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Date:	Thu, 07 Aug 2014 15:12:09 +0200
From:	Jacek Anaszewski <>
To:	Sakari Ailus <>
	Bryan Wu <>,
	Richard Purdie <>
Subject: Re: [PATCH/RFC v4 06/21] leds: add API for setting torch brightness

Hi Sakari,

On 08/04/2014 02:50 PM, Sakari Ailus wrote:
> Hi Jacek,
> Thank you for your continued efforts on this!
> On Mon, Aug 04, 2014 at 02:35:26PM +0200, Jacek Anaszewski wrote:
>> On 07/16/2014 11:54 PM, Sakari Ailus wrote:
>>> Hi Jacek,
>>> Jacek Anaszewski wrote:
>>> ...
>>>> diff --git a/include/linux/leds.h b/include/linux/leds.h
>>>> index 1a130cc..9bea9e6 100644
>>>> --- a/include/linux/leds.h
>>>> +++ b/include/linux/leds.h
>>>> @@ -44,11 +44,21 @@ struct led_classdev {
>>>>   #define LED_BLINK_ONESHOT_STOP    (1 << 18)
>>>>   #define LED_BLINK_INVERT    (1 << 19)
>>>>   #define LED_SYSFS_LOCK        (1 << 20)
>>>> +#define LED_DEV_CAP_TORCH    (1 << 21)
>>>>       /* Set LED brightness level */
>>>>       /* Must not sleep, use a workqueue if needed */
>>>>       void        (*brightness_set)(struct led_classdev *led_cdev,
>>>>                         enum led_brightness brightness);
>>>> +    /*
>>>> +     * Set LED brightness immediately - it is required for flash led
>>>> +     * devices as they require setting torch brightness to have
>>>> immediate
>>>> +     * effect. brightness_set op cannot be used for this purpose because
>>>> +     * the led drivers schedule a work queue task in it to allow for
>>>> +     * being called from led-triggers, i.e. from the timer irq context.
>>>> +     */
>>> Do we need to classify actual devices based on this? I think it's rather
>>> a different API behaviour between the LED and the V4L2 APIs.
>>> On devices that are slow to control, the behaviour should be asynchronous
>>> over the LED API and synchronous when accessed through the V4L2 API. How
>>> about implementing the work queue, as I have suggested, in the
>>> framework, so
>>> that individual drivers don't need to care about this and just implement
>>> the
>>> synchronous variant of this op? A flag could be added to distinguish
>>> devices
>>> that are fast so that the work queue isn't needed.
>>> It'd be nice to avoid individual drivers having to implement multiple
>>> ops to
>>> do the same thing, just for differing user space interfacs.
>> It is not only the matter of a device controller speed. If a flash
>> device is to be made accessible from the LED subsystem, then it
>> should be also compatible with led-triggers. Some of led-triggers
>> call brightness_set op from the timer irq context and thus no
>> locking in the callback can occur. This requirement cannot be
>> met i.e. if i2c bus is to be used. This is probably the primary
>> reason for scheduling work queue tasks in brightness_set op.
>> Having the above in mind, setting a brightness in a work queue
>> task must be possible for all LED Class Flash drivers, regardless
>> whether related devices have fast or slow controller.
>> Let's recap the cost of possible solutions then:
>> 1) Moving the work queues to the LED framework
>>    - it would probably require extending led_set_brightness and
>>      __led_set_brightness functions by a parameter indicating whether it
>>      should call brightness_set op in the work queue task or directly;
>>    - all existing triggers would have to be updated accordingly;
>>    - work queues would have to be removed from all the LED drivers;
>> 2) adding led_set_torch_brightness API
>>    - no modifications in existing drivers and triggers would be required
>>    - instead, only the modifications from the discussed patch would
>>      be required
>> Solution 1 looks cleaner but requires much more modifications.
> How about a combination of the two, i.e. option 1 with the old op remaining
> there for compatibility with the old drivers (with a comment telling it's
> deprecated)?
> This way new drivers will benefit from having to implement this just once,
> and modifications to the existing drivers could be left for later.

It's OK for me, but the opinion from the LED side guys is needed here
as well.

> The downside is that any old drivers wouldn't get V4L2 flash API but that's
> entirely acceptable in my opinion since these would hardly be needed in use
> cases that would benefit from V4L2 flash API.

In the version 4 of the patch set I changed the implementation, so that
a flash led driver must call led_classdev_flash_register to get
registered as a LED Flash Class device and v4l2_flash_init to get
V4L2 Flash API. In effect old drivers will have no chance to get V4L2
Flash API either way.

Best Regards,
Jacek Anaszewski
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