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From: Sekhar Nori <>
To: Bartosz Golaszewski <>,
	David Lechner <>
Cc: Kevin Hilman <>,
	Michael Turquette <>,
	Arnd Bergmann <>,
	Greg Kroah-Hartman <>,
	Linux ARM <>,
	Linux Kernel Mailing List <>,
	Bartosz Golaszewski <>,
	Stephen Boyd <>
Subject: Re: [RFC work-in-progress 0/7] of: platform: use early platform routines instead of OF_DECLARE
Date: Tue, 24 Apr 2018 13:48:00 +0530	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Tuesday 24 April 2018 12:56 PM, Bartosz Golaszewski wrote:
> 2018-04-23 23:38 GMT+02:00 David Lechner <>:
>> FYI: It looks like the CC for Stephen and Arnd was messed up, so I
>> fixed.
> Thanks!
>> On 04/23/2018 01:38 PM, Bartosz Golaszewski wrote:
>>> From: Bartosz Golaszewski <>
>>> 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. :-)
> We already have "syscon" which too is more an implementation detail
> than HW description. I should have probably Cc'ed Rob Herring. I'll do
> it with a more polished version I should have today.

Yeah, we should check with DT maintainers here. Even if there is push
back on this, I suppose we can make of_early_platform_populate() take a
list of compatible strings whose DT nodes should be considered early
platform devices?

>>> 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.
> Yes, it seems we do. I was getting kobj stack dumps too when trying to
> register a device using just platform_device_register() and it went
> away as soon as I switched to early platform.

I agree, it sounds like a good idea to use for clock and timer devices.
Thanks for looking into this. Looking forward to the more polished version.


  reply	other threads:[~2018-04-24  8:18 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <>
2018-04-23 21:38 ` David Lechner
2018-04-24  7:26   ` Bartosz Golaszewski
2018-04-24  8:18     ` Sekhar Nori [this message]
     [not found] ` <>
2018-04-24  8:10   ` [RFC work-in-progress 5/7] !WIP ARM: davinci: convert da850 to using the davinci clocksource driver Sekhar Nori
2018-04-24 16:12     ` Bartosz Golaszewski

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