linux-kernel - Re: [PATCH v2] ARM: Use udiv/sdiv for __aeabi

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Message-ID: <alpine.LFD.2.10.1311120933060.9667@knanqh.ubzr>
Date:	Tue, 12 Nov 2013 09:36:29 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Måns Rullgård <mans@...sr.com>
cc:	Stephen Boyd <sboyd@...eaurora.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	Christopher Covington <cov@...eaurora.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Rob Herring <robherring2@...il.com>
Subject: Re: [PATCH v2] ARM: Use udiv/sdiv for __aeabi_{u}idiv library
 functions

On Tue, 12 Nov 2013, Måns Rullgård wrote:

> Nicolas Pitre <nicolas.pitre@...aro.org> writes:
> 
> > What about this patch which I think is currently your best option.  Note 
> > it would need to use the facilities from asm/opcodes.h to make it endian 
> > agnostic.
> >
> > diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
> > index 6a1b8a81b1..379cffe4ab 100644
> > --- a/arch/arm/kernel/setup.c
> > +++ b/arch/arm/kernel/setup.c
> > @@ -383,6 +383,34 @@ static void __init cpuid_init_hwcaps(void)
> >  		elf_hwcap |= HWCAP_IDIVT;
> >  	}
> >
> > +	/*
> > +	 * Patch our division routines with the corresponding opcode
> > +	 * if the hardware supports it.
> > +	 */
> > +	if (IS_ENABLED(CONFIG_THUMB2_KERNEL) && (elf_hwcap & HWCAP_IDIVT)) {
> > +		extern char __aeabi_uidiv, __aeabi_idiv;
> 
> It would be safer to declare these as arrays of unspecified size.
> Otherwise the compiler might do evil things with what to it looks like
> out of bounds indexing.

Right.

> 
> There should also be some cache maintenance after this patching, or is
> that already happening for some other reason?

This is so early during boot that the MMU isn't even fully initialized 
yet.  The cache will be flushed.


Nicolas