linux-kernel - Re: [PATCH v8 24/25] powerpc: Adopt nvram module for PPC64

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Date:   Wed, 9 Jan 2019 09:51:48 +1100 (AEDT)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     Michael Ellerman <mpe@...erman.id.au>
cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v8 24/25] powerpc: Adopt nvram module for PPC64

On Tue, 8 Jan 2019, Michael Ellerman wrote:

> > The reason why that doesn't work boils down to introspection. (This 
> > was mentioned elsewhere in this email thread.) For example, we 
> > presently have code like this,
> >
> > ssize_t nvram_get_size(void)
> > {
> >        if (ppc_md.nvram_size)
> >                return ppc_md.nvram_size();
> >        return -1;
> > }
> > EXPORT_SYMBOL(nvram_get_size);
> >
> > This construction means we get to decide at run-time which of the NVRAM 
> > functions should be used. (Whereas your example makes a build-time decision.)
> 
> Right, but we only need to make a runtime decision on powerpc (right?).

It's needed in many places outside of powerpc. Otherwise the caller can't 
determine at run-time which ops are implemented.

Hence you have to duplicate the caller for each supported configuration 
that you build.

Already, this precludes a shared misc device implementation and belies the 
"generic" in drivers/char/generic_nvram.c.

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