Re: [PATCH v1] leds: introduce ordered workqueue for leds events instead of system_wq

[Dmitry Rokosov]

On Tue, Sep 03, 2024 at 03:01:19PM +0100, Lee Jones wrote:
> On Tue, 20 Aug 2024, Dmitry Rokosov wrote:
> 
> > This allows to setup ordered workqueue for leds events. This may be
> > useful, because default 'system_wq' does not guarantee execution order
> > of each work_struct, thus for several brightness update requests (for
> > multiple leds), real brightness switch could be in random order.
> > 
> > Yes, for sysfs-based leds we have flush_work() call inside
> > brightness_store() operation, but it's blocking call, so userspace
> > caller can be blocked at a long time, which means leds animation stream
> > can be broken.
> > 
> > Ordered workqueue has the same behaviour as system_wq + flush_work(),
> > but all scheduled works are async and userspace caller is not blocked,
> > which it better for userspace animation scheduling.
> > 
> > Signed-off-by: Alexey Romanov <avromanov@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Dmitry Rokosov <ddrokosov@xxxxxxxxxxxxxxxxx>
> > ---
> >  drivers/leds/led-class.c | 12 +++++++++++-
> >  drivers/leds/led-core.c  |  6 +++---
> >  include/linux/leds.h     |  1 +
> >  3 files changed, 15 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/leds/led-class.c b/drivers/leds/led-class.c
> > index ba1be15cfd8e..fba12471cf1f 100644
> > --- a/drivers/leds/led-class.c
> > +++ b/drivers/leds/led-class.c
> > @@ -25,6 +25,8 @@
> >  static DEFINE_MUTEX(leds_lookup_lock);
> >  static LIST_HEAD(leds_lookup_list);
> >  
> > +static struct workqueue_struct *leds_wq;
> 
> Does this _have_ to be global?
> 

Unfortunately, yes, it has. Each LED described in the DTS, for example,
is represented as led_cdev. We do not have a led_device or a similar
structure to represent the entire LED device (the controller that
integrates all LEDs and manages them).

One option is to move leds_wq to the driver’s private data. However,
from my perspective, this is a more complicated solution, especially
considering that we have the ability to create a global leds_wq variable
in the core implementation, which can resolve the described issue
entirely.

> Isn't there a suitable data structure that we can store it in?
> 

> >  static ssize_t brightness_show(struct device *dev,
> >  		struct device_attribute *attr, char *buf)
> >  {
> > @@ -57,7 +59,6 @@ static ssize_t brightness_store(struct device *dev,
> >  	if (state == LED_OFF)
> >  		led_trigger_remove(led_cdev);
> >  	led_set_brightness(led_cdev, state);
> > -	flush_work(&led_cdev->set_brightness_work);
> >  
> >  	ret = size;
> >  unlock:
> > @@ -549,6 +550,8 @@ int led_classdev_register_ext(struct device *parent,
> >  
> >  	led_update_brightness(led_cdev);
> >  
> > +	led_cdev->wq = leds_wq;
> > +
> >  	led_init_core(led_cdev);
> >  
> >  #ifdef CONFIG_LEDS_TRIGGERS
> > @@ -667,12 +670,19 @@ EXPORT_SYMBOL_GPL(devm_led_classdev_unregister);
> >  
> >  static int __init leds_init(void)
> >  {
> > +	leds_wq = alloc_ordered_workqueue("leds", 0);
> > +	if (!leds_wq) {
> > +		pr_err("failed to create leds ordered workqueue\n");
> 
> Nit: "LEDs"
> 

Okay, I will send v2 patch today with fixed this line.

> > +		return -ENOMEM;
> > +	}
> > +
> >  	return class_register(&leds_class);
> >  }
> >  
> >  static void __exit leds_exit(void)
> >  {
> >  	class_unregister(&leds_class);
> > +	destroy_workqueue(leds_wq);
> >  }
> >  
> >  subsys_initcall(leds_init);
> > diff --git a/drivers/leds/led-core.c b/drivers/leds/led-core.c
> > index 89c9806cc97f..9769ac49be20 100644
> > --- a/drivers/leds/led-core.c
> > +++ b/drivers/leds/led-core.c
> > @@ -266,7 +266,7 @@ void led_blink_set_nosleep(struct led_classdev *led_cdev, unsigned long delay_on
> >  		led_cdev->delayed_delay_on = delay_on;
> >  		led_cdev->delayed_delay_off = delay_off;
> >  		set_bit(LED_SET_BLINK, &led_cdev->work_flags);
> > -		schedule_work(&led_cdev->set_brightness_work);
> > +		queue_work(led_cdev->wq, &led_cdev->set_brightness_work);
> >  		return;
> >  	}
> >  
> > @@ -297,7 +297,7 @@ void led_set_brightness(struct led_classdev *led_cdev, unsigned int brightness)
> >  		 */
> >  		if (!brightness) {
> >  			set_bit(LED_BLINK_DISABLE, &led_cdev->work_flags);
> > -			schedule_work(&led_cdev->set_brightness_work);
> > +			queue_work(led_cdev->wq, &led_cdev->set_brightness_work);
> >  		} else {
> >  			set_bit(LED_BLINK_BRIGHTNESS_CHANGE,
> >  				&led_cdev->work_flags);
> > @@ -333,7 +333,7 @@ void led_set_brightness_nopm(struct led_classdev *led_cdev, unsigned int value)
> >  		set_bit(LED_SET_BRIGHTNESS_OFF, &led_cdev->work_flags);
> >  	}
> >  
> > -	schedule_work(&led_cdev->set_brightness_work);
> > +	queue_work(led_cdev->wq, &led_cdev->set_brightness_work);
> >  }
> >  EXPORT_SYMBOL_GPL(led_set_brightness_nopm);
> >  
> > diff --git a/include/linux/leds.h b/include/linux/leds.h
> > index 6300313c46b7..7c9f1cb12ab9 100644
> > --- a/include/linux/leds.h
> > +++ b/include/linux/leds.h
> > @@ -169,6 +169,7 @@ struct led_classdev {
> >  	int			 new_blink_brightness;
> >  	void			(*flash_resume)(struct led_classdev *led_cdev);
> >  
> > +	struct workqueue_struct *wq; /* LED workqueue */
> >  	struct work_struct	set_brightness_work;
> >  	int			delayed_set_value;
> >  	unsigned long		delayed_delay_on;
> > -- 
> > 2.43.0
> > 
> > 
> 
> -- 
> Lee Jones [李琼斯]

-- 
Thank you,
Dmitry