linux-kernel - Re: [08/36] AArch64: Kernel booting and initialisation

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Date:	Tue, 24 Jul 2012 16:20:23 -0700
From:	Frank Rowand <frank.rowand@...sony.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Catalin Marinas <catalin.marinas@....com>,
	Christopher Covington <cov@...eaurora.org>,
	Stephen Warren <swarren@...dotorg.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Will Deacon <Will.Deacon@....com>,
	Domenico Andreoli <cavokz@...il.com>
Subject: Re: [08/36] AArch64: Kernel booting and initialisation

On 07/24/12 11:53, Arnd Bergmann wrote:
> On Tuesday 24 July 2012, Catalin Marinas wrote:
>> On Mon, Jul 23, 2012 at 09:52:22PM +0100, Christopher Covington wrote:
>>> Hi Catalin and Stephen,
>>>
>>> Catalin Marinas - July 8, 2012, 9:18 a.m.
>>>> On Fri, Jul 06, 2012 at 10:32:54PM +0100, Stephen Warren wrote:
>>>>
>>>>> Also, on Tegra at least and perhaps OMAP too, there are a few parts
>>>>> of earlyprintk serial port setup (i.e. port selection) which happen
>>>>> as part of the decompressor rather than the main kernel image, so
>>>>> we'd have to re-jig that too.
>>>>
>>>> Whatever we did on AArch32, if it was useful and we want to follow a
>>>> similar model it needs porting given that the code cannot be shared. So
>>>> there is nothing to re-jig but rather implement.
>>>>
>>>> As for earlyprintk, you can initialise it in the decompressed kernel
>>>> anyway. But even though it's a useful debugging tool, it goes against
>>>> the single Image aim (at least the current printascii/addruart
>>>> implementation). I don't have a proper implementation yet.
>>>
>>> Would DCC [1] be a reasonably commonly available early printk mechanism
>>> for the single image to provide?
>>
>> Yes, this can be implemented but I think it has its own restrictions,
>> requiring a JTAG connection.
> 
> Right, which means that it's not generic and we still need to make
> a decision for a how we want to solve the underlying problem. The
> options I see are:
> 
> 1. Have no earlyprintk before loading the console drivers
> 2. Make the series from Domenico architecture independent code
> 3. Reimplement a layer like that, differently
> 4. Make it a compile-time option and allow this only for
>    low level debugging purposes but disable it in distro kernels.

I have found earlyprintk to be very valuable for debugging the many
times that boot fails for me.

Not having earlyprintk does not seem a good idea to me.  Option 4 is
perfectly acceptable for my use cases.

-Frank

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