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

[Nicolas Ferre]

On 18/11/2021 at 11:09, Russell King (Oracle) wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>
> 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.

Thanks for the technical demonstration Russell. With this in mind:
NACK to the patch, sorry Changcheng Deng.

Best regards,
  Nicolas


--
Nicolas Ferre