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

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Message-ID: <52823877.60806@codethink.co.uk>
Date:	Tue, 12 Nov 2013 14:17:27 +0000
From:	Ben Dooks <ben.dooks@...ethink.co.uk>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Måns Rullgård 
	<mans@...sr.com>, Stephen Boyd <sboyd@...eaurora.org>,
	linux-kernel@...r.kernel.org,
	Christopher Covington <cov@...eaurora.org>,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2] ARM: Use udiv/sdiv for __aeabi_{u}idiv library functions

On 12/11/13 14:04, 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?

I was also going to add a note about endian-ness.

Given these are single instructoins for ARM, is it possible we could
make a table of all the callers and fix them up when we initialise
as we do for the SMP/UP case and for page-offset?


-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius
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