linux-kernel - Re: [PATCH v4 2/2] leds: bd2606mvv: Driver for the Rohm 6 Channel i2c LED driver

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Date:   Sat, 15 Apr 2023 00:05:33 +0200
From:   Andreas Kemnade <andreas@...nade.info>
To:     Pavel Machek <pavel@....cz>
Cc:     lee@...nel.org, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, linux-leds@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Matti Vaittinen <mazziesaccount@...il.com>
Subject: Re: [PATCH v4 2/2] leds: bd2606mvv: Driver for the Rohm 6 Channel
 i2c LED driver

Hi Pavel,

On Fri, 14 Apr 2023 14:18:56 +0200
Pavel Machek <pavel@....cz> wrote:

> Hi!
> 
> > The device provides 6 channels which can be individually
> > turned off and on but groups of two channels share a common brightness
> > register.  
> 
> Yeah, well.. Turn it into 3-channel controller with brightness or
> 6-channel on/off one... You can't really share brightness.
> 
No, I cannot change the hardware, so it is a 6-channel with limitations.
And the devicetree has to describe the hardware and not the driver.

What is discussable is just how the driver should deal with that:

I see 5 possibilities.
a) ignore the shared brightness problem (status quo)
b) never set a brightness other than full on/off
c) ignore one led of each pair (not register it at all{
d) couple also the on/off of the pairs, so present to
   userspace only max. 3 leds.
e) allow full brightness control where independently possible,
   if LEDs are defined where that leads to conflicts,
   register them with max_brightness=1 and use them
   in on/off mode.

My preference were a) or e), the most possible usages.
e) has a cleaner interface to the userspace.

I would not upstream b)

Regards,
Andreas

> > +++ b/drivers/leds/Kconfig
> > @@ -551,6 +551,17 @@ config LEDS_REGULATOR
> >  	help
> >  	  This option enables support for regulator driven LEDs.
> >  
> > +config LEDS_BD2606MVV
> > +	tristate "LED driver for BD2606MVV"
> > +	depends on LEDS_CLASS
> > +	depends on I2C
> > +	select REGMAP_I2C
> > +	help
> > +	  This option enables support for BD2606MVV LED driver chips
> > +	  accessed via the I2C bus. It supports setting brightness, with
> > +	  the limitiation that there are groups of two channels sharing
> > +	  a brightness setting, but not the on/off setting.
> > +  
> 
> This driver can be used as a module...
> 
> 
> > +static int
> > +bd2606mvv_brightness_set(struct led_classdev *led_cdev,
> > +		      enum led_brightness brightness)
> > +{
> > +	struct bd2606mvv_led *led = ldev_to_led(led_cdev);
> > +	struct bd2606mvv_priv *priv = led->priv;
> > +	int err;
> > +
> > +	if (brightness == 0)
> > +		return regmap_update_bits(priv->regmap,
> > +					  BD2606_REG_PWRCNT,
> > +					  1 << led->led_no,
> > +					  0);
> > +
> > +	/* shared brightness register */
> > +	err = regmap_write(priv->regmap, led->led_no / 2,
> > +			   brightness);
> > +	if (err)
> > +		return err;  
> 
> Yeah. No.
> 
> Best regards,
> 							Pavel