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Subject: Re: [PATCH 3/4] backlight: add led-backlight driver
To:     Daniel Thompson <daniel.thompson@linaro.org>
CC:     <jacek.anaszewski@gmail.com>, <pavel@ucw.cz>, <robh+dt@kernel.org>,
        <mark.rutland@arm.com>, <lee.jones@linaro.org>,
        <jingoohan1@gmail.com>, <dmurphy@ti.com>,
        <linux-leds@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <dri-devel@lists.freedesktop.org>, <tomi.valkeinen@ti.com>
References: <20190701151423.30768-1-jjhiblot@ti.com>
 <20190701151423.30768-4-jjhiblot@ti.com>
 <20190702095434.d426lichmaffz7a5@holly.lan>
 <531e237c-b570-5270-6fc3-6629a8bf7acd@ti.com>
 <20190702130434.frbx7jkec27ejbpo@holly.lan>
From:   Jean-Jacques Hiblot <jjhiblot@ti.com>
Message-ID: <72c45311-c710-dc2d-a6de-68e44ea8436a@ti.com>
Date:   Tue, 2 Jul 2019 17:17:21 +0200
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Daniel,

On 02/07/2019 15:04, Daniel Thompson wrote:
> On Tue, Jul 02, 2019 at 12:59:53PM +0200, Jean-Jacques Hiblot wrote:
>> Hi Daniel,
>>
>> On 02/07/2019 11:54, Daniel Thompson wrote:
>>> On Mon, Jul 01, 2019 at 05:14:22PM +0200, Jean-Jacques Hiblot wrote:
>>>> From: Tomi Valkeinen <tomi.valkeinen@ti.com>
>>>>
>>>> This patch adds a led-backlight driver (led_bl), which is mostly similar to
>>>> pwm_bl except the driver uses a LED class driver to adjust the brightness
>>>> in the HW.
>>>>
>>>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
>>>> Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
>>>> ---
>>>>    drivers/video/backlight/Kconfig  |   7 +
>>>>    drivers/video/backlight/Makefile |   1 +
>>>>    drivers/video/backlight/led_bl.c | 217 +++++++++++++++++++++++++++++++
>>>>    3 files changed, 225 insertions(+)
>>>>    create mode 100644 drivers/video/backlight/led_bl.c
>>>>
>>>> diff --git a/drivers/video/backlight/Kconfig b/drivers/video/backlight/Kconfig
>>>> index 8b081d61773e..585a1787618c 100644
>>>> --- a/drivers/video/backlight/Kconfig
>>>> +++ b/drivers/video/backlight/Kconfig
>>>> @@ -458,6 +458,13 @@ config BACKLIGHT_RAVE_SP
>>>>    	help
>>>>    	  Support for backlight control on RAVE SP device.
>>>> +config BACKLIGHT_LED
>>>> +	tristate "Generic LED based Backlight Driver"
>>>> +	depends on LEDS_CLASS && OF
>>>> +	help
>>>> +	  If you have a LCD backlight adjustable by LED class driver, say Y
>>>> +	  to enable this driver.
>>>> +
>>>>    endif # BACKLIGHT_CLASS_DEVICE
>>>>    endmenu
>>>> diff --git a/drivers/video/backlight/Makefile b/drivers/video/backlight/Makefile
