Message-ID: <56F2C60B.7040801@gmail.com>
Date:	Wed, 23 Mar 2016 17:36:27 +0100
From:	Heiner Kallweit <hkallweit1@...il.com>
To:	Jacek Anaszewski <j.anaszewski@...sung.com>
Cc:	"linux-leds@...r.kernel.org" <linux-leds@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] leds: triggers: add support for RGB triggers

Am 23.03.2016 um 17:02 schrieb Jacek Anaszewski:
> On 03/23/2016 12:57 PM, Heiner Kallweit wrote:
>> Am 23.03.2016 um 09:32 schrieb Jacek Anaszewski:
>>> On 03/22/2016 11:06 PM, Heiner Kallweit wrote:
>>>> Am 22.03.2016 um 17:00 schrieb Jacek Anaszewski:
>>>>> On 03/22/2016 12:47 PM, Heiner Kallweit wrote:
>>>>>> Am 22.03.2016 um 09:05 schrieb Jacek Anaszewski:
>>>>>>> On 03/21/2016 06:34 PM, Heiner Kallweit wrote:
>>>>>>>> Am 21.03.2016 um 16:35 schrieb Jacek Anaszewski:
>>>>>>>>> On 03/19/2016 08:11 PM, Heiner Kallweit wrote:
>>>>>>>>>> Am 18.03.2016 um 14:10 schrieb Jacek Anaszewski:
>>>>>>>>>>> On 03/17/2016 08:53 PM, Heiner Kallweit wrote:
>>>>>>>>>>>> Am 17.03.2016 um 14:41 schrieb Jacek Anaszewski:
>>>>>>>>>>>>> Hi Heiner,
>>>>>>>>>>>>>
>>>>>>>>>>>>> On 03/13/2016 06:14 PM, Heiner Kallweit wrote:
>>>>>>>>>>>>>> Add basic support for RGB triggers. Triggers with flag LED_TRIG_CAP_RGB
>>>>>>>>>>>>>> set are available to RGB LED devices only.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Signed-off-by: Heiner Kallweit <hkallweit1@...il.com>
>>>>>>>>>>>>>> ---
>>>>>>>>>>>>>>         drivers/leds/led-triggers.c | 15 ++++++++++++---
>>>>>>>>>>>>>>         include/linux/leds.h        |  3 +++
>>>>>>>>>>>>>>         2 files changed, 15 insertions(+), 3 deletions(-)
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> diff --git a/drivers/leds/led-triggers.c b/drivers/leds/led-triggers.c
>>>>>>>>>>>>>> index 2181581..3ccf88b 100644
>>>>>>>>>>>>>> --- a/drivers/leds/led-triggers.c
>>>>>>>>>>>>>> +++ b/drivers/leds/led-triggers.c
>>>>>>>>>>>>>> @@ -30,6 +30,13 @@ static LIST_HEAD(trigger_list);
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>          /* Used by LED Class */
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> +static inline bool led_trig_check_rgb(struct led_trigger *trig,
>>>>>>>>>>>>>> +                      struct led_classdev *led_cdev)
>>>>>>>>>>>>>> +{
>>>>>>>>>>>>>> +    return !(trig->flags & LED_TRIG_CAP_RGB) ||
>>>>>>>>>>>>>> +           led_cdev->flags & LED_DEV_CAP_RGB;
>>>>>>>>>>>>>> +}
>>>>>>>>>>>>>
>>>>>>>>>>>>> Could you explain what is the purpose of this function?
>>>>>>>>>>>>> What actually do we want to check here?
>>>>>>>>>>>>>
>>>>>>>>>>>> Triggers using RGB functionality can't be used with non-RGB LED's.
>>>>>>>>>>>> This check checks for such unsupported combinations:
>>>>>>>>>>>> It returns false if the trigger uses RGB functionality but LED doesn't
>>>>>>>>>>>> support the RGB extension.
>>>>>>>>>>>
>>>>>>>>>>> We need more meaningful name for it. Maybe led_trigger_is_supported() ?
>>>>>>>>>>> And let's make it no-op for !CONFIG_LEDS_CLASS_RGB case.
>>>>>>>>>>>
>>>>>>>>>> OK, led_trigger_is_supported() is better.
>>>>>>>>>>
