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

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Date:	Mon, 5 Dec 2011 14:15:56 -0700 (MST)
From:	Paul Walmsley <paul@...an.com>
To:	Mike Turquette <mturquette@...com>
cc:	linux@....linux.org.uk, 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, aul@...an.com,
	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

Hi

a brief comment concerning clock rates:

On Mon, 21 Nov 2011, Mike Turquette wrote:

> +unsigned long clk_get_rate(struct clk *clk)

...

> +long clk_round_rate(struct clk *clk, unsigned long rate)

...

> +int clk_set_rate(struct clk *clk, unsigned long rate)

...

> +struct clk {
...
> +	unsigned long		rate;
...
> +};

The types associated with clock rates in the clock interface 
(include/linux/clk.h) are inconsistent, and we should fix this. 
clk_round_rate() is the problematic case, returning a signed long rather 
than an unsigned long.  So clk_round_rate() won't work correctly with any 
rates higher than 2 GiHz.

We could fix the immediate problem by changing the prototype of 
clk_round_rate() to pass the rounded rate back to the caller via a pointer 
in one of the arguments, and return an error code (if any) via the return 
value:

int clk_round_rate(struct clk *clk, unsigned long rate, unsigned long 
                   *rounded_rate);

But I'd propose that we instead increase the size of struct clk.rate to be 
s64:

s64 clk_round_rate(struct clk *clk, s64 desired_rate);
int clk_set_rate(struct clk *clk, s64 rate);
s64 clk_get_rate(struct clk *clk);

struct clk {
...
     s64 rate;
...
};

That way the clock framework can accommodate current clock rates, as well 
as any conceivable future clock rate.  (Some production CPUs are already 
running at clock rates greater than 4 GiHZ[1].  RF devices with 4 GiHz+ 
clock rates are also common, such as 802.11a devices running in the 5.8 
GHz band, and drivers for those may eventually wish to use the clock 
framework.)



- Paul

1. www.cpu-world.com, "Intel Xeon X5698 - AT80614007314AA" 
   http://www.cpu-world.com/CPUs/Xeon/Intel-Xeon%20X5698%20-%20AT80614007314AA.html
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