...il.com>,
        Helge Deller <deller@....de>, Pavel Machek <pavel@....cz>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-leds@...r.kernel.org" <linux-leds@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v3 2/2] backlight: mp3309c: Add support for MPS MP3309C

Hi Daniel,

...

> > +static int mp3309c_bl_update_status(struct backlight_device *bl) {
> > +	struct mp3309c_chip *chip = bl_get_data(bl);
> > +	int brightness = backlight_get_brightness(bl);
> > +	struct pwm_state pwmstate;
> > +	unsigned int analog_val, bits_val;
> > +	int i, ret;
> > +
> > +	if (chip->pdata->dimming_mode == DIMMING_PWM) {
> > +		/*
> > +		 * PWM dimming mode
> > +		 */
> > +		pwm_get_state(chip->pwmd, &pwmstate);
> > +		pwm_set_relative_duty_cycle(&pwmstate, brightness,
> > +					    chip->pdata->max_brightness);
> > +		pwmstate.enabled = true;
> > +		ret = pwm_apply_state(chip->pwmd, &pwmstate);
> > +		if (ret)
> > +			return ret;
> > +
> > +		switch (chip->pdata->status) {
> > +		case FIRST_POWER_ON:
> > +		case BACKLIGHT_OFF:
> > +			/*
> > +			 * After 20ms of low pwm signal level, the chip turns
> > +			   off automatically. In this case, before enabling the
> > +			   chip again, we must wait about 10ms for pwm signal
> to
> > +			   stabilize.
> > +			 */
> > +			if (brightness > 0) {
> > +				msleep(10);
> > +				mp3309c_enable_device(chip);
> > +				chip->pdata->status = BACKLIGHT_ON;
> > +			} else {
> > +				chip->pdata->status = BACKLIGHT_OFF;
> > +			}
> > +			break;
> > +		case BACKLIGHT_ON:
> > +			if (brightness == 0)
> > +				chip->pdata->status = BACKLIGHT_OFF;
> > +			break;
> > +		}
> > +	} else {
> > +		/*
> > +		 * Analog dimming (by I2C command) dimming mode
> > +		 *
> > +		 * The first time, before setting brightness, we must enable the
> > +		 * device
> > +		 */
> > +		if (chip->pdata->status == FIRST_POWER_ON)
> > +			mp3309c_enable_device(chip);
> > +
> > +		/*
> > +		 * Dimming mode I2C command
> > +		 *
> > +		 * The 5 bits of the dimming analog value D4..D0 is allocated
> > +		 * in the I2C register #0, in the following way:
> > +		 *
> > +		 *     +--+--+--+--+--+--+--+--+
> > +		 *     |EN|D0|D1|D2|D3|D4|XX|XX|
> > +		 *     +--+--+--+--+--+--+--+--+
> > +		 */
> > +		analog_val = DIV_ROUND_UP(ANALOG_MAX_VAL *
> brightness,
> > +					  chip->pdata->max_brightness);
> 
> Sorry to only notice after sharing a Reviewed-by:[1] but...
> 
> Scaling brightness here isn't right. When running in I2C dimming mode then
> max_brightness *must* be 31 or lower, meaning the value in brightness can
> be applied directly to the hardware without scaling.

ok, right; max brightness is 31, fixed

> 
> Quoting the DT binding docs about how max-brightness should be
> interpretted:
> 
>   Normally the maximum brightness is determined by the hardware and this
>   property is not required. This property is used to put a software
>   limit on the brightness apart from what the driver says, as it could
>   happen that a LED can be made so bright that it gets damaged or causes
>   damage due to restrictions in a specific system, such as mounting
>   conditions.
> 

ok

> 
> Daniel.
> 
> 
> [1] I remember checking if this code could overflow the field but I was
>     so distracted by that I ended up missing the obvious!

Thanks and best regards,
Flavio