linux-kernel - Re: RFC: oftree based setup of composite board devices

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Message-ID: <CAHp75VfcpS4TNhCG2bGRQzPtqyWZ8UotKs2DVRBE_gDLZRQLoA@mail.gmail.com>
Date:   Mon, 15 Feb 2021 17:35:00 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Frank Rowand <frowand.list@...il.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        "Enrico Weigelt, metux IT consult" <info@...ux.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Johan Hovold <johan@...nel.org>
Subject: Re: RFC: oftree based setup of composite board devices

On Mon, Feb 15, 2021 at 3:15 AM Frank Rowand <frowand.list@...il.com> wrote:
> On 2/8/21 5:48 PM, Rob Herring wrote:
> > On Mon, Feb 8, 2021 at 4:22 PM Enrico Weigelt, metux IT consult
> > <info@...ux.net> wrote:
> >>
> >> Hello folks,
> >>
> >> here's an RFC for using compiled-in dtb's for initializing board devices
> >> that can't be probed via bus'es or firmware.
>
> I've just been monitoring this thread for several days, hoping that the
> discussion would make things more clear for me.

You beat me up to it. I support your comments.

I have to comment to Enrico and others that under overlays for
ACPI-based platforms I meant SSDT overlays, no DT.

Also I have to point out that we have swnode API for the cases where
we need quirks for either ACPI or DT or whatever (not yet present)
firmware quirks.

> Disclaimer: I know essentially nothing about ACPI, please excuse improper
> naming and misunderstandings on my part.
>
> Why not compile in ACPI data (tables?) instead of devicetree description?
>
> >
> > I'm not convinced compiled in is the mechanism we want.
> >
> >> Use cases are boards with non-oftree firmware (ACPI, etc) where certain
> >> platform devices can't be directly enumerated via firmware. Traditionally
> >> we had to write board specific drivers that check for board identification
> >> (DMI strings, etc), then initialize the actual devices and their links
> >> (eg. gpio<->leds/buttons, ...). Often this can be expressed just by DT.
> >
> > This is something I've wanted to see for a while. There's use cases
> > for DT based systems too. The example I'd like to see supported are
> > USB serial adapters with downstream serdev, GPIO, I2C, SPI, etc. Then
> > plug more than one of those in.
>
> My understanding from the past is that the experts (those who understand both
> devicetree and ACPI) regard trying to mix devicetree and ACPI in a single
> running Linux kernel image is insanity, or at least likely to be confusing,
> difficult, and problematic.
>
> From the devicetree side, I expect nightmares for me if devicetree and ACPI
> are mixed in a single running kernel image.
>
> >
> >> This patch queue does a bunch of preparations in oftree code, so we can
> >> support multiple fully independent DT's (not using DT overlays). And then
> >> adds a generic driver parses compiled-in fdt blobs, checks for mathing
> >> DMI strings and initializes the devices. As an example, the last patch
> >> adds an alternative implementation for the PC engines APU2/3/4 boa> Disclaimer: I know essentially nothing about ACPI, please excuse improper
rd
> >> family based on device tree.
> >
> > I think there's a couple of approaches we could take. Either support
> > multiple root nodes as you have done or keep a single root and add
> > child nodes to them. I think the latter would be less invasive. In the
> > non-DT cases, we'd just always create an empty skeleton DT. A 3rd
> > variation on a DT system is we could want to create parent nodes if
> > they don't exist to attach this DT to so we have a full hierarchy.
> >
> > I'm not saying which one we should do, just laying out some of the options.
> >
>
> Multiple root nodes and disjoint trees both seem problematic.  Existing
> subsystems and drivers expect a single cohesive tree.  Changing that
> architecture looks to me to be a painful exercise.
>
> >> The approach can be easily be extended to other kinds of composite devices,
> >> eg. PCI cards or USB dongles.
> >>
> >>
> >> Yet some drawbacks of the current implementation:
> >>
> >>  * individual FDT's can't be modularized yet (IMHO, we don't have DMI-based
> >>    modprobing anyways)
> >
> > I think we need to use either firmware loading or udev mechanisms to
> > load the FDTs.
> >
> >>  * can't reconfigure or attach to devices outside the individual DT's
> >>    (eg. probed by PCI, etc)
> >
> > Not sure I follow.
> >
> > Rob
> >
>


-- 
With Best Regards,
Andy Shevchenko