linux-kernel - Re: [PATCH v3 1/2] bootconfig: Prohibit re-defining value on same key

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Date:   Sat, 22 Feb 2020 23:41:47 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Randy Dunlap <rdunlap@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Borislav Petkov <bp@...en8.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 1/2] bootconfig: Prohibit re-defining value on same
 key

On Sat, 22 Feb 2020 10:31:17 +0100
Geert Uytterhoeven <geert@...ux-m68k.org> wrote:

> Hi Randy,
> 
> On Sat, Feb 22, 2020 at 5:30 AM Randy Dunlap <rdunlap@...radead.org> wrote:
> > On 2/21/20 12:13 AM, Masami Hiramatsu wrote:
> > > --- a/Documentation/admin-guide/bootconfig.rst
> > > +++ b/Documentation/admin-guide/bootconfig.rst
> > > @@ -62,7 +62,16 @@ Or more shorter, written as following::
> > >  In both styles, same key words are automatically merged when parsing it
> > >  at boot time. So you can append similar trees or key-values.
> > >
> > > -Note that a sub-key and a value can not co-exist under a parent key.
> > > +Same-key Values
> > > +---------------
> > > +
> > > +It is prohibited that two or more values or arraies share a same-key.
> >
> > I think (?):                                   arrays
> >
> > > +For example,::
> > > +
> > > + foo = bar, baz
> > > + foo = qux  # !ERROR! we can not re-define same key
> > > +
> > > +Also, a sub-key and a value can not co-exist under a parent key.
> > >  For example, following config is NOT allowed.::
> > >
> > >   foo = value1
> >
> >
> > I'm pretty sure that the kernel command line allows someone to use
> >   key=value1 ... key=value2
> > and the first setting is just overwritten with value2 (for most "key"s).
> >
> > Am I wrong?  and is this patch saying that bootconfig won't operate like that?
> 
> I think so. Both are retained.
> A typical example is "console=ttyS0 console=tty", to have the kernel output
> on both the serial and the graphical console.

Right, it actually depends on how the option is defined and its handler.
If the option is defined with module_param*() macros, those will be 
overwritten.
But if it is defined with __setup() or early_param(), the handler function
will be called repeatedly. Thus, overwrite or append or skip later one
depends on the option handler.

I think the bootconfig is a bit different from legacy command line at
this moment. The legacy command line can be modified by bootloader,
whereas the bootconfig is a single text file which user can update
each value. Of course bootloader will support the bootconfig to append
some key-values in the future.
So I would like to introduce another "overwrite" operator (":=") and
"assign default" operator ("?=") too. With those operators, the
bootloader can just add their own key-value without decoding the
current bootconfig.

Thank you,

-- 
Masami Hiramatsu <mhiramat@...nel.org>