Re: [PATCH/RFC v9 02/19] Documentation: leds: Add description of LED Flash class extension

From: Jacek Anaszewski
Date: Thu Dec 04 2014 - 04:43:10 EST

Hi Sakari,

Thanks for the review.

On 12/03/2014 06:08 PM, Sakari Ailus wrote:
Hi Jacek,

On Wed, Dec 03, 2014 at 05:06:37PM +0100, Jacek Anaszewski wrote:
The documentation being added contains overall description of the
LED Flash Class and the related sysfs attributes.

Signed-off-by: Jacek Anaszewski <j.anaszewski@xxxxxxxxxxx>
Acked-by: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
Cc: Bryan Wu <cooloney@xxxxxxxxx>
Cc: Richard Purdie <rpurdie@xxxxxxxxx>
Documentation/leds/leds-class-flash.txt | 50 +++++++++++++++++++++++++++++++
1 file changed, 50 insertions(+)
create mode 100644 Documentation/leds/leds-class-flash.txt

diff --git a/Documentation/leds/leds-class-flash.txt b/Documentation/leds/leds-class-flash.txt
new file mode 100644
index 0000000..82e58b1
--- /dev/null
+++ b/Documentation/leds/leds-class-flash.txt
@@ -0,0 +1,50 @@
+Flash LED handling under Linux
+Some LED devices support two modes - torch and flash. The modes are
+supported by the LED class (see Documentation/leds/leds-class.txt)
+and LED Flash class respectively.
+In order to enable support for flash LEDs CONFIG_LEDS_CLASS_FLASH symbol
+must be defined in the kernel config. A flash LED driver must register
+in the LED subsystem with led_classdev_flash_register to gain flash
+Following sysfs attributes are exposed for controlling flash led devices:
+ - flash_brightness - flash LED brightness in microamperes (RW)
+ - max_flash_brightness - maximum available flash LED brightness (RO)
+ - flash_timeout - flash strobe duration in microseconds (RW)
+ - max_flash_timeout - maximum available flash strobe duration (RO)
+ - flash_strobe - flash strobe state (RW)
+ - flash_sync_strobe - one flash device can control more than one
+ sub-led; when this atrribute is set to 1


+ the flash led will be strobed synchronously
+ with the other one controlled by the same
+ device; flash timeout setting is inherited
+ from the led being strobed explicitly and
+ flash brightness setting of a sub-led's
+ being synchronized is used (RW)

The flash brightness shouldn't be determined by the strobed LED. If this is
a property of the hardware, then be it, but in general no, it it shouldn't
be an interface requirement. I think this should just say that the strobe is

I intended this to sound exactly as you laid it out above, but maybe it
is obscure English. "and flash brightness setting of a sub-led >>>being
synchronized<<< is used" - from my point of view the led being
synchronized is the one that isn't strobed explicitly. But I'm ok with
confining ourselves only to saying that strobe is synchronized.

How does the user btw. figure out which flash LEDs may be strobed
synchronously using the LED flash interface?

The flash_sync_strobe argument is absent if synchronized strobe
is not available for a LED. The driver defines this by setting
newly added LED_DEV_CAP_COMPOUND flag.

Best Regards,
Jacek Anaszewski
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