linux-kernel - Re: [RESEND PATCH v17 00/17] Multi Color LED Framework

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Message-ID: <20200228074235.GA2900429@kroah.com>
Date:   Fri, 28 Feb 2020 08:42:35 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Jacek Anaszewski <jacek.anaszewski@...il.com>
Cc:     Dan Murphy <dmurphy@...com>, Pavel Machek <pavel@...x.de>,
        linux-leds@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v17 00/17] Multi Color LED Framework

On Thu, Feb 27, 2020 at 10:22:43PM +0100, Jacek Anaszewski wrote:
> On 2/27/20 2:07 PM, Dan Murphy wrote:
> > Pavel
> > 
> > On 2/27/20 6:43 AM, Greg KH wrote:
> >> On Thu, Feb 27, 2020 at 11:58:08AM +0100, Pavel Machek wrote:
> >>> Hi, Jacek!
> >>>
> >>> (and thanks for doing this).
> >>>
> >>>> We have here long lasting discussion related to LED multicolor class
> >>>> sysfs interface design. We went through several iterations and worked
> >>>> out the solution with individual file per each color sub-LED in the
> >>>> color directory as shown below:
> >>>>
> >>>> /sys/class/leds/<led>/colors/<color>_intensity
> >>>>
> >>>> This is in line with one-value-per-file sysfs rule, that is being
> >>>> frequently highlighted, and we even had not so long ago a patch
> >>>> for led cpu trigger solving the problem caused by this rule not
> >>>> being adhered to.
> >>> Yep. One of the problems is that it is nice to change all the hardware
> >>> channels at once to produce color (it is often on i2c -- and slow), so
> >>> current proposals use "interesting" kind of latching.
> >>>
> >>>> Now we have the voice below bringing to attention another caveat
> >>>> from sysfs documentation:
> >>>>
> >>>> "it is socially acceptable to express an array of values of the same
> >>>> type"
> >>>>
> >>>> and proposing the interface in the form of two files:
> >>>>
> >>>> channel_intensity (file containing array of u32's)
> >>>> channel_names (usually containing "red green blue")
> >>> And thus I want to have it in one file, so it is naturaly atomic. RGB
> >>> leds with 3 channels are common; I have not user yet, but there are
> >>> RGBW with 4 channels (and some more exotic stuff). I don't expect to
> >>> have more than 5 channels.
> > 
> > This is not an accurate statement.  Right now a user can have up to 8
> > channels to cover all the LEDs defined in the LED core
> > 
> > And if the led_colors array expands then this array can expand.
> > 
> > We have no control on how many entries the user will put in their DT so
> > again this number is completely arbitrary.
> 
> I believe that some of mechanisms that were devised for the most
> recent implementation proposal of LED mc class will need
> to be reused for the array approach. E.g. available_colors bitmask
> will make the parsing resistant to duplicates.
> 
> Of course LED multicolor DT node design should be applicable as well
> to the array approach.
> 
> >> Writing 3 or 4 or 5 numbers all at once in a single sysfs file to
> >> represent a single output should be fine.
> >> thanks,
> 
> Thank you for making this clear.
> 
> Effectively, the way to go as I see it now is just moving from
> colors directory to channel_intensity and channel_names files.

Wait, we already have an interface for this and you want to create a
competing one?  Why?  What's wrong with what you have now?

Do you have a pointer to the Documentation/ABI/ entries that you have
now that you feel do not work well?

thanks,

greg k-h