linux-kernel - Re: early init dt for earlyprintk (was Re: [RFC PATCH v1 25/31] ARC: [plat-arcfpga] Hooking up platform to ARC UART)

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Date:	Mon, 14 Jan 2013 15:39:21 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	James Hogan <james.hogan@...tec.com>
CC:	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Arnd Bergmann <arnd@...db.de>,
	Grant Likely <grant.likely@...retlab.ca>,
	<linux-arch@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<tglx@...utronix.de>, Paul Mundt <lethal@...ux-sh.org>
Subject: Re: early init dt for earlyprintk (was Re: [RFC PATCH v1 25/31] ARC:
 [plat-arcfpga] Hooking up platform to ARC UART)

On Monday 14 January 2013 03:18 PM, James Hogan wrote:
> Hi Vineet
> 
> On 14/01/13 07:35, Vineet Gupta wrote:
>> 2. We really need a Documentation/dt-for-new-linux-arches.txt, as the existing
>> refs - while very helpful - fail to mention subtleties such as you absolutely need
>> a intc instance and a default irq domain ..... before the driver can start using
>> the DT.
> 
> Sounds like a good idea. I'm curious about what you say about default
> IRQ domains though. Do you mean irq_set_default_host? In what way is
> that required to use DT?
> 
> Cheers
> James
> 

Nope, the fact that we need a irq_domain_add_*( ) call corresponding to atleast
one interrupt-controller node in the DT. In absence of that, the following
callstack returns - with RESOURCE_IRQ unpopulated - causing driver to eventually
fail the probe.

of_platform_populate
...
   of_irq_to_resource
      irq_of_parse_and_map
        irq_create_of_mapping  -> Needs "some" irq domain.

Prior to introducing DT in ARC Port, I didn't have any of CONFIG_IRQ_DOMAIN and
still it used to work fine. Enabling CONFIG_OF forced me to enable
CONFIG_IRQ_DOMAIN (merely to resolve link errors) but I had no clue (call me
stupid if you will) that I needed an actual domain setup.

The irq_set_default_host( ) is a nice design pattern though which avoids exporting
the root domain outside of the specific compilation unit.

-Vineet
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