Re: [PATCH v2 6/7] cmdline: Gives architectures opportunity to use generically defined boot cmdline manipulation

[Will Deacon <will@kernel.org>]

On Thu, Mar 25, 2021 at 12:18:38PM +0100, Christophe Leroy wrote:
> 
> 
> Le 03/03/2021 à 18:57, Will Deacon a écrit :
> > On Tue, Mar 02, 2021 at 05:25:22PM +0000, Christophe Leroy wrote:
> > > Most architectures have similar boot command line manipulation
> > > options. This patchs adds the definition in init/Kconfig, gated by
> > > CONFIG_HAVE_CMDLINE that the architectures can select to use them.
> > > 
> > > In order to use this, a few architectures will have to change their
> > > CONFIG options:
> > > - riscv has to replace CMDLINE_FALLBACK by CMDLINE_FROM_BOOTLOADER
> > > - architectures using CONFIG_CMDLINE_OVERRIDE or
> > > CONFIG_CMDLINE_OVERWRITE have to replace them by CONFIG_CMDLINE_FORCE.
> > > 
> > > Architectures also have to define CONFIG_DEFAULT_CMDLINE.
> > > 
> > > Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> > > ---
> > >   init/Kconfig | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> > >   1 file changed, 56 insertions(+)
> > > 
> > > diff --git a/init/Kconfig b/init/Kconfig
> > > index 22946fe5ded9..a0f2ad9467df 100644
> > > --- a/init/Kconfig
> > > +++ b/init/Kconfig
> > > @@ -117,6 +117,62 @@ config INIT_ENV_ARG_LIMIT
> > >   	  Maximum of each of the number of arguments and environment
> > >   	  variables passed to init from the kernel command line.
> > > +config HAVE_CMDLINE
> > > +	bool
> > > +
> > > +config CMDLINE_BOOL
> > > +	bool "Default bootloader kernel arguments"
> > > +	depends on HAVE_CMDLINE
> > > +	help
> > > +	  On some platforms, there is currently no way for the boot loader to
> > > +	  pass arguments to the kernel. For these platforms, you can supply
> > > +	  some command-line options at build time by entering them here.  In
> > > +	  most cases you will need to specify the root device here.
> > 
> > Why is this needed as well as CMDLINE_FROM_BOOTLOADER? IIUC, the latter
> > will use CONFIG_CMDLINE if it fails to get anything from the bootloader,
> > which sounds like the same scenario.
> > 
> > > +config CMDLINE
> > > +	string "Initial kernel command string"
> > 
> > s/Initial/Default
> > 
> > which is then consistent with the rest of the text here.
> > 
> > > +	depends on CMDLINE_BOOL
> > 
> > Ah, so this is a bit different and I don't think lines-up with the
> > CMDLINE_BOOL help text.
> > 
> > > +	default DEFAULT_CMDLINE
> > > +	help
> > > +	  On some platforms, there is currently no way for the boot loader to
> > > +	  pass arguments to the kernel. For these platforms, you can supply
> > > +	  some command-line options at build time by entering them here.  In
> > > +	  most cases you will need to specify the root device here.
> > 
> > (same stale text)
> > 
> > > +choice
> > > +	prompt "Kernel command line type" if CMDLINE != ""
> > > +	default CMDLINE_FROM_BOOTLOADER
> > > +	help
> > > +	  Selects the way you want to use the default kernel arguments.
> > 
> > How about:
> > 
> > "Determines how the default kernel arguments are combined with any
> >   arguments passed by the bootloader"
> > 
> > > +config CMDLINE_FROM_BOOTLOADER
> > > +	bool "Use bootloader kernel arguments if available"
> > > +	help
> > > +	  Uses the command-line options passed by the boot loader. If
> > > +	  the boot loader doesn't provide any, the default kernel command
> > > +	  string provided in CMDLINE will be used.
> > > +
> > > +config CMDLINE_EXTEND
> > 
> > Can we rename this to CMDLINE_APPEND, please? There is code in the tree
> > which disagrees about what CMDLINE_EXTEND means, so that will need be
> > to be updated to be consistent (e.g. the EFI stub parsing order). Having
> > the generic option with a different name means we won't accidentally end
> > up with the same inconsistent behaviours.
> 
> Argh, yes. Seems like the problem is even larger than that IIUC:
> 
> - For ARM it means to append the bootloader arguments to the CONFIG_CMDLINE
> - For Powerpc it means to append the CONFIG_CMDLINE to the bootloader arguments
> - For SH  it means to append the CONFIG_CMDLINE to the bootloader arguments
> - For EFI it means to append the bootloader arguments to the CONFIG_CMDLINE
> - For OF it means to append the CONFIG_CMDLINE to the bootloader arguments
> 
> So what happens on ARM for instance when it selects CONFIG_OF for instance ?

I think ARM gets different behaviour depending on whether it uses ATAGs or
FDT.

> Or should we consider that EXTEND means APPEND or PREPEND, no matter which ?
> Because EXTEND is for instance used for:
> 
> 	config INITRAMFS_FORCE
> 		bool "Ignore the initramfs passed by the bootloader"
> 		depends on CMDLINE_EXTEND || CMDLINE_FORCE

Oh man, I didn't spot that one :(

I think I would make the generic options explicit: either APPEND or PREPEND.
Then architectures which choose to define CMDLINE_EXTEND in their Kconfigs
can select the generic option that matches their behaviour.

INITRAMFS_FORCE sounds like it should depend on APPEND (assuming that means
CONFIG_CMDLINE is appended to the bootloader arguments).

Will