Re: [PATCH 3/4] backlight: add led-backlight driver

Jean-Jacques Hiblot
2019-07-02T15:17:21Z

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
>>>>