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

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Message-ID: <20230926131039.GC4356@aspen.lan>
Date:   Tue, 26 Sep 2023 14:10:39 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Flavio Suligoi <f.suligoi@...m.it>
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, linux-leds@...r.kernel.org,
        devicetree@...r.kernel.org, linux-fbdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] backlight: mp3309c: Add support for MPS MP3309C

On Mon, Sep 25, 2023 at 02:26:09PM +0200, Flavio Suligoi wrote:
> diff --git a/drivers/video/backlight/mp3309c.c b/drivers/video/backlight/mp3309c.c
> new file mode 100644
> index 000000000000..923ac7f7b291
> --- /dev/null
> +++ b/drivers/video/backlight/mp3309c.c
> @@ -0,0 +1,398 @@
> ...
> +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.

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.


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!