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

From: Russell King (Oracle)
Date: Thu Nov 18 2021 - 05:11:43 EST

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@xxxxxxxxx wrote:
> > From: Changcheng Deng <deng.changcheng@xxxxxxxxxx>
> >
> > 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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