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

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Date:	Mon, 11 Nov 2013 18:34:08 -0800
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Nicolas Pitre <nico@...xnic.net>
CC:	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>,
	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 11/09/13 21:03, Nicolas Pitre wrote:
> Bah..... NAK. We are doing runtime patching of the kernel for many
> many things already. So why not do the same here?

static keys are a form of runtime patching, albeit not as extreme as
you're suggesting.

>
> The obvious strategy is to simply overwrite the start of the existing 
> __aeabi_idiv code with the "sdiv r0, r0, r1" and "bx lr" opcodes.  
>
> Similarly for the unsigned case.

I was thinking the same thing when I wrote this, but I didn't know how
to tell the compiler to either inline this function or to let me inilne
an assembly stub with some section magic.

>
> That let you test the hardware capability only once during boot instead 
> of everytime a divide operation is performed.

The test for hardware capability really isn't done more than once during
boot. The assembly is like so at compile time

00000000 <__aeabi_idiv>:
   0:   nop     {0}
   4:   b       0 <___aeabi_idiv>
   8:   sdiv    r0, r0, r1
   c:   bx      lr

and after we test and find support for the instruction it will be
replaced with

00000000 <__aeabi_idiv>:
   0:   b       8
   4:   b       0 <___aeabi_idiv>
   8:   sdiv    r0, r0, r1
   c:   bx      lr

Unfortunately we still have to jump to this function. It would be great
if we could inline this function at the call site but as I already said
I don't know how to do that.

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by The Linux Foundation

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