linux-kernel - Re: [PATCH 00/11] platform: device tree support for early platform drivers

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Date:   Tue, 24 Apr 2018 13:59:37 -0400
From:   Rich Felker <dalias@...c.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Bartosz Golaszewski <brgl@...ev.pl>, Sekhar Nori <nsekhar@...com>,
        Kevin Hilman <khilman@...nel.org>,
        David Lechner <david@...hnology.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Frank Rowand <frowand.list@...il.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Arend van Spriel <aspriel@...il.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Michal Suchanek <msuchanek@...e.de>,
        Jan Kiszka <jan.kiszka@...mens.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH 00/11] platform: device tree support for early platform
 drivers

On Tue, Apr 24, 2018 at 06:55:45PM +0100, Mark Rutland wrote:
> Hi bartosz,
> 
> On Tue, Apr 24, 2018 at 07:30:40PM +0200, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bgolaszewski@...libre.com>
> > 
> > Device tree based systems often use OF_DECLARE() macros for devices
> > that need to be initialized early in the boot process such as clocks,
> > timers etc. However platform devices are more desirable.
> > 
> > This series aims at introducing support for early platform drivers for
> > OF-based systems.
> > 
> > The idea is to have a special compatible fallback string: "earlydev"
> > that similarily to "syscon" would be added to device nodes that need
> > early probing. We then need to call of_early_platform_populate() early
> > in the boot process to actually probe the registered devices.
> > 
> > I am aware that this new compatible does not strictly correspond with
> > the "hardware description only" rule of DT, but we already have many
> > functionalities that break this rule: syscon, gpio hogs etc.
> 
> I don't think we need the "earlydev" string at all, and can still keep
> the rest of this infrastrcuture if it is helpful.
> 
> So while I'm certainly oppposed to the "earlydev" string, I think that
> the infrastrcuture could be useful.

Yes. I think this agrees with my objection -- I'm not opposed to the
infrastructure in the kernel, but to semantically-wrong and
backwards-incompatible changes to the device tree contract.

> > The first user of this new functionality will be the DaVinci platform
> > for which we want to probe the pll, psc and timer drivers early in the
> > boot sequence but keep them implemented as platform drivers.
> 
> Can we have early_paltform_driver() take a reference to the driver's
> of_match_table, and put that in a special section?
> 
> Then instead of looking for "earlydev", we look for any compatible
> string in that section. No magic string required in the DT, and we can
> always mess around with probe ordering in future.

Yes, this is pretty much exactly the change I had in mind.

Rich