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

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Date:	Tue, 12 Nov 2013 09:16:32 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
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>,
	Måns Rullgård <mans@...sr.com>,
	Rob Herring <robherring2@...il.com>
Subject: Re: [PATCH v2] ARM: Use udiv/sdiv for __aeabi_{u}idiv library
 functions

On Tue, 12 Nov 2013, Russell King - ARM Linux wrote:

> On Tue, Nov 12, 2013 at 09:01:16AM -0500, Nicolas Pitre wrote:
> > 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;
> > +		u16 *uidiv = (u16 *)&__aeabi_uidiv;
> > +		u16 *idiv = (u16 *)&__aeabi_idiv;
> > +
> > +		uidiv[0] = 0xfbb0;  /* udiv r0, r0, r1 */
> > +		uidiv[1] = 0xf0f1;
> > +		uidiv[2] = 0x4770;  /* bx lr */
> > +
> > +		idiv[0] = 0xfb90;  /* sdiv r0, r0, r1 */
> > +		idiv[1] = 0xf0f1;  
> > +		idiv[2] = 0x4770;  /* bx lr */
> > +	} else if (!IS_ENABLED(CONFIG_THUMB2_KERNEL) && (elf_hwcap & HWCAP_IDIVA)) {
> > +		extern char __aeabi_uidiv, __aeabi_idiv;
> > +		u32 *uidiv = (u32 *)&__aeabi_uidiv;
> > +		u32 *idiv = (u32 *)&__aeabi_idiv;
> > +
> > +		uidiv[0] = 0xe730f110;  /* udiv r0, r0, r1 */
> > +		uidiv[1] = 0xe12fff1e;  /* bx lr */
> > +
> > +		idiv[0] = 0xe710f110;  /* sdiv r0, r0, r1 */
> > +		idiv[1] = 0xe12fff1e;  /* bx lr */
> > +	}
> 
> What about endianness, and what if XIP is enabled?

Just as I said above the diff: this needs refined.

Obviously XIP can't use this and doesn't need it either as a XIP kernel 
should be optimized for the very platform it will run onto i.e. gcc 
should already emit those div opcodes inline if appropriate.


Nicolas
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