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

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Message-ID: <f5e74774-226a-4678-a6f9-b4f17250ad91@alliedtelesis.co.nz>
Date: Thu, 22 Feb 2024 21:53:42 +0000
From: Chris Packham <Chris.Packham@...iedtelesis.co.nz>
To: "andy.shevchenko@...il.com" <andy.shevchenko@...il.com>
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 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.

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.