linux-kernel - Re: [PATCH v2 3/7] powerpc: convert config files to generic cmdline

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Message-ID: <e4899874-1684-fa1b-443e-f4e478e05e31@csgroup.eu>
Date:   Wed, 24 Mar 2021 17:59:59 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Daniel Walker <danielwa@...co.com>
Cc:     Will Deacon <will@...nel.org>, Rob Herring <robh@...nel.org>,
        Daniel Gimpelevich <daniel@...pelevich.san-francisco.ca.us>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        xe-linux-external@...co.com, Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/7] powerpc: convert config files to generic cmdline



Le 09/03/2021 à 22:29, Daniel Walker a écrit :
> On Tue, Mar 09, 2021 at 08:47:09AM +0100, Christophe Leroy wrote:
>>
>>
>> Le 09/03/2021 à 01:02, Daniel Walker a écrit :
>>> This is a scripted mass convert of the config files to use
>>> the new generic cmdline. There is a bit of a trim effect here.
>>> It would seems that some of the config haven't been trimmed in
>>> a while.
>>
>> If you do that in a separate patch, you loose bisectability.
>>
>> I think it would have been better to do things in a different way, more or less like I did in my series:
>> 1/ Provide GENERIC cmdline at the same functionnality level as what is
>> spread in the different architectures
>> 2/ Convert architectures to the generic with least churn.
>> 3/ Add new features to the generic
> 
> You have to have the churn eventually, no matter how you do it. The only way you
> don't have churn is if you never upgrade the feature set.
> 
> 
>>>
>>> The bash script used to convert is as follows,
>>>
>>> if [[ -z "$1" || -z "$2" ]]; then
>>>           echo "Two arguments are needed."
>>>           exit 1
>>> fi
>>> mkdir $1
>>> cp $2 $1/.config
>>> sed -i 's/CONFIG_CMDLINE=/CONFIG_CMDLINE_BOOL=y\nCONFIG_CMDLINE_PREPEND=/g' $1/.config
>>
>> This is not correct.
>>
>> By default, on powerpc the provided command line is used only if the bootloader doesn't provide one.
>>
>> Otherwise:
>> - the builtin command line is appended to the one provided by the bootloader
>> if CONFIG_CMDLINE_EXTEND is selected
>> - the builtin command line replaces to the one provided by the bootloader if
>> CONFIG_CMDLINE_FORCE is selected
> 
> I think my changes maintain most of this due to the override of
> CONFIG_CMDLINE_PREPEND. This is an upgrade and the inflexibility in powerpc is
> an example of why these changes were created in the first place.

"inflexibility in powerpc" : Can you elaborate ?

> 
> For example , say the default command line is "root=/dev/issblk0" from iss476
> platform. And the bootloader adds "root=/dev/sda1"
> 
> The result is <prepend><bootloader><append>.


I'm still having hard time understanding the benefit of having both <prepend> and <append>.
Could you please provide a complete exemple from real life, ie what exactly the problem is and what 
it solves ?

> 
> Then you have,
> 
> root=/dev/issblk0 root=/dev/sda1
> 
> and the bootloader has precedent over the default command line. So root= in the
> above cases is defined by the bootloader.
> 
> The only issue would be if a person wants to override the default command line
> with an unrelated bootloader command line. I don't know how many people do this,
> but I doubt it's many. Can you think of any use cases like this?
> 
> I would imagine there are many more people who have to entirely duplicate the
> default command line in the boot loader when they really just want to change a
> single part of it like the root= device or console device or speed.
> 
> Daniel
> 

Christophe