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

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Message-Id: <CZXPQZY8PUGE.QZM8XSOUNMT4@kernel.org>
Date: Tue, 19 Mar 2024 13:08:33 +0100
From: "Michael Walle" <mwalle@...nel.org>
To: "Ayush Singh" <ayushdevel1325@...il.com>, "Krzysztof Kozlowski"
 <krzysztof.kozlowski@...aro.org>, "open list"
 <linux-kernel@...r.kernel.org>
Cc: <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>, "Vaishnav M A" <vaishnav.a@...com>, "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

On Tue Mar 19, 2024 at 12:36 PM CET, Ayush Singh wrote:
> >> Regardless, this patch actually does not contain any code for EEPROM
> >> support I have just mentioned it to give more context on why mikroBUS
> >> manifest is the focus of this patch instead of DT overlay or something
> >> else.
> > Right, and I think this is the crux here. Why can't you use DT
> > overlays? The manifest files, seem to be yet another hardware
> > description (method) and we already have DT. Can't we have some kind
> > of userspace helper that could translate them to DT overlays? That
> > way, you could also handle the EEPROM vs non-EEPROM case, or have
> > some other kind of method to load a DT overlay.
> >
> > Admittedly, I've never worked with in-kernel overlays, but AFAIK
> > they work with some subsystems.
> >
> > -michael
>
>
> So let me 1st go over 3 cases that the driver needs to support:
>
> 1. Non EEPROM boards:
>
> Using overlays should be pretty similar to current solution. If the 
> manifest is converted to overlay in userspace, then we do not even need 
> to do manifest parsing, setting up spi, i2c etc in the kernel driver.
>
>
> 2. EEPROM boards
>
> How do you propose handling these. If you are proposing storing dt 
> overlay in EEPROM, then this raises some questions regarding support 
> outside of Linux.
>
> The other option would be generating overlay from manifest in the kernel 
> driver, which I'm not sure is significantly better than registering the 
> i2c, spi, etc. interfaces separately using standard kernel APIs.

You did answer that yourself in (1): you could use a user space
helper to translate it to a DT overlay, I don't think this has to be
done in the kernel. Also how do you know whether there is an EEPROM
or not?

> 3. Over Greybus
>
> It is quite important to have mikroBUS over greybus for BeagleConnect. 
> This is one of the major reasons why greybus manifest was chosen for the 
> manifest format.
>
> Also, it is important to note that mikroBUS manifest is being used since 
> 2020 now and thus manifests for a lot of boards (both supporting clickID 
> and not supporting it exist). So I would prefer using it, unless of 
> course there are strong reasons not to.

And also here, I'm not really familiar with greybus. Could you give
a more complex example? My concern is that some driver might need
additional properties from DT (or software nodes) to function
properly. It might not only be a node with a compatible string but
also more advanced bindings. How would that play together with this?
My gut feeling is that you can handle any missing properties
easier/better (eg. for existing modules) in user space. But maybe
that is already solved in/with greybus?

Here's a random one: the manifest [1] just lists the compatible
string apparently, but the actual DT binding has also reset-gpios,
some -supply and interrupt properties.

-michael

[1] https://github.com/MikroElektronika/click_id/blob/main/manifests/WEATHER-CLICK.mnfs

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