linux-kernel - Re: [PATCH v8 0/5] auxdisplay: add support for TI LP5812 4x3 Matrix LED driver

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Message-Id: <20250507164219.10083-1-trannamatk@gmail.com>
Date: Wed,  7 May 2025 23:42:19 +0700
From: Nam Tran <trannamatk@...il.com>
To: andy@...nel.org
Cc: geert@...ux-m68k.org,
	pavel@....cz,
	robh@...nel.org,
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	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v8 0/5] auxdisplay: add support for TI LP5812 4x3 Matrix LED driver

On Tue, 29 Apr 2025 Andy Shevchenko wrote:

> On Tue, Apr 29, 2025 at 8:02 PM Nam Tran <trannamatk@...il.com> wrote:
> > On Mon, 28 Apr 2025 Pavel Machek wrote:
> > > > On Mon, 28 Apr 2025 Geert Uytterhoeven wrote:
> > >
> > > > > > > > - Move driver to drivers/auxdisplay/ instead of drivers/leds/.
> > > > > > > > - Rename files from leds-lp5812.c/.h to lp5812.c/.h.
> > > > > > > > - Move ti,lp5812.yaml binding to auxdisplay/ directory,
> > > > > > > >   and update the title and $id to match new path.
> > > > > > > > - No functional changes to the binding itself (keep Reviewed-by).
> > > > > > > > - Update commit messages and patch titles to reflect the move.
> > > > > > > > - Link to v7: https://lore.kernel.org/linux-leds/20250422190121.46839-1-trannamatk@gmail.com/
> > > > > > >
> > > > > > > Out of sudden without discussing with auxdisplay maintainers/reviewers?
> > > > > > > Thanks, no.
> > > > > > > Please, put into the cover letter the meaningful summary of what's
> > > > > > > going on and why this becomes an auxdisplay issue. Brief review of the
> > > > > > > bindings sounds more likely like LEDS or PWM subsystems.
> > > > > >
> > > > > > It is 4x3 matrix. That means it is not suitable for LEDs. I don't
> > > > > > believe it is suitable for PWM, either -- yes, it is 36 PWM outputs,
> > > > > > but...
> > > > >
> > > > > Is it intended to be used as a 4x3 matrix, or is this just an internal
> > > > > wiring detail, and should it be exposed as 12 individual LEDs instead?
> > > >
> > > > The 4×3 matrix is a real and fundamental aspect of the LP5812’s operation.
> > > > It is not just an internal wiring detail.
> > > > The device adopts a Time-Cross-Multiplexing (TCM) structure, where 4 output
> > > > pins control 12 LED dots individually through scanning. Each pin includes
> > > > both high-side and low-side drive circuits, meaning matrix multiplexing is
> > > > required for proper operation — it cannot be treated as 12 completely
> > > > independent LEDs.
> > >
> > > Scanning is really a detail.
> > >
> > > If this is used as rectangular 4x3 display, then it goes to auxdisplay.
> > >
> > > If this is used as a power LED, SD activity LED, capslock and numlock
> > > ... placed randomly all around the device, then it goes LED subsystem.
> >
> > The LP5812 is used for LED status indication in devices like smart speakers,
> > wearables, and routers, not as a structured rectangular display.
> >
> > Given that, it seems to match the LED subsystem better than auxdisplay, doesn't it?
> 
> I have mixed feelings about all this. As per hardware organisation it
> sounds more like a matrix (for example. keyboard), where all entities
> are accessed on a scanline, but at the same time each of the entities
> may have orthogonal functions to each other. Have you checked with DRM
> for the sake of completeness?
> Personally I lean more to the something special, which doesn't fit
> existing subsystems. Auxdisplay subsystem more or less about special
> alphanumeric displays (with the exception of some FB kinda devices,
> that were even discussed to have drivers be removed). Also maybe FB
> might have something suitable, but in any case it looks quite
> non-standard...

I understand your mixed feelings about where the LP5812 fits within
the existing subsystems.

While the LP5812 uses a matrix-based structure for controlling LEDs,
it is not intended for displaying structured text or graphics. Instead,
it controls up to 4 RGB LEDs for status indication, where each RGB LED
consists of 3 individual color LEDs: red, green, and blue. Based on this,
I think it aligns more closely with the LED subsystem rather than DRM or FB.

Best regards,
Nam Tran