Re: [PATCH] pwm: Use div64_ul instead of do_div

From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: cgel.zte@gmail.com, linux-pwm@vger.kernel.org,
	alexandre.belloni@bootlin.com, Zeal Robot <zealci@zte.com.cn>,
	linux-kernel@vger.kernel.org, ludovic.desroches@microchip.com,
	thierry.reding@gmail.com,
	Changcheng Deng <deng.changcheng@zte.com.cn>,
	lee.jones@linaro.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] pwm: Use div64_ul instead of do_div
Date: Thu, 18 Nov 2021 10:09:41 +0000	[thread overview]
Message-ID: <YZYmZecp8WPkFY2F@shell.armlinux.org.uk> (raw)
In-Reply-To: <20211117112400.bkscb2pyavonpfsn@pengutronix.de>

On Wed, Nov 17, 2021 at 12:24:00PM +0100, Uwe Kleine-König wrote:
> Hello,
> 
> On Wed, Nov 17, 2021 at 02:04:26AM +0000, cgel.zte@gmail.com wrote:
> > From: Changcheng Deng <deng.changcheng@zte.com.cn>
> > 
> > do_div() does a 64-by-32 division. If the divisor is unsigned long, using
> > div64_ul can avoid truncation to 32-bit.
> 
> After some research I understood your commit log. I'd write:
> 
> 	do_div() does a 64-by-32 division. Here the divsor is an
> 	unsigned long which on some platforms is 64 bit wide. So use
> 	div64_ul instead of do_div to avoid a possible truncation.
> 
> The priority of this patch seems to be low, as the device seems to exist
> only on (32bit) arm.

... where unsigned long is 32-bit.

In any case, for this to overflow, we would need to have a clock in
excess of 2^32-1 Hz, or around 4GHz - and if we had such a situation
on 32-bit devices, we need to change the type for holding the frequency
in the clk API, and probably a lot of code in the CCF as well.

Unless there is a real reason for this change, I would suggest leaving
the code as is - there is absolutely no point in making these divisions
more expensive unless there is a real reason.

-- 
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