linux-kernel - Re: [PATCH v4 1/5] dt-bindings: misc: Add mikrobus-connector

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Date: Thu, 21 Mar 2024 10:38:23 +0100
From: "Michael Walle" <mwalle@...nel.org>
To: "Vaishnav Achath" <vaishnav.a@...com>, "Andrew Lunn" <andrew@...n.ch>
Cc: "Russell King (Oracle)" <linux@...linux.org.uk>, "Ayush Singh"
 <ayushdevel1325@...il.com>, "open list" <linux-kernel@...r.kernel.org>,
 <jkridner@...gleboard.org>, <robertcnelson@...gleboard.org>,
 <lorforlinux@...gleboard.org>, "Rob Herring" <robh@...nel.org>, "Krzysztof
 Kozlowski" <krzysztof.kozlowski+dt@...aro.org>, "Conor Dooley"
 <conor+dt@...nel.org>, "Nishanth Menon" <nm@...com>, "Vignesh Raghavendra"
 <vigneshr@...com>, "Tero Kristo" <kristo@...nel.org>, "Derek Kiernan"
 <derek.kiernan@....com>, "Dragan Cvetic" <dragan.cvetic@....com>, "Arnd
 Bergmann" <arnd@...db.de>, "Greg Kroah-Hartman"
 <gregkh@...uxfoundation.org>, "Mark Brown" <broonie@...nel.org>, "Johan
 Hovold" <johan@...nel.org>, "Alex Elder" <elder@...nel.org>, "open
 list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS"
 <devicetree@...r.kernel.org>, "moderated list:ARM/TEXAS INSTRUMENTS K3
 ARCHITECTURE" <linux-arm-kernel@...ts.infradead.org>, "open list:SPI
 SUBSYSTEM" <linux-spi@...r.kernel.org>, "moderated list:GREYBUS SUBSYSTEM"
 <greybus-dev@...ts.linaro.org>, "Vaishnav M A" <vaishnav@...gleboard.org>
Subject: Re: [PATCH v4 1/5] dt-bindings: misc: Add mikrobus-connector

Hi,

> > Is that because the current software support is too limited? Are there
> > manufactures who want to create more complex designed, but are limited
> > by what can be described in the manifest?
> > 
>
> most mikroBUS add-on boards in production lies in the category of 
> sensors, displays, connectivity, mixed signal (ADC/DAC .etc) and if you 
> look at the existing bindings under bindings/iio/ , most devices need 
> only simple descriptions and the properties are mainly standard bus 
> properties (SPI/I2C properties), IRQ, named-gpios, named properties, 
> regulators, clocks the extension to manifest was made taking this into 
> account and the named property description interface just maps the 
> manifest entries to the unified device property interface under 
> include/linux/property.h

How will the ethernet boards ([1], [2]) work? Where do they get
their MAC address from, for example. The DT has some nice properties
for that, but I doubt that will be possible with the manifest files.
I've looked at the manifest file for the w5500 board [3] and to me
it looks like that board will come up with a random MAC address on
each start. Thus, even today, you have some boards which require
a more complex description.

Apart from the discussion whether the manifest is a suitable or
sufficient mechanism to describe the hardware, I think the main
problem with the proposed binding, is that it doesn't follow the
binding Rob was proposing for a socket. If I want to use DT
overlays, how would you describe an add-on board?

The proposal was that the base board has something like

mikrobus: socket {
	compatible = "mikrobus-socket";
	i2c-parent = <&i2c0>;
	spi-parent = <&spi0>;

	i2c {};
	spi {};
};

an add-on board can then have a DT snippet/overlay like the
following:

&mikrobus {
	i2c {
		eeprom@52: {
			reg = <52>;
			compatible = <atmel,at24..>;
		}
	};

	spi {
		sensor@0: {
			reg = <0>;
			compatible = <foobar>;
		};
	};
};

That should be possible with a binding for the mikrobus, which
in fact it is just a pin header with a standard pinout. Also as
Russell pointed out in v3, the EEPROM/manifest is not part of the
mikrobus standard. So maybe that deserves an own compatible, like

   compatible = "mikroe,click", "mikrobus-socket";

Or maybe click-eeprom? Although click seems to be the brand name of
MikroElektronika.

-michael

[1] https://www.mikroe.com/eth-3-click
[2] https://www.mikroe.com/eth-wiz-click
[3] https://github.com/MikroElektronika/click_id/blob/main/manifests/ETH-WIZ-CLICK.mnfs