Re: [PATCH 2/2 v1] backlight: Add Kinetic KTD253 backlight driver

[Sam Ravnborg <sam@ravnborg.org>]

Hi Linus.

On Mon, Jul 20, 2020 at 10:35:06PM +0200, Linus Walleij wrote:
> The Kinetic KTD253 backlight driver is controlled with a
> single GPIO line, but still supports a range of brightness
> settings by sending fast pulses on the line.
> 
> This is based off the source code release for the Samsung
> GT-S7710 mobile phone.
> 
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

In -next there is a few updated to backlight stuff that this driver
could benefit from.
My comments in the following assumes you have the latest -next.

	Sam

> ---
>  MAINTAINERS                                |   6 +
>  drivers/video/backlight/Kconfig            |   8 +
>  drivers/video/backlight/Makefile           |   1 +
>  drivers/video/backlight/ktd253-backlight.c | 254 +++++++++++++++++++++
>  4 files changed, 269 insertions(+)
>  create mode 100644 drivers/video/backlight/ktd253-backlight.c
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index b4a43a9e7fbc..ea6fcc5bb79e 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -9610,6 +9610,12 @@ F:	Documentation/admin-guide/auxdisplay/ks0108.rst
>  F:	drivers/auxdisplay/ks0108.c
>  F:	include/linux/ks0108.h
>  
> +KTD253 BACKLIGHT DRIVER
> +M:	Linus Walleij <linus.walleij@linaro.org>
> +S:	Maintained
> +F:	Documentation/devicetree/bindings/leds/backlight/kinetic,ktd253.yaml
> +F:	drivers/video/backlight/ktd253-backlight.c
> +
>  L3MDEV
>  M:	David Ahern <dsahern@kernel.org>
>  L:	netdev@vger.kernel.org
> diff --git a/drivers/video/backlight/Kconfig b/drivers/video/backlight/Kconfig
> index 7d22d7377606..6a74c60707b4 100644
> --- a/drivers/video/backlight/Kconfig
> +++ b/drivers/video/backlight/Kconfig
> @@ -190,6 +190,14 @@ config BACKLIGHT_IPAQ_MICRO
>  	  computers. Say yes if you have one of the h3100/h3600/h3700
>  	  machines.
>  
> +config BACKLIGHT_KTD253
> +	tristate "Backlight Driver for Kinetic KTD253"
> +	depends on GPIOLIB || COMPILE_TEST
> +	help
> +	  Say y to enabled the backlight driver for the Kinetic KTD253
> +	  which is a 1-wire GPIO-controlled backlight found in some mobile
> +	  phones.
> +
>  config BACKLIGHT_LM3533
>  	tristate "Backlight Driver for LM3533"
>  	depends on MFD_LM3533
> diff --git a/drivers/video/backlight/Makefile b/drivers/video/backlight/Makefile
> index 0c1a1524627a..d50cd12574ae 100644
> --- a/drivers/video/backlight/Makefile
> +++ b/drivers/video/backlight/Makefile
> @@ -36,6 +36,7 @@ obj-$(CONFIG_BACKLIGHT_GPIO)		+= gpio_backlight.o
>  obj-$(CONFIG_BACKLIGHT_HP680)		+= hp680_bl.o
>  obj-$(CONFIG_BACKLIGHT_HP700)		+= jornada720_bl.o
>  obj-$(CONFIG_BACKLIGHT_IPAQ_MICRO)	+= ipaq_micro_bl.o
> +obj-$(CONFIG_BACKLIGHT_KTD253)		+= ktd253-backlight.o
>  obj-$(CONFIG_BACKLIGHT_LM3533)		+= lm3533_bl.o
>  obj-$(CONFIG_BACKLIGHT_LM3630A)		+= lm3630a_bl.o
>  obj-$(CONFIG_BACKLIGHT_LM3639)		+= lm3639_bl.o
> diff --git a/drivers/video/backlight/ktd253-backlight.c b/drivers/video/backlight/ktd253-backlight.c
> new file mode 100644
> index 000000000000..d460d1fef329
> --- /dev/null
> +++ b/drivers/video/backlight/ktd253-backlight.c
> @@ -0,0 +1,254 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Backlight driver for the Kinetic KTD253
> + * Based on code and know-how from the Samsung GT-S7710
> + * Gareth Phillips <gareth.phillips@samsung.com>
> + */
> +#include <linux/backlight.h>
> +#include <linux/err.h>
> +#include <linux/fb.h>
> +#include <linux/gpio/consumer.h>
> +#include <linux/init.h>
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/of.h>
> +#include <linux/platform_device.h>
> +#include <linux/property.h>
> +#include <linux/slab.h>
> +#include <linux/delay.h>
> +#include <linux/limits.h>

In drm land these needs to be sorted.
I do not think backlight demands it.

