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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>
 <72c45311-c710-dc2d-a6de-68e44ea8436a@ti.com>
 <20190703094457.etmbbjhhssbdkveo@holly.lan>
From:   Jean-Jacques Hiblot <jjhiblot@ti.com>
Message-ID: <a8886ae9-31ec-de4c-0a83-5f681582a0b9@ti.com>
Date:   Wed, 3 Jul 2019 12:02:23 +0200
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Daniel,

On 03/07/2019 11:44, Daniel Thompson wrote:
> On Tue, Jul 02, 2019 at 05:17:21PM +0200, Jean-Jacques Hiblot wrote:
>> 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())
> led_sysfs_disable() sounds like a sensible change but that's not quite
> what I mean.
>
> It is more a thought experiment to see if the power control *should* be
> implemented by the backlight. Consider what happens if we *don't*
> enable CONFIG_BACKLIGHT_LED in the kernel: we would still have an LED
> device and it would not work correctly.
>
> In other words I naively expect turning on an LED using the LED API
> (any of them, sysfs or kernel) to result in the LED turning on.
> Implementing a workaround in the client for what appears to be
> something missing in the LED driver strikes me as odd. Why shouldn't
> the regulator be managed in the LED driver?

I see your point. Indeed having the regulator handled in the LED-core 
makes sense in a lot of situations

I'll think about it.

>
>
>>> 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.
> I'm afraid I don't follow this. If you have a backlight composed of mutliple
> strings why wouldn't each string be attached directly to the output an of LED
> driver chip such as the TLC591xx family.

OK. It makes sense.

I'll rework the series in this direction: multiple LED devices handled 
by one backlight device

Thanks,

JJ

>
>
> Daniel.
>
>
>>
>> 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
>>>>>>