>>>> index 63c507c07437..2a67642966a5 100644
>>>> --- a/drivers/video/backlight/Makefile
>>>> +++ b/drivers/video/backlight/Makefile
>>>> @@ -57,3 +57,4 @@ obj-$(CONFIG_BACKLIGHT_TPS65217)	+= tps65217_bl.o
>>>>    obj-$(CONFIG_BACKLIGHT_WM831X)		+= wm831x_bl.o
>>>>    obj-$(CONFIG_BACKLIGHT_ARCXCNN) 	+= arcxcnn_bl.o
>>>>    obj-$(CONFIG_BACKLIGHT_RAVE_SP)		+= rave-sp-backlight.o
>>>> +obj-$(CONFIG_BACKLIGHT_LED)		+= led_bl.o
>>>> diff --git a/drivers/video/backlight/led_bl.c b/drivers/video/backlight/led_bl.c
>>>> new file mode 100644
>>>> index 000000000000..e699924cc2bc
>>>> --- /dev/null
>>>> +++ b/drivers/video/backlight/led_bl.c
>>>> @@ -0,0 +1,217 @@
>>>> +// SPDX-License-Identifier: GPL-2.0
>>>> +/*
>>>> + * Copyright (C) 2015-2018 Texas Instruments Incorporated -  http://www.ti.com/
>>>> + * Author: Tomi Valkeinen <tomi.valkeinen@ti.com>
>>>> + *
>>>> + * Based on pwm_bl.c
>>>> + */
>>>> +
>>>> +#include <linux/backlight.h>
>>>> +#include <linux/gpio/consumer.h>
>>>> +#include <linux/leds.h>
>>>> +#include <linux/module.h>
>>>> +#include <linux/platform_device.h>
>>>> +#include <linux/regulator/consumer.h>
>>>> +#include <linux/slab.h>
>>>> +
>>>> +struct led_bl_data {
>>>> +	struct device		*dev;
>>>> +	struct backlight_device	*bl_dev;
>>>> +
>>>> +	unsigned int		*levels;
>>>> +	bool			enabled;
>>>> +	struct regulator	*power_supply;
>>>> +	struct gpio_desc	*enable_gpio;
>>> For the PWM driver the power_supply and enable_gpio are part of managing
>>> a dumb LED driver device that is downstream of the PWM.
>>>
>>> What is their purpose when we wrap an LED device? Put another why why isn't
>>> the LED device driver responsible for this?
>> This question came up when the patch was first proposed in 2015. Here is the
>> answer from Tomi at the time. It is still relevant.
>>
>> "These are for the backlight, not for the LED chip. So "LED" here is a
>> chip that produces (most likely) a PWM signal, and "backlight" is the
>> collection of components that use the PWM to produce the backlight
>> itself, and use the power-supply and gpios."
> Expanded significantly in the associated thread, right?
> https://patchwork.kernel.org/patch/7293991/
>
> Also still relevant is whether the LED device is being correctly
> modelled if the act of turning on the LED doesn't, in fact, turn the LED
> on. Is it *really* a correct implementation of an LED device that
> setting it to LED_FULL using sysfs doesn't cause it to light up?

