On Thu, Jun 18, 2015 at 09:58:59PM +0800, Yingjoe Chen wrote: > On Thu, 2015-06-18 at 18:19 +0800, YH Huang wrote: > > On Fri, 2015-06-12 at 12:20 +0200, Thierry Reding wrote: > > > > +/* Shift log2(PWM_PERIOD_MAX + 1) as divisor */ > > > > +#define PWM_PERIOD_BIT_SHIFT 12 > > > > > > I wasn't very clear about this in my earlier review, so let me try to > > > explain why I think this is confusing. You use this as a divisor, but > > > you encode it as a shift. It's also PWM_PERIOD_MAX + 1, so I think it > > > would make more sense to drop this, keep PWM_PERIOD_MAX as above and > > > then replace the > > > > > > >> PWM_PERIOD_BIT_SHIFT > > > > > > below by > > > > > > / (PWM_PERIOD_MAX + 1) > > > > > > > Maybe I can change in this way: > > Remove this: #define PWM_PERIOD_MAX 0x00000fff > > Using ">> PWM_PERIOD_BIT_SHIFT" is faster than "/ (PWM_PERIOD_MAX + 1)" > > Is this right? > > > The place which use this shift is: > > clk_div = div_u64(rate * period_ns, NSEC_PER_SEC) >> > PWM_PERIOD_BIT_SHIFT; > > div_u64 return u64. If we change >> to /, and somehow compiler didn't > optimize that div into shift, it will cause build error.
Good point. I think every compiler should be able to optimize this, but the shift isn't any worse than a divide and if we can proactively avoid portability issues, let's go with the shift. Thierry
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