linux-kernel - Re: [PATCH v2 18/18] auxdisplay: ht16k33: Add segment display LED support

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Date:   Mon, 28 Jun 2021 12:15:51 +0200
From:   Marek Behun <marek.behun@....cz>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Robin van der Gracht <robin@...tonic.nl>,
        Rob Herring <robh+dt@...nel.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Paul Burton <paulburton@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Pavel Machek <pavel@....cz>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        linux-leds <linux-leds@...r.kernel.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 18/18] auxdisplay: ht16k33: Add segment display LED
 support

On Mon, 28 Jun 2021 11:21:04 +0200
Geert Uytterhoeven <geert@...ux-m68k.org> wrote:

> Hi Marek,
> 
> On Fri, Jun 25, 2021 at 10:39 PM Marek Behun <marek.behun@....cz> wrote:
> > On Fri, 25 Jun 2021 14:59:02 +0200
> > Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> >  
> > > Instantiate a single LED for a segment display.  This allows the user to
> > > control display brightness and blinking through the LED class API and
> > > triggers, and exposes the display color.
> > > The LED will be named "auxdisplay:<color>:backlight".  
> >
> > What if there are multiple "auxdisplay"s ?  
> 
> I understand the LED core will just add a suffix on a name collision.
> 
> > Doesn't this subsystem have IDs? So that you can use auxdisplayN for
> > device name, for example?  
> 
> Auxdisplay does not have IDs, as there is no subsystem to register
> with.  It's just a collection of drivers for auxiliary displays with
> no common API.  Some drivers use fbdev, others use a chardev, or an
> attribute file in sysfs.
> 
> BTW, I just followed Pavel's advice in naming.

Very well.

> > > +     of_property_read_u32(node, "color", &color);
> > > +     seg->led.name = devm_kasprintf(dev, GFP_KERNEL,
> > > +                     "auxdisplay:%s:" LED_FUNCTION_BACKLIGHT,
> > > +                     color < LED_COLOR_ID_MAX ? led_colors[color] : "");  
> >
> > If you use devm_led_classdev_register_ext and pass struct
> > led_init_data, LED core will generate name of the LED itself.  
> 
> Will that make any difference, except for adding more code?

You are hardcoding the backlight function. Using the _ext() registering
function will make it so that the function and color are parsed from
fwnode by LED core. I understand that the function will always be
"backlight" in this case, but this should be specified in the
device-tree anyway, so why not use it?

> Looking at the implementation, I still have to use devm_kasprintf()
> to combine color and function for led_init_data.default_label?

AFAIK you don't have to fill in default_label. (If the needed OF
properties are not present so that default_label is tried, it means the
device-tree does not correctly specify the device. In that case I don't
think it is a problem if the default_label is not present and LED
core will use the OF node name as the LED name.)

The code could look like this

  struct led_init_data init_data = {};

  init_data.fwnode = of_fwnode_handle(node);
  init_data.devicename = "auxdisplay";
  init_data.devname_mandatory = true;

  ...register_ext();

But if you still don't want to do this then ignore me :)

Marek