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

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Date:   Fri, 4 Jan 2019 19:45:58 +1100 (AEDT)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     Arnd Bergmann <arnd@...db.de>
cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        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 Mon, 31 Dec 2018, Arnd Bergmann wrote:

> On Sun, Dec 30, 2018 at 4:29 AM Finn Thain <fthain@...egraphics.com.au> wrote:
> >
> > On Sat, 29 Dec 2018, Arnd Bergmann wrote:
> >
> > > With the current method, it does seem odd to have a single 
> > > per-architecture instance of the exported structure containing 
> > > function pointers. This doesn't give us the flexibility of having 
> > > multiple copies in the kernel the way that ppc_md does, but it adds 
> > > overhead compared to simply exporting the functions directly.
> > >
> >
> > You're right, there is overhead here.
> >
> > With a bit of auditing, wrappers like the one you quoted (which merely 
> > checks whether or not a ppc_md method is implemented) could surely be 
> > avoided.
> >
> > The arch_nvram_ops methods are supposed to optional (that is, they are 
> > allowed to be NULL).
> >
> > We could call exactly the same function pointers though either ppc_md 
> > or arch_nvram_ops. That would avoid the double indirection.
> 
> I think you can have a 'const' structure in the __ro_after_init section, 
> so without changing anything else, powerpc could just copy the function 
> pointers from ppc_md into the arch_nvram_ops at early init time, which 
> should ideally simplify your implementation as well.
> 

Does this require removing the 'const' from the powerpc arch_nvram_ops 
definition? That would mean removing the 'const' from the declaration in 
nvram.h, which means removing 'const' for every other instance of that 
struct too.

That's what happened when I tried removing the ppc_md.nvram_* methods 
entirely and assigning the same function pointers to arch_nvram_ops 
methods instead. Apparently all instances of arch_nvram_ops have to be 
const or none of them. Otherwise gcc says, "error: conflicting type 
qualifiers for 'arch_nvram_ops'".

-- 

>         Arnd
>