linux-kernel - Re: [PATCH v8 20/25] powerpc, fbdev: Use arch_nvram_ops methods instead of nvram_read_byte() and nvram_write

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Message-ID: <CAK8P3a3645im4aFDi2mNjwcCe=jvmDLkKK_ZSwG1POOLUq=+QQ@mail.gmail.com>
Date:   Mon, 31 Dec 2018 13:29:46 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Finn Thain <fthain@...egraphics.com.au>
Cc:     LEROY Christophe <christophe.leroy@....fr>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v8 20/25] powerpc, fbdev: Use arch_nvram_ops methods
 instead of nvram_read_byte() and nvram_write_byte()

On Sun, Dec 30, 2018 at 12:43 AM Finn Thain <fthain@...egraphics.com.au> wrote:

>
> Is there some benefit, or is that just personal taste?
>
> Avoiding changes to call sites avoids code review, but I think 1) the
> thinkpad_acpi changes have already been reviewed and 2) the fbdev changes
> need review anyway.
>
> Your suggesion would add several new entities and one extra layer of
> indirection.
>
> I think indirection harms readability because now the reader now has to go
> and look up the meaning of the new entities.
>
> It's not the case that we need to choose between definitions of
> nvram_read_byte() at compile time, or stub them out:
>
> #ifdef CONFIG_FOO
> static inline unsigned char nvram_read_byte(int addr)
> {
>         return arch_nvram_ops.read_byte(addr);
> }
> #else
> static inline unsigned char nvram_read_byte(int addr) { }
> #endif
>
> And I don't anticipate a need for a macro here either:
>
> #define nvram_read_byte(a) random_nvram_read_byte_impl(a)
>
> I think I've used the simplest solution.

Having the indirection would help if the inline function can
encapsulate the NULL pointer check, like

static inline unsigned char nvram_read_byte(loff_t addr)
{
       char data;

       if (!IS_ENABLED(CONFIG_NVRAM))
                 return 0xff;

       if (arch_nvram_ops.read_byte)
                 return arch_nvram_ops.read_byte(addr);

       if (arch_nvram_ops.read)
                 return arch_nvram_ops.read(char, 1, &addr);

      return 0xff;
}

(the above assumes no #ifdef in the structure definition, if you
keep the #ifdef there they have to be added here as well).

      Arnd