linux-kernel - Re: [PATCH v3] leds: Introduce userspace leds driver

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Date:   Fri, 16 Sep 2016 10:32:16 -0500
From:   David Lechner <david@...hnology.com>
To:     Pavel Machek <pavel@....cz>
Cc:     Jacek Anaszewski <j.anaszewski@...sung.com>,
        Richard Purdie <rpurdie@...ys.net>,
        linux-kernel@...r.kernel.org, linux-leds@...r.kernel.org,
        Marcel Holtmann <marcel@...tmann.org>
Subject: Re: [PATCH v3] leds: Introduce userspace leds driver

On 09/16/2016 12:59 AM, Pavel Machek wrote:
> Hi!
>
> +The current brightness is found by reading a single byte from the
> character
> +device. Values are unsigned: 0 to 255. Reading does not block and
> always returns
> +the most recent brightness value. The device node can also be polled
> to notify
> +when the brightness value changes.
>
> What is going on there? We have O_NONBLOCK, user should be able to
> select if he wants blocking behaviour or not.

I will look into this.

>
> And yes, there's interface for creating a LED, but not one for
> deleting it?

uleds.txt says "The LED class device will be removed when the open file 
handle to /dev/uleds is closed."

Is this not clear?

>
> And... how is it going to work with multiple LEDs? Userspace gets
> single bytes with brightness. How does it know to which LED the
> brightness belongs?

It looks like this is missing from uleds.txt. By opening multiple file 
handles to /dev/uleds, you can create multiple devices. Each file handle 
will return a different value when read that corresponds to the LEDs 
class device that belongs to it.