linux-kernel - Re: [PATCH v2 0/3] leds: blink resolution improvements

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Message-id: <5540F573.1040908@samsung.com>
Date:	Wed, 29 Apr 2015 17:14:59 +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>
Subject: Re: [PATCH v2 0/3] leds: blink resolution improvements

On 04/29/2015 01:26 PM, Stas Sergeev wrote:
> 28.04.2015 15:58, Jacek Anaszewski пишет:
>> On 04/28/2015 12:12 PM, Stas Sergeev wrote:
>>> 28.04.2015 11:57, Jacek Anaszewski пишет:
>>>> Hi Stas,
>>>>
>>>> Have you tested it?
>>> Of course I did.
>>> Works with gpio driver and provides up to 10usec precision on
>>> armada-xp board.
>>> This is 1000 times better than without my patch - the precision
>>> was 10ms (jiffy).
>>
>> Please take into account that this could work reliably only for gpio
>> LEDs. For the LEDs driven though a bus (e.g. I2C) delays below 1ms
>> might be hard to achieve. The minimum available delay would vary from
>> driver to driver.
>>
>> We could think of adding the hr_timer mode to the led-class.
>> The mode could be turned on with use of a new led_set_high_res_timer
>> API. The API would be called by drivers/leds/leds-gpio.c driver when
>> a dedicated sysfs attribute was set adequately.
>> The other drivers could also set this mode if they controlled device
>> with a suitable LED switching rate. The minimum delay value could
>> be made configurable by the driver and readable through sysfs
>> when in hr_timer mode.
> I've found the SET_BRIGHTNESS_ASYNC and SET_BRIGHTNESS_SYNC flags.
> Sounds interesting for my patch, but the only "documentation" I was
> able to find, is this:
> ---
>          /* Setting a torch brightness needs to have immediate effect */
>          led_cdev->flags &= ~SET_BRIGHTNESS_ASYNC;
>          led_cdev->flags |= SET_BRIGHTNESS_SYNC;
> ---
> Aren't these flags mutually exclusive, and so just one could have
> been used?

Yes they are.

> Anyway, from that comment I can try to guess that if the driver
> supports ASYNC mode, it should be fast enough and without sleeps.
> The drivers that do i2c transfers with sleeps, should be using SYNC
> mode. Or was the intention for these flags entirely different?

Sync mode was designed for flash LED devices, for which turning
torch immediately has a big importance because they are often
synchronized with camera sensor.

Triggers use led_set_brightness_async API anyway.

> My intention is to use either a work-queue or a direct hrtimer
> callback, depending on whether the driver supports sync or async
> mode. This is instead of the driver being able to set the minimum
> delay - much simpler to implement. Makes sense?
> Can I use the above flags for that purpose, or will I need
> a new one?
>

These flags are intended for internal LED core use.


-- 
Best Regards,
Jacek Anaszewski
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