linux-kernel - Re: [PATCH] ARM: fix vdsomunge depends on glibc specific byteswap.h

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Message-Id: <771A393B-5BCF-4DEB-9C93-53EFEA91B41A@goldelico.com>
Date:	Wed, 30 Sep 2015 19:25:30 +0200
From:	"H. Nikolaus Schaller" <hns@...delico.com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	Szabolcs Nagy <szabolcs.nagy@....com>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Nathan Lynch <nathan_lynch@...tor.com>,
	Will Deacon <will.deacon@....com>,
	Marek Belisko <marek@...delico.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: fix vdsomunge depends on glibc specific byteswap.h

Hi,

Am 30.09.2015 um 18:17 schrieb Ard Biesheuvel <ard.biesheuvel@...aro.org>:

> On 30 September 2015 at 18:13, H. Nikolaus Schaller <hns@...delico.com> wrote:
>> 
>> Am 30.09.2015 um 18:02 schrieb Ard Biesheuvel <ard.biesheuvel@...aro.org>:
>> 
>>> On 30 September 2015 at 17:56, H. Nikolaus Schaller <hns@...delico.com> wrote:
>>>> If the host toolchain is not glibc based then the arm kernel build
>>>> fails with
>>>> 
>>>> HOSTCC  arch/arm/vdso/vdsomunge
>>>> arch/arm/vdso/vdsomunge.c:48:22: fatal error: byteswap.h: No such file or directory
>>>> 
>>>> Observed with omap2plus_defconfig and compile on Mac OS X with arm ELF
>>>> cross-compiler.
>>>> 
>>>> Reason: byteswap.h is a glibc only header.
>>>> 
>>>> Changed to detect the host and include the right file as described at:
>>>> 
>>>> https://bugs.freedesktop.org/show_bug.cgi?id=8882
>>>> 
>>>> Tested to compile on Mac OS X 10.9.5 host.
>>>> 
>>>> Signed-off-by: H. Nikolaus Schaller <hns@...delico.com>
>>>> ---
>>>> arch/arm/vdso/vdsomunge.c | 18 ++++++++++++++++++
>>>> 1 file changed, 18 insertions(+)
>>>> 
>>>> diff --git a/arch/arm/vdso/vdsomunge.c b/arch/arm/vdso/vdsomunge.c
>>>> index aedec81..5364cea 100644
>>>> --- a/arch/arm/vdso/vdsomunge.c
>>>> +++ b/arch/arm/vdso/vdsomunge.c
>>>> @@ -45,7 +45,25 @@
>>>> * it does.
>>>> */
>>>> 
>>>> +#if defined(__linux__)
>>>> #include <byteswap.h>
>>>> +#elif defined(__OpenBSD__)
>>>> +#include <sys/endian.h>
>>>> +#define bswap_16 __swap16
>>>> +#define bswap_32 __swap32
>>>> +#define bswap_64 __swap64
>>>> +#elif defined(__APPLE__)
>>>> +#include <libkern/OSByteOrder.h>
>>>> +#define bswap_16 OSSwapInt16
>>>> +#define bswap_32 OSSwapInt32
>>>> +#define bswap_64 OSSwapInt64
>>>> +#else
>>>> +#include <sys/endian.h>
>>>> +#define bswap_16 bswap16
>>>> +#define bswap_32 bswap32
>>>> +#define bswap_64 bswap64
>>>> +#endif
>>>> +
>>> 
>>> Have you tried this?
>>> 
>>> #define bswap_16 __builtin_bswap16
>>> #define bswap_32 __builtin_bswap32
>>> #define bswap_64 __builtin_bswap64
>>> 
>>> https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
>> 
>> OS X host uses llvm and I am not sure if these builtins are
>> always available.
>> 
> 
> I am pretty sure recent clang supports these as well. Could you please try it?

What release date (= feature set) of a compiler would you no longer consider as "recent"?

> 
>> So it replaces an include file dependency by a compiler
>> built-in dependency. IMHO my approach is more general
>> and should cover a broader range of compile hosts.
>> 
>> This is a tool compiled by HOSTCC to be run on the build
>> host - so I think we can't assume to have a gcc host compiler
>> (while for the CC cross-compiler we can).
>> 
> 
> If my suggestion works, we'll support host side GCC and clang without
> decorating the source code with lots of #ifdefs for OSes few people
> care about. That would be an improvement imo

Indeed it would be the best solution.

But at least my Apple LLVM version 6.0 (clang-600.0.57) (based on LLVM 3.5svn)
it does *not* work (see also for example http://sourceforge.net/p/flac/bugs/416/ ).

Clang 3.5 appears to provide all features of GCC 4.7.

I have checked and it appears that __builtin_bswap16 only became available in
GCC 4.8 (released 2013-03-22). And maybe in Clang 3.7.

Unfortunately I could not find a reference what the minimum required gcc for
the Linux HOSTCC currently is.

So if we require gcc >= 4.8 just by this small tool, this should be made well
known and well decided.

Therefore I would propose to use the #ifdef approach now and revise 5 years
after gcc 4.8 release (2013 ==> 2018) if __builtin_bswap16 (or something else)
has become a widely available standard.

I can add this to the patch commit V2 for reference.

BR and thanks,
Nikolaus Schaller

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