> +
> +/* Current ratio is n/32 from 1/32 to 32/32 */
> +#define KTD253_MIN_RATIO 1
> +#define KTD253_MAX_RATIO 32
> +#define KTD253_DEFAULT_RATIO 13
> +
> +/* With the table we use this is 24/32 current ratio actually */
> +#define KTD253_MAX_BRIGHTNESS 255
> +#define KTD253_DEFAULT_BRIGHTNESS 160
> +
> +#define KTD253_T_LOW_NS (200 + 10) /* Additional 10ns as safety factor */
> +#define KTD253_T_HIGH_NS (200 + 10) /* Additional 10ns as safety factor */
> +#define KTD253_T_OFF_MS 3
> +
> +struct ktd253_backlight {
> +	struct device *dev;
> +	struct gpio_desc *gpiod;
> +	u16 ratio;

> +	unsigned int brightness;
brightness is not used - delete.
> +};

I had expected to see a backlight pointer in the above structure.
Like we do in most drivers.

> +
> +/*
> + * The following table is used to convert brightness level to the LED
> + * Current Ratio expressed as (full current) /(n * 32).
> + * i.e. 1 = 1/32 full current. Zero indicates LED is powered off.
> + * The table is intended to allow the brightness level to be "tuned"
> + * to compensate for non-linearity of brightness relative to current.
> + */
> +static const u16 ktd253_brightness_to_current_ratio[] = {
> +	0,      /* (0/32) KTD253_BACKLIGHT_OFF */
> +	39,     /* (1/32) KTD253_MIN_RATIO */
> +	58,     /* (2/32) */
> +	67,     /* (3/32) */
> +	76,     /* (4/32) */
> +	85,     /* (5/32) */
> +	94,     /* (6/32) */
> +	104,    /* (7/32) */
> +	113,    /* (8/32) */
> +	122,    /* (9/32) */
> +	131,    /* (10/32) */
> +	145,    /* (11/32) */
> +	159,    /* (12/32) */
> +	169,    /* (13/32) */
> +	179,    /* (14/32) */
> +	189,    /* (15/32) */
> +	196,    /* (16/32) */
> +	203,    /* (17/32) */
> +	210,    /* (18/32) */
> +	217,    /* (19/32) */
> +	224,    /* (20/32) */
> +	231,    /* (21/32) */
> +	238,    /* (22/32) */
> +	245,    /* (23/32) */
> +	255,    /* (24/32) */
> +	300,    /* (25/32) */
> +	300,    /* (26/32) */
> +	300,    /* (27/32) */
> +	300,    /* (28/32) */
> +	300,    /* (29/32) */
> +	300,    /* (30/32) */
> +	300,    /* (31/32) */
> +	300     /* (32/32) KTD253_MAX_RATIO */
> +
> +};
> +
> +/* Inspired by gpio_bl.c */
> +static int ktd253_backlight_get_next_brightness(struct backlight_device *bl)
> +{
> +	int brightness = bl->props.brightness;
> +
> +	if (bl->props.power != FB_BLANK_UNBLANK ||
> +	    bl->props.fb_blank != FB_BLANK_UNBLANK ||
> +	    bl->props.state & (BL_CORE_SUSPENDED | BL_CORE_FBBLANK))
> +		brightness = 0;
> +
> +	return brightness;
> +}
> +
> +static int ktd253_backlight_update_status(struct backlight_device *bl)
> +{
> +	struct ktd253_backlight *ktd253 = bl_get_data(bl);
> +	int brightness = ktd253_backlight_get_next_brightness(bl);
Use backligt_get_brightness() to get the brightness.
Then you can delete ktd253_backlight_get_next_brightness()

> +	u16 target_ratio;
> +	u16 current_ratio = ktd253->ratio;
> +	unsigned long flags;
> +
> +	dev_dbg(ktd253->dev, "new brightness: %d\n", brightness);
> +
> +	/* Look up the current ratio */
> +	for (target_ratio = KTD253_MAX_RATIO; target_ratio > 0; target_ratio--) {
> +		if (brightness > ktd253_brightness_to_current_ratio[target_ratio - 1])
> +			break;
> +	}
> +
> +	dev_dbg(ktd253->dev, "new ratio: %d/32\n", target_ratio);
Maybe only one print with both brightness and ratio?

