[RFC work-in-progress 0/7] of: platform: use early platform routines instead of OF_DECLARE

[David Lechner]

FYI: It looks like the CC for Stephen and Arnd was messed up, so I
fixed.

On 04/23/2018 01:38 PM, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bgolaszewski at baylibre.com>
> 
> Hi David, Sekhar,
> 
> since platform devices are generally considered more desirable than
> CLK_OF_DECLARE, TIMER_OF_DECLARE etc. and we need to figure out how to
> handle the clocks that need to be initialized early in the boot
> process on DaVinci, I thought that I could give the early_platform
> mechanism a try.
> 
> This API is only used on one architecture (sh) but seems to work just
> fine on ARM. It allows to register early platform drivers and then
> probe them early in the boot process. So far only machine code is
> supported but with a bit of hacking I was able to probe a DT device.
> 
> This is a very dirty and far-from-upstream proof of concept that allows
> to probe the (so far dummy) davinci timer platform device during the
> call to init_time (from machine_desc).
> 
> 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. Then we'd call the of_early_platform_populate()
> function that would find all compatible nodes and populate them
> long before all the "normal" nodes.

FWIW, "earlydev" sounds like a driver implementation detail, so not
something that should be included in the device tree. We only need
this because Linux needs a clocksource early on, but that doesn't
mean that all device tree users need to do the same.

I'm sure it makes things easier for a proof of concept though. :-)

> 
> This would allow us to make the davinci timer a normal platform device
> and possibly also probe the psc and pll drivers earlier than we do now.
> 
> The early platform API even allows us to check if we're being probed
> early in probe() so we can possibly probe the driver twice if needed:
> only doing the critical stuff first and then completing the process
> later.
> 
> If you think this is a good idea, I would like to continue on that
> and eventually make it an alternative to OF_DECLARE macros.
> 
> For a quick conversion of the davinci timer to a platform driver
> I image we'd need to use platform data lookup that would be passed
> to of_early_platform_populate().

On the surface, it certainly sounds like a good idea to me. Do we have
access to struct device of the platform device when using this early
platform device? I remember when I was working on the clock drivers, I
tried registering a platform device in the init_time callback but the
kernel crashed because kobj stuff was not initialized yet. I'm guessing
that the early platform device somehow works around this.