linux-kernel - Re: [PATCH v2 2/2] backlight: lm3630a: add an enable gpio for the HWEN pin

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Message-ID: <20190912091811.evlmeeuoai6phy3l@holly.lan>
Date:   Thu, 12 Sep 2019 10:18:11 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Dan Murphy <dmurphy@...com>
Cc:     Andreas Kemnade <andreas@...nade.info>, lee.jones@...aro.org,
        jingoohan1@...il.com, jacek.anaszewski@...il.com, pavel@....cz,
        robh+dt@...nel.org, mark.rutland@....com, b.zolnierkie@...sung.com,
        dri-devel@...ts.freedesktop.org, linux-leds@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fbdev@...r.kernel.org,
        "H. Nikolaus Schaller" <hns@...delico.com>
Subject: Re: [PATCH v2 2/2] backlight: lm3630a: add an enable gpio for the
 HWEN pin

On Wed, Sep 11, 2019 at 01:48:36PM -0500, Dan Murphy wrote:
> 
> On 9/11/19 5:25 AM, Daniel Thompson wrote:
> > On Tue, Sep 10, 2019 at 11:29:09PM +0200, Andreas Kemnade wrote:
> > > For now just enable it in the probe function to allow i2c
> > > access. Disabling also means resetting the register values
> > > to default and according to the datasheet does not give
> > > power savings
> > > 
> > > Tested on Kobo Clara HD.
> > > 
> > > Signed-off-by: Andreas Kemnade <andreas@...nade.info>
> > > ---
> > > changes in v2:
> > > - simplification
> > > - correct gpio direction initialisation
> > > 
> > >   drivers/video/backlight/lm3630a_bl.c | 10 ++++++++++
> > >   1 file changed, 10 insertions(+)
> > > 
> > > diff --git a/drivers/video/backlight/lm3630a_bl.c b/drivers/video/backlight/lm3630a_bl.c
> > > index 8f84f3684f04..9d0639d4202d 100644
> > > --- a/drivers/video/backlight/lm3630a_bl.c
> > > +++ b/drivers/video/backlight/lm3630a_bl.c
> > > @@ -12,6 +12,8 @@
> > >   #include <linux/uaccess.h>
> > >   #include <linux/interrupt.h>
> > >   #include <linux/regmap.h>
> > > +#include <linux/gpio/consumer.h>
> > > +#include <linux/gpio.h>
> > Nitpicking... but I don't think linux/gpio.h is used anymore.
> > 
> > 
> > >   #include <linux/pwm.h>
> > >   #include <linux/platform_data/lm3630a_bl.h>
> > > @@ -48,6 +50,7 @@ struct lm3630a_chip {
> > >   	struct lm3630a_platform_data *pdata;
> > >   	struct backlight_device *bleda;
> > >   	struct backlight_device *bledb;
> > > +	struct gpio_desc *enable_gpio;
> > >   	struct regmap *regmap;
> > >   	struct pwm_device *pwmd;
> > >   };
> > > @@ -535,6 +538,13 @@ static int lm3630a_probe(struct i2c_client *client,
> > >   	}
> > >   	pchip->pdata = pdata;
> > > +	pchip->enable_gpio = devm_gpiod_get_optional(&client->dev, "enable",
> > > +						GPIOD_OUT_HIGH);
> > > +	if (IS_ERR(pchip->enable_gpio)) {
> > > +		rval = PTR_ERR(pchip->enable_gpio);
> > > +		return rval;
> 
> the enable gpio is optional so if it fails you log the error and move on

Isn't the effect of this to cope gracefully if enable-gpios is absent
but to fail with an error if enable-gpios exists and is broken. I
thought this code pattern is fairly common.


> Also on driver removal did you want to set the GPIO to low to disable the
> device to save power?

As it happens I offered to opposite feedback for v1:
https://lists.freedesktop.org/archives/dri-devel/2019-September/234918.html

Basically if the power matters then we should take care of things in the
PM code path (which for this driver means reacting properly to
suspended flag when updating the brightness). If the power doesn't matter
then, given unallocated GPIO pins are in an unknown state anyway, there
is no point in tidying up because we don't know what value to restore.


Daniel.