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

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Message-ID: <CAHp75Vfq3Pc+Eo3Z5mhORZwwuKF+Y7_47dTO8qARcfba4nmj2w@mail.gmail.com>
Date: Thu, 22 Feb 2024 23:59:34 +0200
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Chris Packham <Chris.Packham@...iedtelesis.co.nz>, 
	Geert Uytterhoeven <geert@...ux-m68k.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>
Cc: 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 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).

> 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