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

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Message-ID: <20111205234849.GA14547@n2100.arm.linux.org.uk>
Date:	Mon, 5 Dec 2011 23:48:49 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Paul Walmsley <paul@...an.com>
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, 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

On Mon, Dec 05, 2011 at 02:15:56PM -0700, Paul Walmsley wrote:
> The types associated with clock rates in the clock interface 
> (include/linux/clk.h) are inconsistent, and we should fix this. 

Rubbish.  They're different with good reason.  Rates are primerily
unsigned quantities - and should be treated as such.

The exception is clk_round_rate() which returns the rate, but also
_may_ return an error.  Therefore, its return type has to be signed.

> 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);

Yes that might have been a better solution.

> 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.)

Yuck.  You are aware that 64-bit math on 32-bit CPUs sucks?  So burdening
_everything_ with 64-bit rate quantities is absurd.  As for making then
64-bit signed quantities, that's asking for horrid code from gcc for the
majority of cases.
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