linux-kernel - Re: [PATCH] zlib: Put get_unaligned16() inside #ifdef block

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Message-ID: <20170523180316.GO141096@google.com>
Date:   Tue, 23 May 2017 11:03:16 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] zlib: Put get_unaligned16() inside #ifdef block

Hi Christoph,

El Tue, May 23, 2017 at 12:36:01AM -0700 Christoph Hellwig ha dit:

> On Mon, May 22, 2017 at 02:13:26PM -0700, Matthias Kaehlcke wrote:
> > The function is not used when CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y.
> > Adding the #ifdef fixes the following warning when building with clang:
> > 
> > lib/zlib_inflate/inffast.c:31:1: error: unused function 'get_unaligned16'
> >     [-Werror,-Wunused-function]
> > 
> > Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> > ---
> > Note: Usually we would use the __maybe_unused attribute to silence the
> > warning. Since this code is used in the kernel decompression stub rather
> > than in the kernel itself we can't include <linux/compiler.h> with the
> > definition of __maybe_unused (it would be possible for some platforms,
> > however for powerpc the build fails with a compiler error). We could
> > redefine __maybe_unused or use the raw __attribute__((unused)), but
> > using the #ifdef is a simpler solution.
> 
> Usually one would take a look at the root cause.  And then remove
> get_unaligned16 entirely and replace it with the get_unaligned
> helper provided by the kernel.

Thanks for the suggestion to use get_unaligned().

Unfortunately that doesn't work without hackery, at least for
powerpc. We can add arch/powerpc/include to the include paths of
the decompression stub to be able to include <asm/unaligned.h>,
however the build then fails with "undefined reference to
`get_unaligned'", because on powerpc get/put_unaligned are defined
inside an #ifdef __KERNEL__ block. We probably don't want to define
__KERNEL__ for the decompression stub just to avoid an #ifndef in the
zlib code.