What I understood from the discussion between Rob and Tomi is that the 
child-node of the LED controller should be considered a backlight 
device, not a simple LED. I'm not sure if the sysfs interface is still 
relevant in that case. Maybe it should just be disabled by the backlight 
driver (possible with led_sysfs_disable())

>
> Actually there is another area where I think an LED backlight should
> perhaps be held to a higher standard than a PWM backlight and that is
> handling backlights composed of multiple LEDs.
>
> Using the TLC591xx examples from the thread above... these are
> multi-channel (8 or 16) LED controllers and I don't think its
> speculative to assume that a backlight could constructed using
> one of these could require multiple LEDs.

In that case, the device-tree model must be quite different.

Actually the best way to do that is to use the model from Tomi 
https://patchwork.kernel.org/patch/7293991/ and modify it to handle more 
than one LED
<https://patchwork.kernel.org/patch/7293991/>

I'm not completely sure that people would start making real backlight 
this way though. It is much more probable that the ouput of the led ctrl 
is connected to a single control input of a real backlight than to 
actual LEDs.


JJ

>
>
> Daniel.
>
>
>>>> +
>>>> +	struct led_classdev *led_cdev;
>>>> +
>>>> +	unsigned int max_brightness;
>>>> +	unsigned int default_brightness;
>>>> +};
>>>> +
>>>> +static void led_bl_set_brightness(struct led_bl_data *priv, int brightness)
>>>> +{
>>>> +	int err;
>>>> +
>>>> +	if (!priv->enabled) {
>>>> +		err = regulator_enable(priv->power_supply);
>>>> +		if (err < 0)
>>>> +			dev_err(priv->dev, "failed to enable power supply\n");
>>>> +
>>>> +		if (priv->enable_gpio)
>>>> +			gpiod_set_value_cansleep(priv->enable_gpio, 1);
>>>> +	}
>>>> +
>>>> +	led_set_brightness(priv->led_cdev, priv->levels[brightness]);
>>>> +
>>>> +	priv->enabled = true;
>>>> +}
>>>> +
>>>> +static void led_bl_power_off(struct led_bl_data *priv)
>>>> +{
>>>> +	if (!priv->enabled)
>>>> +		return;
>>>> +
>>>> +	led_set_brightness(priv->led_cdev, LED_OFF);
>>>> +
>>>> +	if (priv->enable_gpio)
>>>> +		gpiod_set_value_cansleep(priv->enable_gpio, 0);
>>>> +
>>>> +	regulator_disable(priv->power_supply);
>>>> +
>>>> +	priv->enabled = false;
>>>> +}
>>>> +
>>>> +static int led_bl_update_status(struct backlight_device *bl)
>>>> +{
>>>> +	struct led_bl_data *priv = bl_get_data(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_FBBLANK)
>>>> +		brightness = 0;
>>>> +
>>>> +	if (brightness > 0)
>>>> +		led_bl_set_brightness(priv, brightness);
>>>> +	else
>>>> +		led_bl_power_off(priv);
>>>> +
>>>> +	return 0;
>>>> +}
>>>> +
>>>> +static const struct backlight_ops led_bl_ops = {
>>>> +	.update_status	= led_bl_update_status,
>>>> +};
>>>> +
>>>> +static int led_bl_parse_dt(struct device *dev,
>>>> +			   struct led_bl_data *priv)
>>>> +{
>>>> +	struct device_node *node = dev->of_node;
>>>> +	int num_levels;
>>>> +	u32 *levels;
>>>> +	u32 value;
>>>> +	int ret;
>>>> +
>>>> +	if (!node)
>>>> +		return -ENODEV;
>>>> +
>>>> +	num_levels = of_property_count_u32_elems(node, "brightness-levels");
>>> Is there any reason that this function cannot use the (more generic)
>>> device property API throughout this function?
>> No reason. The code is a bit old, and can do with an update.
>>
>> Are you thinking of of_property_read_u32_array(), or another function ?
>>
