linux-kernel - Re: [microblaze-uclinux] Rethinking MicroBlaze commandline precedence

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Message-ID: <4A7FE78F.4040504@petalogix.com>
Date:	Mon, 10 Aug 2009 11:25:35 +0200
From:	Michal Simek <michal.simek@...alogix.com>
To:	microblaze-uclinux@...e.uq.edu.au
CC:	John Linn <John.Linn@...inx.com>,
	David DeBonis <david.debonis@...inx.com>,
	Linux Kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [microblaze-uclinux] Rethinking MicroBlaze commandline precedence

Hi John,

John Williams wrote:
> Hi Michal,
> 
> On Mon, Aug 10, 2009 at 5:29 PM, Michal Simek<michal.simek@...alogix.com> wrote:
> 
>> John Williams wrote:
> 
>>> Currently, MicroBlaze commandline handling in order of lowest to
>>> highest priority, looks like this:
>>>
>>> 1. pointer in r5 from bootloader
>>> 2. CONFIG_CMDLINE=...
>>> 3. "chosen" section in DTS/DT
>>> 4. CONFIG_CMDLINE=... && CONFIG_CMDLINE_FORCE
>>>
>>> I'm wondering if a cmdline in r5 should override the DTS.  My thinking
>>> is based on two observations:
>>>
>>> (a) not everyone will use a bootloader like u-boot that can manipulate
>>> DTBs easily before kernel boot
>>> (b) a custom cmdline string in r5 allows the latest possible binding
>>> (runtime), where as the DTB is typically created at compile time.
>>>
>>> So, how about this order instead:
>>>
>>> 1. CONFIG_CMDLINE=...
>>> 2. "chosen" section in DTS/DT
>>> 3. pointer in r5 from bootloader
>>> 4. CONFIG_CMDLINE=... and CONFIG_CMDLINE_FORCE
>>>
>>> Then, apart from CMDLINE_FORCE, the precedence goes from earliest
>>> binding (kernel build) to latest (runtime via bootloader/r5).
>>>
>>> Thoughts?
>> I see there one big problem which can easily arise. Kernel expect that r5 points to
>> NULL string and DTS/DTB and CMDLINE will contain correct string. Kernel just copy it and use
>> it. There will be undefined behavior for more cases than for current handling. It will be
>> less error prune.
> 
> This expectation currently exists anyway,  giving precendence to DTS
> cmdline is only less error-prone on the assumption that it's more
> common.
> 
> What about a move to an ARM-style ATAGs arrangement, where r5 points
> not to an ASCIIZ string but instead to some sort of loosely structured
> object, with tag codes to signify the commandline etc.
> 
> That way, we can error check r5 - if it doesn't have the right tags
> then we don't use the (probably bogus) cmdline string?

Not sure if is help - but seems to me more complicated than we need.

> 
>> Ad observation a)
>>
>> My expectation is that users will use s.....I.... format (I don't like that name - What about
>> renaming it?) with dtb - they setup commandline at once and they don't change it.
> 
> 
> Ah what's wrong with simpleImage?  It's simple to boot, and make-fu
> makes it simple to build! :)

It is more complicated than origin file. Your suggestion for simple build/boot is fine. :-)

> 
>> Ad observation b) - for final product they will use only one command line. For testing you can setup
>> kernel to use only r5.
> 
> How?  Do we add CONFIG_CMDLINE_IGNORE_DTB?

> 
> Or just remove the commandline section from DTS "chosen" part?

yes.

> 
>> I understand that you are trying to pass to kernel for example MTD map which could be possible but
>> IMHO better to do this in script which one with sed erase/comment/setup command line in DTS before
>> compilation.
> 
> It's still a very early binding, and I think there are plenty of
> reasons to want to override it at boot time.  If there weren't, why do
> u-boot and PPC simpleBoot add the capability to tweak at boot time,
> the cmdline passed via the DTB?

I understand that make sense to change it - especially for development.
It should be possible to do it in u-boot. Not sure if is work to extend command line but maybe yes.
Worth to ask on u-boot mailing list.

> 
> Setting MAC addresses for example - you don't want to compile a new
> DTS for every board you ship, just to provide unique values. You just
> want to be able to tweak the cmdline in the config flash or whatever.

You should be able to change it in u-boot. The size of mac addr is the same. Shorten is possible too.


Cheers,
Michal

> 
> Cheers,
> 
> John

-- 
Michal Simek, Ing. (M.Eng)
PetaLogix - Linux Solutions for a Reconfigurable World
w: www.petalogix.com p: +61-7-30090663,+42-0-721842854 f: +61-7-30090663
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