>>>>>>>>>> Making the function a no-op in the non-RGB case would have some impact:
>>>>>>>>>> We'd have to make sure that all public trigger functions are a de-facto no-op
>>>>>>>>>> for RGB triggers (at least register / unregister). Means we would need
>>>>>>>>>> something like this in each public trigger function:
>>>>>>>>>>
>>>>>>>>>> #if !IS_ENABLED(CONFIG_LEDS_CLASS_RGB)
>>>>>>>>>>         if (trig->flags & LED_TRIG_CAP_RGB))
>>>>>>>>>>             return;
>>>>>>>>>> #endif
>>>>>>>>>>
>>>>>>>>>> I think this would add a lot of overhead and therefore IMHO it's better to
>>>>>>>>>> not make the check function a no-op.
>>>>>>>>>
>>>>>>>>> Wouldn't it suffice to make the no-op returning true?
>>>>>>>>> Preventing RGB trigger registration for non-RGB LED class configuration
>>>>>>>>> seems to be different thing, also to be considered.
>>>>>>>>>
>>>>>>>> No, it's not sufficient. Let's say the RGB extension is disabled and we have a RGB trigger.
>>>>>>>> The check is a no-op now (returns always true), therefore the RGB trigger would be displayed
>>>>>>>> in the list of available triggers also for all non-RGB LED's.
>>>>>>>
>>>>>>> If RGB trigger was made dependent on LED RGB class, then the related
>>>>>>> Kconfig symbol would remain undefined in !CONFIG_LEDS_CLASS_RGB case.
>>>>>>>
>>>>>> Making a RGB trigger dependent on LED RGB class would mean to enclose all calls to trigger
>>>>>> functions in the RGB trigger like this:
>>>>>> #if IS_ENABLED(CONFIG_LEDS_CLASS_RGB)
>>>>>>       trigger_function()
>>>>>> #endif
>>>>>
>>>>> You probably think about the case when we have two triggers in
>>>>> single module, like in the planned {rgb-}heartbeat case?
>>>>>
>>>>> If so this is an argument for having RGB triggers in separate files.
>>>>>
>>>> I mean the case of triggers implemented outside drivers/leds. There the trigger code
>>>> often is not separated from other functionality (e.g. drivers/tty/vt/keyboard.c)
>>>> and it's not directly under our (LED core) control.
>>>>
>>>>>> This would apply to led_trigger_(un)register, led_trigger_event, led_trigger_blink, etc.
>>>>>> And I think it wouldn't be too nice to force other kernel modules wanting to implement
>>>>>> a RGB trigger to add these conditional compile statements.
>>>>>
>>>>> What other modules do you have on mind? LED triggers are implemented in
>>>>> their own files.
>>>>>
>>>> That's true for the triggers under drivers/leds/trigger, but not necessarily for triggers
>>>> implemented in other parts of the kernel.
>>>
>>> In this case surrounding all the trigger implementation  with
>>> IS_ENABLED(CONFIG_LEDS_CLASS_RGB) guard would do.
>>
>> Yes, that's what would need to be done. But IMHO it's not nice to force trigger implementations
>> in other parts of the kernel to guard each trigger-related call this way.
> 
> My main objection is that led_trigger_is_supported() would be redundant
> in led_trigger_store() and led_trigger_show() for non-RGB LED subsystem
> configuration.
> 
Yes, it's redundant for non-RGB configurations. But it affects sysfs access only
and overhead / impact should be minimal to negligible.

