linux-kernel - Re: [PATCH-RFC 6/7] drivers: of: ifdef out cmdline section

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Message-ID: <561EAA1F.7040406@cisco.com>
Date:	Wed, 14 Oct 2015 12:16:47 -0700
From:	Daniel Walker <danielwa@...co.com>
To:	Rob Herring <robh@...nel.org>
Cc:	Daniel Walker <dwalker@...o99.com>, xe-kernel@...ernal.cisco.com,
	Frank Rowand <frowand.list@...il.com>,
	Grant Likely <grant.likely@...aro.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH-RFC 6/7] drivers: of: ifdef out cmdline section

On 10/14/2015 10:57 AM, Rob Herring wrote:
> On Wed, Oct 14, 2015 at 9:48 AM, Daniel Walker <danielwa@...co.com> wrote:
>>
>> There's one last little wrinkle .. In the current setup the defconfig
>> CONFIG_CMDLINE="" is used as a default in case the device tree has nothing
>> in it. In my changes, there is no identical functionality. The only similar
>> thing I have is the the CONFIG_CMDLINE_APPEND="" . The main difference is
>> that in the current implementation CONFIG_CMDLINE="" doesn't get added at
>> all if there is a device tree bootargs, but with my implementation this line
>> would be added unconditionally. It would represent a subtle change where
>> people would have to add into the DT bootargs something to override what
>> might be in the default command line.
> So CMDLINE_EXTEND would be equivalent to your version, but it looks
> like CMDLINE_EXTEND is not used in the DT case. Perhaps you can add
> the option? You already have OVERRIDE which is equivalent to FORCE.

I have better than CONFIG_CMDLINE_EXTEND.. My changes allow the 
bootloader arguments to be appended or prepended to the default, which 
ever works for you. It works in a slightly different way, your appending 
or prepending the default values the bootloader arugments (or DT 
bootargs), opposed to the other way around.

This is what I was suggesting we do, i.e. to use something like 
CONFIG_CMDLINE_EXTEND. If we put everything from CONFIG_CMDLINE into 
something like CONFIG_CMDLINE_EXTEND, then it ends up setting default I 
think people won't expect.


>
>> For example,
>>
>> if a config has CONFIG_CMDLINE_APPEND="debug" then they would have to add a
>> "loglevel=7" into the DT bootargs to get back to normal. I wouldn't think
>> people would want "debug" as the default, but oddly enough some of the
>> configs do have this. Some of them also have default ip address setting,
>> nfsroot= settings, and loglevel= settings.
> Or they would have to remove the kernel default from their config.
> That might be acceptable. You could have a case where you have 1
> kernel binary and 2 different bootloaders where you expect the
> bootloader's cmdline used in one case and the kernel's in the other.
> Seems unlikely, but it would be an ABI break.
>
>

I suppose it would depend if it works or not with my changes. If we do 
the EXTEND thing, then people might end up with "nfsroot=" lines they 
didn't expend as defaults.

I'm not sure this is ABI breakage, because the kernel code isn't stuck 
on ABI's internally. If someone wants specific boot arguments in there 
kernel config this doesn't prevent it, it just changes the behavior.


>> What are your thoughts on this ? I think using the append type default makes
>> more sense because it's actually setting up global defaults. The current
>> complete replacement scheme seems to set the stage for people to make an
>> entirely custom default for a single development machine, which IMO doesn't
>> make sense. However, I'm not sure what the intent is with the current setup.
> People will want a path to support up to the current 3 options (use
> bootloader's cmdline, append bootloader cmdline to default, and force
> kernel default) and you have to assume changing bootloader is not an
> option.
>

Yeah we have those 3 with my changes. Maybe I'm drilling down too much 
on this and we should just do the CONFIG_CMDLINE_EXTEND equivalent and 
when they find there's a new "nfsroot=" line in there bootargs they can 
just send a patch.

Daniel
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