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

Jean-Jacques Hiblot
2019-07-03T10:02:23Z

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
>>>>>>