> +
> +	if (target_ratio == current_ratio)
> +		/* This is already right */
> +		return 0;
> +
> +	if (target_ratio == 0) {
> +		gpiod_set_value_cansleep(ktd253->gpiod, 0);
> +		/*
> +		 * We need to keep the GPIO low for at least this long
> +		 * to actually switch the KTD253 off.
> +		 */
> +		msleep(KTD253_T_OFF_MS);
> +		ktd253->ratio = 0;
> +		return 0;
> +	}
> +
> +	if (current_ratio == 0) {
> +		gpiod_set_value_cansleep(ktd253->gpiod, 1);
> +		ndelay(KTD253_T_HIGH_NS);
> +		/* We always fall back to this when we power on */
> +		current_ratio = KTD253_MAX_RATIO;
> +	}
> +
> +	/*
> +	 * WARNING:
> +	 * The loop to set the correct current level is performed
> +	 * with interrupts disabled as it is timing critical.
> +	 * The maximum number of cycles of the loop is 32
> +	 * so the time taken will be (T_LOW_NS + T_HIGH_NS + loop_time) * 32,
> +	 */
> +	local_irq_save(flags);
> +	while (current_ratio != target_ratio) {
> +		/*
> +		 * These GPIO operations absolutely can NOT sleep so no
> +		 * _cansleep suffixes, and no using GPIO expanders on
> +		 * slow buses for this!
> +		 */
> +		gpiod_set_value(ktd253->gpiod, 0);
> +		ndelay(KTD253_T_LOW_NS);
> +		gpiod_set_value(ktd253->gpiod, 1);
> +		ndelay(KTD253_T_HIGH_NS);
> +		/* After 1/32 we loop back to 32/32 */
> +		if (current_ratio == KTD253_MIN_RATIO)
> +			current_ratio = KTD253_MAX_RATIO;
> +		else
> +			current_ratio--;
> +	}
> +	local_irq_restore(flags);
> +	ktd253->ratio = current_ratio;
> +
> +	dev_dbg(ktd253->dev, "new ratio set to %d/32\n", target_ratio);
> +
> +	return 0;
> +}
> +
> +static const struct backlight_ops ktd253_backlight_ops = {
> +	.options	= BL_CORE_SUSPENDRESUME,
> +	.update_status	= ktd253_backlight_update_status,
> +};
> +
> +static int ktd253_backlight_probe(struct platform_device *pdev)
> +{
> +	struct device *dev = &pdev->dev;
> +	struct backlight_device *bl;
> +	struct ktd253_backlight *ktd253;
> +	u32 max_brightness;
> +	u32 brightness;
> +	int ret;
> +
> +	ktd253 = devm_kzalloc(dev, sizeof(*ktd253), GFP_KERNEL);
> +	if (!ktd253)
> +		return -ENOMEM;
> +	ktd253->dev = dev;
> +
> +	ret = device_property_read_u32(dev, "max-brightness", &max_brightness);
> +	if (ret)
> +		max_brightness = KTD253_MAX_BRIGHTNESS;
> +
> +	ret = device_property_read_u32(dev, "default-brightness", &brightness);
> +	if (ret)
> +		brightness = KTD253_DEFAULT_BRIGHTNESS;
> +
> +	if (brightness)
> +		/* This will be the default ratio when the KTD253 is enabled */
> +		ktd253->ratio = KTD253_MAX_RATIO;
> +	else
> +		ktd253->ratio = 0;
> +
> +	ktd253->gpiod = devm_gpiod_get(dev, NULL,
> +				       brightness ? GPIOD_OUT_HIGH :
> +				       GPIOD_OUT_LOW);
> +	if (IS_ERR(ktd253->gpiod)) {
> +		ret = PTR_ERR(ktd253->gpiod);
> +		if (ret != -EPROBE_DEFER)
> +			dev_err(dev, "gpio line missing or invalid.\n");
> +		return ret;
> +	}
> +	gpiod_set_consumer_name(ktd253->gpiod, dev_name(dev));
> +
> +	bl = devm_backlight_device_register(dev, dev_name(dev), dev, ktd253,
> +					    &ktd253_backlight_ops, NULL);
> +	if (IS_ERR(bl)) {
> +		dev_err(dev, "failed to register backlight\n");
> +		return PTR_ERR(bl);
> +	}
> +	bl->props.max_brightness = max_brightness;
> +	/* When we just enable the GPIO line we set max brightness */
> +	if (brightness) {
> +		bl->props.brightness = brightness;
> +		bl->props.power = FB_BLANK_UNBLANK;
> +		ktd253_backlight_update_status(bl);
> +	} else {
> +		bl->props.brightness = 0;
> +		bl->props.power = FB_BLANK_POWERDOWN;
> +	}
Pass a backlight_properties to devm_backlight_device_register.
So this is correct at init time.

FB_BLANK_* are constant used by the fb_blank icotl - and should not be
used here.
Do not assign props.power - as there is no change in power state to
report.

In other words:
Init backlight_properties with:
- max_brightness
- brightness
- Type (RAW)
Call devm_backlight_device_register()

Then unconditionally call backlight_update_status()
(Not the local variant, go via backlight core)

The above is my understandig - but let the backlight people chime in
too.

I would love if we could make it simpler to register a backlight
device...

	Sam

> +
> +	platform_set_drvdata(pdev, bl);
> +
> +	return 0;
> +}
> +
> +static const struct of_device_id ktd253_backlight_of_match[] = {
> +	{ .compatible = "kinetic,ktd253" },
> +	{ /* sentinel */ }
> +};
> +MODULE_DEVICE_TABLE(of, ktd253_backlight_of_match);
> +
> +static struct platform_driver ktd253_backlight_driver = {
> +	.driver = {
> +		.name = "ktd253-backlight",
> +		.of_match_table = ktd253_backlight_of_match,
> +	},
> +	.probe		= ktd253_backlight_probe,
> +};
> +module_platform_driver(ktd253_backlight_driver);
> +
> +MODULE_AUTHOR("Linus Walleij <linus.walleij@linaro.org>");
> +MODULE_DESCRIPTION("Kinetic KTD253 Backlight Driver");
> +MODULE_LICENSE("GPL");
> +MODULE_ALIAS("platform:ktd253-backlight");
> -- 
> 2.26.2
> 
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