linux-kernel - Re: Linux support for a 7 segment LED display

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Message-ID: <CAHp75VdnXtVwrLrcGjnYMfNx7roBvQm9DMr6_ndjZeAbRDbs_Q@mail.gmail.com>
Date: Fri, 23 Feb 2024 00:13:47 +0200
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Chris Packham <Chris.Packham@...iedtelesis.co.nz>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, Pavel Machek <pavel@....cz>, 
	"lee@...nel.org" <lee@...nel.org>, "linux-leds@...r.kernel.org" <linux-leds@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Linux support for a 7 segment LED display

On Fri, Feb 23, 2024 at 12:08 AM Chris Packham
<Chris.Packham@...iedtelesis.co.nz> wrote:
> On 23/02/24 10:59, Andy Shevchenko wrote:
> > On Thu, Feb 22, 2024 at 11:53 PM Chris Packham
> > <Chris.Packham@...iedtelesis.co.nz> wrote:
> >> On 23/02/24 10:34, andy.shevchenko@...il.com wrote:
> >>> Sun, Feb 11, 2024 at 08:46:12PM +0000, Chris Packham kirjoitti:
> >>>> Hi Blinkenlight enthusiasts,
> >>>>
> >>>> I'm looking for something that I figured must exists but maybe it's so
> >>>> niche that no-one has bothered to upstream a driver for it.
> >>>>
> >>>> I have a requirement to support a 7-segment LED display[1] (one that can
> >>>> display a single digit from 0-9). Hardware wise it's just a bunch of
> >>>> individual GPIOs connected to each segment (plus an extra one for a
> >>>> dot). I can't see anything obvious in drivers/leds but maybe I'm looking
> >>>> in the wrong place. Or maybe it's the kind of thing on PC hardware that
> >>>> is just driven by the BIOS without the operating system knowing about it.
> >>>>
> >>>> Is there an existing in-kernel driver for such a thing?
> >>> No, and can't be. Here is just a mapping table and other drivers that use
> >>> 7-segment LED displays to be connected to.
> >>>
> >>> What you need is something else, i.e. special case of leds-gpio (which should
> >>> be somewhere else) that does something like this. To me it sounds like a
> >>> mixture between line-display.h (from auxdisplay) and gpio-aggregator.
> >>>
> >>> How many digits do you want to connect? How are they going to be connected
> >>> (static display, or dynamic when you need to refresh in certain periods of
> >>> time)? Depending on the answer it might take one or another approach.
> >> It sounds like a auxdisplay driver might be the way to go. My hardware
> >> happens to have a single 7seg block but there's no reason the driver
> >> needs to be restricted to that. At some point it obviously becomes
> >> better to fit something like the ht16k33 to offload the character
> >> display but for one or 2 digits a PCA953x plus the LED block would do
> >> just fine.
> > I have hc595 (SPI GPIO) connected to a single digit 7-segment LED.
> > Since it can be also serialized, line display APIs seem plausible to
> > fit. What we need is a proxy between the two. And I think
> > gpio-aggregator is the best for that. It needs an additional
> > compatible string and the registration for line display (overall
> > something like 50 LoCs). We can even call that hardware compatible as
> > line-display-gpio (or so).
> >
> > Cc: Geert and Krzysztof (for the comments on the idea above).
>
> Would the gpio-aggregator be necessary?

Probably not. we just need to consume GPIOs, no need to provide...

> I was thinking something like
> this in the devicetree
>
> \{
>      led-7seg {
>          compatible = "kingbright,sa36";
>          seg-gpios = <&gpioN 0 GPIO_ACTIVE_HIGH>,
>              <&gpioN 1 GPIO_ACTIVE_HIGH>,
>              <&gpioN 2 GPIO_ACTIVE_HIGH>,
>              <&gpioN 3 GPIO_ACTIVE_HIGH>,
>              <&gpioN 4 GPIO_ACTIVE_HIGH>,
>              <&gpioN 5 GPIO_ACTIVE_HIGH>,
>              <&gpioN 6 GPIO_ACTIVE_HIGH>;
>      };
> };

Right, but also with some additional properties, like
- type of the indicator (7, 14, etc segments, for now we have only
these two supported)
- use-dot (+1 led per each digit)
- characters (how many digits we have, so we need respective amount of GPIOs)

With this it's indeed belongs directly to auxdisplay as we have almost
everything is done already there.

> >> The information we want to display is mostly static (basically a numeric
> >> unit identifier) but there are cases where we may want to alternate
> >> between this and "F" to indicate some fault condition

-- 
With Best Regards,
Andy Shevchenko