>> Also it might happen
>> that a trigger is implemented w/o this guarding and w/o informing you.
>> Then this (RGB) trigger would show up also for all non-RGB LED's.
> 
> It is likely that it wouldn't compile without led-rgb-core.o.
> 
It would compile because the only relevant difference between a RGB and a non-RGB trigger is a flag
being set in struct led_trigger.

>> I still think that not making led_trigger_is_supported() a no-op in the non-RGB case is a small
>> price for preventing such potential issues.
> 
> We could avoid the issues by adding a guard in led_trigger_register(),
> that would prevent RGB trigger registration in !CONFIG_LEDS_CLASS_RGB
> case.
> 
With "preventing registration" most likely you mean registering being a no-op.
I'm afraid we'd need the same also in all other public trigger functions, because it may cause
problems if registering is a no-op and we call e.g. led_trigger_event then (not being a no-op).
That's what I meant when I wrote earlier in this thread that we might need something like this
in all exported trigger functions:

#if !IS_ENABLED(CONFIG_LEDS_CLASS_RGB)
	if (trig->flags & LED_TRIG_CAP_RGB))
		return;
#endif

And this seems to be much more overhead than the one check in sysfs access not being a no-op
in the non-RGB case.

>>> In the aformentioned drivers/tty/vt/keyboard.c we have even more generic
>>> IS_ENABLED(CONFIG_LEDS_TRIGGERS) guard anyway.
>>>
>>>>>> Alternatively, as mentioned before, we would have to add this to all public trigger functions:
>>>>>>
>>>>>> #if !IS_ENABLED(CONFIG_LEDS_CLASS_RGB)
>>>>>>       if (trig->flags & LED_TRIG_CAP_RGB))
>>>>>>           return;
>>>>>> #endif
>>>>>> I think this would add significant overhead w/o gaining really something.
>>>>>>
>>>>>>>> We could maximum remove the "|| led_cdev->flags & LED_DEV_CAP_RGB" from the check if
>>>>>>>> the RGB extension is disabled. But it's open whether this minimal gain in a non-critical
>>>>>>>> code path justifies this.
>>>>>>>>
>>>>>>>>>>>>>>         ssize_t led_trigger_store(struct device *dev, struct device_attribute *attr,
>>>>>>>>>>>>>>                 const char *buf, size_t count)
>>>>>>>>>>>>>>         {
>>>>>>>>>>>>>> @@ -52,12 +59,12 @@ ssize_t led_trigger_store(struct device *dev, struct device_attribute *attr,
>>>>>>>>>>>>>>             down_read(&triggers_list_lock);
>>>>>>>>>>>>>>             list_for_each_entry(trig, &trigger_list, next_trig) {
>>>>>>>>>>>>>>                 if (sysfs_streq(buf, trig->name)) {
>>>>>>>>>>>>>> +            if (!led_trig_check_rgb(trig, led_cdev))
>>>>>>>>>>>>>> +                break;
>>>>>>>>>>>>
>>>>>>>>>>>> Check for the case that userspace wants to set a RGB trigger for a non-RGB LED via sysfs.
>>>>>>>>>>>>
>>>>>>>>>>>>>>                     down_write(&led_cdev->trigger_lock);
>>>>>>>>>>>>>>                     led_trigger_set(led_cdev, trig);
>>>>>>>>>>>>>>                     up_write(&led_cdev->trigger_lock);
>>>>>>>>>>>>>> -
>>>>>>>>>>>>>> -            up_read(&triggers_list_lock);
>>>>>>>>>>>>>> -            goto unlock;
>>>>>>>>>>>>>> +            break;
>>>>>>>>>>>
>>>>>>>>>>> This seems to be an unrelated cleanup. Please submit it separately.
>>>>>>>>>>>
>>>>>>>>>> OK
>>>>>>>>>>
>>>>>>>>>>>>>>                 }
>>>>>>>>>>>>>>             }
>>>>>>>>>>>>>>             up_read(&triggers_list_lock);
>>>>>>>>>>>>>> @@ -84,6 +91,8 @@ ssize_t led_trigger_show(struct device *dev, struct device_attribute *attr,
>>>>>>>>>>>>>>                 len += sprintf(buf+len, "none ");
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>             list_for_each_entry(trig, &trigger_list, next_trig) {
>>>>>>>>>>>>>> +        if (!led_trig_check_rgb(trig, led_cdev))
>>>>>>>>>>>>>> +            continue;
>>>>>>>>>>>>
>>>>>>>>>>>> Omit RGB triggers when listing the available triggers for a non-RGB LED via sysfs.
>>>>>>>>>>>>
>>>>>>>>>>>>>>                 if (led_cdev->trigger && !strcmp(led_cdev->trigger->name,
>>>>>>>>>>>>>>                                     trig->name))
>>>>>>>>>>>>>>                     len += sprintf(buf+len, "[%s] ", trig->name);
>>>>>>>>>>>>>> diff --git a/include/linux/leds.h b/include/linux/leds.h
>>>>>>>>>>>>>> index 58e22e6..07eb074 100644
>>>>>>>>>>>>>> --- a/include/linux/leds.h
>>>>>>>>>>>>>> +++ b/include/linux/leds.h
>>>>>>>>>>>>>> @@ -248,6 +248,9 @@ enum led_brightness led_hsv_to_rgb(enum led_brightness hsv);
>>>>>>>>>>>>>>         struct led_trigger {
>>>>>>>>>>>>>>             /* Trigger Properties */
>>>>>>>>>>>>>>             const char     *name;
>>>>>>>>>>>>>> +    u8        flags;
>>>>>>>>>>>>>> +#define LED_TRIG_CAP_RGB    BIT(0)
>>>>>>>>>>>>>> +
>>>>>>>>>>>>>>             void        (*activate)(struct led_classdev *led_cdev);
>>>>>>>>>>>>>>             void        (*deactivate)(struct led_classdev *led_cdev);
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>
>>
>>
> 
> 

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