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Date:   Tue, 1 Jan 2019 12:10:11 +1100 (AEDT)
From:   Finn Thain <fthain@telegraphics.com.au>
To:     Arnd Bergmann <arnd@arndb.de>
cc:     LEROY Christophe <christophe.leroy@c-s.fr>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
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        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        linux-m68k <linux-m68k@lists.linux-m68k.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Paul Mackerras <paulus@samba.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH v8 20/25] powerpc, fbdev: Use arch_nvram_ops methods
 instead of nvram_read_byte() and nvram_write_byte()
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On Mon, 31 Dec 2018, Arnd Bergmann wrote:

> On Sun, Dec 30, 2018 at 12:43 AM Finn Thain <fthain@telegraphics.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 semantics of .read_byte and .read are subtly different. For CONFIG_X86 
and CONFIG_ATARI, .read implies checksum validation and .read_byte does 
not.

In particular, in the thinkpad_acpi case, checksum validation isn't used, 
but in the atari_scsi case, it is.

So I like to see drivers explicitly call the method they want. I didn't 
want to obscure this distinction in a helper.

--