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

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Date:   Mon, 31 Dec 2018 13:32:56 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Finn Thain <fthain@...egraphics.com.au>
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 Sun, Dec 30, 2018 at 4:29 AM Finn Thain <fthain@...egraphics.com.au> wrote:
>
> On Sat, 29 Dec 2018, Arnd Bergmann wrote:
>
> > On Wed, Dec 26, 2018 at 1:43 AM Finn Thain <fthain@...egraphics.com.au> wrote:
> >
> > > +static ssize_t ppc_nvram_get_size(void)
> > > +{
> > > +       if (ppc_md.nvram_size)
> > > +               return ppc_md.nvram_size();
> > > +       return -ENODEV;
> > > +}
> >
> > > +const struct nvram_ops arch_nvram_ops = {
> > > +       .read           = ppc_nvram_read,
> > > +       .write          = ppc_nvram_write,
> > > +       .get_size       = ppc_nvram_get_size,
> > > +       .sync           = ppc_nvram_sync,
> > > +};
> >
> > Coming back to this after my comment on the m68k side, I notice that
> > there is now a double indirection through function pointers. Have you
> > considered completely removing the operations from ppc_md instead by
> > having multiple copies of nvram_ops?
> >
>
> I considered a few alternatives. I figured that it was refactoring that
> could be deferred, as it would be confined to arch/powerpc. I was more
> interested in the cross-platform API.

Fair enough.

> > 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.

        Arnd