>> JJ
>>
>>>
>>>
>>> Daniel.
>>>
>>>
>>>> +	if (num_levels < 0)
>>>> +		return num_levels;
>>>> +
>>>> +	levels = devm_kzalloc(dev, sizeof(u32) * num_levels, GFP_KERNEL);
>>>> +	if (!levels)
>>>> +		return -ENOMEM;
>>>> +
>>>> +	ret = of_property_read_u32_array(node, "brightness-levels",
>>>> +					 levels,
>>>> +					 num_levels);
>>>> +	if (ret < 0)
>>>> +		return ret;
>>>> +
>>>> +	ret = of_property_read_u32(node, "default-brightness-level", &value);
>>>> +	if (ret < 0)
>>>> +		return ret;
>>>> +
>>>> +	if (value >= num_levels) {
>>>> +		dev_err(dev, "invalid default-brightness-level\n");
>>>> +		return -EINVAL;
>>>> +	}
>>>> +
>>>> +	priv->levels = levels;
>>>> +	priv->max_brightness = num_levels - 1;
>>>> +	priv->default_brightness = value;
>>>> +
>>>> +	return 0;
>>>> +}
>>>> +
>>>> +static int led_bl_probe(struct platform_device *pdev)
>>>> +{
>>>> +	struct backlight_properties props;
>>>> +	struct led_bl_data *priv;
>>>> +	int ret;
>>>> +
>>>> +	priv = devm_kzalloc(&pdev->dev, sizeof(*priv), GFP_KERNEL);
>>>> +	if (!priv)
>>>> +		return -ENOMEM;
>>>> +
>>>> +	platform_set_drvdata(pdev, priv);
>>>> +
>>>> +	priv->dev = &pdev->dev;
>>>> +	priv->led_cdev = to_led_classdev(pdev->dev.parent);
>>>> +
>>>> +	ret = led_bl_parse_dt(&pdev->dev, priv);
>>>> +	if (ret < 0) {
>>>> +		if (ret != -EPROBE_DEFER)
>>>> +			dev_err(&pdev->dev, "failed to parse DT data\n");
>>>> +		return ret;
>>>> +	}
>>>> +
>>>> +	priv->enable_gpio = devm_gpiod_get_optional(&pdev->dev, "enable",
>>>> +			    GPIOD_OUT_LOW);
>>>> +	if (IS_ERR(priv->enable_gpio)) {
>>>> +		ret = PTR_ERR(priv->enable_gpio);
>>>> +		goto err;
>>>> +	}
>>>> +
>>>> +	priv->power_supply = devm_regulator_get(&pdev->dev, "power");
>>>> +	if (IS_ERR(priv->power_supply)) {
>>>> +		ret = PTR_ERR(priv->power_supply);
>>>> +		goto err;
>>>> +	}
>>>> +
>>>> +	memset(&props, 0, sizeof(struct backlight_properties));
>>>> +	props.type = BACKLIGHT_RAW;
>>>> +	props.max_brightness = priv->max_brightness;
>>>> +	priv->bl_dev = backlight_device_register(dev_name(&pdev->dev),
>>>> +			&pdev->dev, priv, &led_bl_ops, &props);
>>>> +	if (IS_ERR(priv->bl_dev)) {
>>>> +		dev_err(&pdev->dev, "failed to register backlight\n");
>>>> +		ret = PTR_ERR(priv->bl_dev);
>>>> +		goto err;
>>>> +	}
>>>> +
>>>> +	priv->bl_dev->props.brightness = priv->default_brightness;
>>>> +	backlight_update_status(priv->bl_dev);
>>>> +
>>>> +	return 0;
>>>> +
>>>> +err:
>>>> +
>>>> +	return ret;
>>>> +}
>>>> +
>>>> +static int led_bl_remove(struct platform_device *pdev)
>>>> +{
>>>> +	struct led_bl_data *priv = platform_get_drvdata(pdev);
>>>> +	struct backlight_device *bl = priv->bl_dev;
>>>> +
>>>> +	backlight_device_unregister(bl);
>>>> +
>>>> +	led_bl_power_off(priv);
>>>> +
>>>> +	return 0;
>>>> +}
>>>> +
>>>> +static const struct of_device_id led_bl_of_match[] = {
>>>> +	{ .compatible = "led-backlight" },
>>>> +	{ }
>>>> +};
>>>> +
>>>> +MODULE_DEVICE_TABLE(of, led_bl_of_match);
>>>> +
>>>> +static struct platform_driver led_bl_driver = {
>>>> +	.driver		= {
>>>> +		.name		= "led-backlight",
>>>> +		.of_match_table	= of_match_ptr(led_bl_of_match),
>>>> +	},
>>>> +	.probe		= led_bl_probe,
>>>> +	.remove		= led_bl_remove,
>>>> +};
>>>> +
>>>> +module_platform_driver(led_bl_driver);
>>>> +
>>>> +MODULE_DESCRIPTION("LED based Backlight Driver");
>>>> +MODULE_LICENSE("GPL");
>>>> +MODULE_ALIAS("platform:led-backlight");
>>>> -- 
>>>> 2.17.1
>>>>