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

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Message-Id: <8B76E3E8-52BF-4803-B1F9-E5704A858870@goldelico.com>
Date:	Wed, 30 Sep 2015 19:47:39 +0200
From:	"H. Nikolaus Schaller" <hns@...delico.com>
To:	Nathan Lynch <Nathan_Lynch@...tor.com>
Cc:	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Szabolcs Nagy <szabolcs.nagy@....com>,
	Russell King <rmk+kernel@....linux.org.uk>,
	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


Am 30.09.2015 um 19:37 schrieb Nathan Lynch <Nathan_Lynch@...tor.com>:

> On 09/30/2015 11:17 AM, Ard Biesheuvel wrote:
>> 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>:
>>>> 
>>>> 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?
> 
> Well, I think GCC did not provide __builtin_bswap16 consistently until
> the 4.8 release:
> 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52624
> 
> That seems too recent to me.

Same for me.

> 
> vdsomunge makes only three or four potentially byteswapped accesses to
> the ELF header.  It's not worth a lot of effort to try to use the most
> optimal implementation available.

Especially as it does not run on the target device but the build host.

>  Why not just use a generic
> implementation like is found in mips' elf2ecoff?

Do you have a reference?
I can't find byte swapping in

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/arch/mips/boot/elf2ecoff.c?id=refs/tags/v4.3-rc3

BR and thanks,
Nikolaus Schaller--
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