linux-kernel - Re: [PATCH v3 3/5] clk: introduce the common clock framework

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Date:	Mon, 5 Dec 2011 23:28:08 -0700 (MST)
From:	Paul Walmsley <paul@...an.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
cc:	Mike Turquette <mturquette@...com>, linux-kernel@...r.kernel.org,
	linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	jeremy.kerr@...onical.com, broonie@...nsource.wolfsonmicro.com,
	tglx@...utronix.de, linus.walleij@...ricsson.com,
	amit.kucheria@...aro.org, dsaxena@...aro.org, patches@...aro.org,
	linaro-dev@...ts.linaro.org, grant.likely@...retlab.ca,
	sboyd@...cinc.com, shawn.guo@...escale.com, skannan@...cinc.com,
	magnus.damm@...il.com, arnd.bergmann@...aro.org,
	eric.miao@...aro.org, richard.zhao@...aro.org,
	Mike Turquette <mturquette@...aro.org>
Subject: Re: [PATCH v3 3/5] clk: introduce the common clock framework

On Mon, 5 Dec 2011, Paul Walmsley wrote:

> For example, here's a trivial implementation for rate recalculation for a 
> integer divider clock node (that can't be handled with a right shift):
> 
> s64 div(struct clk *clk, u32 div) {
> 	if (clk->flags & CLK_PARENT_RATE_MAX_U32)
> 		return ((u32)(clk->parent->rate & 0xffffffff)) / div;
> 
> 	clk->rate = clk->parent->rate;
> 	do_div(clk->rate, div);
> 	return clk->rate;
> }

(removing some useless cruft from the above function, for clarity's sake)

s64 div(struct clk *clk, u32 div) {
	s64 r;

	if (clk->flags & CLK_PARENT_RATE_MAX_U32)
		return ((u32)clk->parent->rate) / div;

	r = clk->parent->rate;
	do_div(r, div);
	return r;
}

00000000 <div>:
   0:	e92d4010 	push	{r4, lr}
   4:	e1a04001 	mov	r4, r1
   8:	e5d03010 	ldrb	r3, [r0, #16]
   c:	e3130001 	tst	r3, #1
  10:	1a000005 	bne	2c <div+0x2c>

  14:	e5903000 	ldr	r3, [r0]
  18:	e1c300d8 	ldrd	r0, [r3, #8]
  1c:	ebfffffe 	bl	0 <__do_div64>
  20:	e1a00002 	mov	r0, r2
  24:	e1a01003 	mov	r1, r3
  28:	e8bd8010 	pop	{r4, pc}

  2c:	e5900000 	ldr	r0, [r0]
  30:	e5900008 	ldr	r0, [r0, #8]
  34:	ebfffffe 	bl	0 <__aeabi_uidiv>
  38:	e3a01000 	mov	r1, #0
  3c:	e8bd8010 	pop	{r4, pc}


- Paul
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