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

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Date:   Sun, 30 Dec 2018 14:28:56 +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 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.

> 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.

-- 

>        Arnd
>