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

[Rich Felker <dalias@libc.org>]

On Tue, Apr 24, 2018 at 07:30:40PM +0200, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bgolaszewski@baylibre.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.

This makes no sense to me. It's not the device tree that should
contain the knowledge that these need to be registered/probed early in
the boot. That need is a property of the operating system/device
drivers, and so the *drivers* should be the ones that contain the
knowledge.

> 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.
> 
> 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.
> 
> In the future, if this series gets accepted, this could become and
> alternative to TIMER_OF_DECLARE, CLK_OF_DECLARE etc. and could lead
> to deprecating of those macros in favor of early platform drivers.

This seems completely unacceptable, in that it would render new
kernels unable to boot on existing device trees (which may be
hard-coded in rom on existing devices).

Rich