Message-id: <556C69EA.10000@samsung.com>
Date:	Mon, 01 Jun 2015 16:19:22 +0200
From:	Jacek Anaszewski <j.anaszewski@...sung.com>
To:	Stas Sergeev <stsp@...t.ru>
Cc:	linux-leds@...r.kernel.org,
	Linux kernel <linux-kernel@...r.kernel.org>,
	Stas Sergeev <stsp@...rs.sourceforge.net>,
	Bryan Wu <cooloney@...il.com>,
	Richard Purdie <rpurdie@...ys.net>,
	Kyungmin Park <kyungmin.park@...sung.com>
Subject: Re: [PATCH 01/20] leds: implement LED_BRIGHTNESS_FAST flag

On 06/01/2015 01:56 PM, Stas Sergeev wrote:
> 01.06.2015 11:31, Jacek Anaszewski пишет:
>> With this approach, the LED will remain in its current blink state, in
>> case LED_BRIGHTNESS_FAST flag is not set and delay to be set is below LED_SLOW_MIN_PERIOD. This is because timer is deleted at the beginning
>> of led_blink_set. Effectively this can result in the LED staying turned
>> on when e.g. "echo 1 > delay_on" fails.
>>
>> I propose to change this part of code as follows:
>>
>>          if (!(led_cdev->flags & LED_BRIGHTNESS_FAST)) {
>>          if (delay_on < LED_SLOW_MIN_PERIOD ||
>>              delay_off < LED_SLOW_MIN_PERIOD)
>>              led_set_brightness_async(led_cdev, LED_OFF);
>>              return -ERANGE;
>>          }
>>      }
> Sounds good.
>
>> By this occasion it turned out that we have inconsistency:
>> led_set_brightness_async calls brightness_set op, which doesn't
>> set LED brightness in an asynchronous way in case of every driver.
>>
>> Before introduction of SET_BRIGHTNESS_SYNC and SET_BRIGHTNESS_ASYNC
>> flags there was only brightness_set op. The flags and
>> brightness_set_sync op was added for flash LED controllers to avoid
>> delegating the job to work queue and assure that brightness is set
>> as quickly as possible.
>>
>> I mistakenly assumed that all drivers set the brightness with use
>> of a work queue.
> Indeed.
>
>> Now, to fix the issue all drivers that set brightness in the
>> asynchronous way need to set the SET_BRIGHTNESS_SYNC flag and rename
>> their brightness_set ops to brightness_set_sync.
>> It would be also nice to rename brightness_set op to
>> brightness_set_async.
>>
>> Also, led_set_brightness_async shouldn't be called directly
>> anywhere, but led_set_brightness should be used, as it
>> selects the appropriate op basing on the SET_BRIGHTNESS_* flag.
>>
>> I'd prefer to do these modifications before merging this patch
> Sounds good: I wonder if the new flag for FAST drivers will then
> be needed at all: maybe it would be enough for the driver to define
> a SYNC method, and we assume the SYNC methods are always fast?

Flash LED controllers also set SYNC flag, but they are not fast.
They require to implement async method (current brightness_set),
for staying compatible with triggers.

> In fact, the things are more complicated: some drivers do small
> udelay()'s but do not use a work-queue. I was not marking them as
> FAST, although perhaps they could still be marked as SYNC?

This could be handled by adding a property to struct led_classdev
for defining minimum acceptable delay. Then FAST flag should not
be needed.

>> set. I can produce the patch set within a few days. Or, maybe you
>> would like to take care of it?
> I would appreciate if you do.
> I very much hate to do any non-trivial changes to the code I can't
> even properly test (sometimes not even compile-test!).

OK, I'll do that.

> Since you are at it, I'd also suggest to kill the work-queue in
> led-core.c. Now that we fixed a scenario where it was mistakenly
> used, I again think it is not needed for what it was initially advertised.
>

OK, I'll check this.

-- 
Best Regards,
Jacek Anaszewski
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