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From: Bartosz Golaszewski <brgl@bgdev.pl>
To: David Lechner <david@lechnology.com>
Cc: Sekhar Nori <nsekhar@ti.com>, Kevin Hilman <khilman@kernel.org>,
	Michael Turquette <mturquette@baylibre.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Bartosz Golaszewski <bgolaszewski@baylibre.com>,
	Stephen Boyd <sboyd@kernel.org>
Subject: Re: [RFC work-in-progress 0/7] of: platform: use early platform routines instead of OF_DECLARE
Date: Tue, 24 Apr 2018 09:26:37 +0200	[thread overview]
Message-ID: <CAMRc=Md0vKTtYXOWAHv7mEZUqBWG7U4oF26xMwSL_u_wPZn9rg@mail.gmail.com> (raw)
In-Reply-To: <b50d1a0d-4585-5c2b-bfc7-4f2ad023b753@lechnology.com>

2018-04-23 23:38 GMT+02:00 David Lechner <david@lechnology.com>:
> 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 <bgolaszewski@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. :-)
>

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.

>>
>> 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.

Best regards,
Bartosz Golaszewski

  reply	other threads:[~2018-04-24  7:26 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20180423183814.20615-1-brgl@bgdev.pl>
2018-04-23 21:38 ` David Lechner
2018-04-24  7:26   ` Bartosz Golaszewski [this message]
2018-04-24  8:18     ` Sekhar Nori
     [not found] ` <20180423183814.20615-6-brgl@bgdev.pl>
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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