Hi,
On Fri, Dec 04, 2020 at 12:42:15AM +0100, Lino Sanfilippo wrote:
> > You're storing an unsigned long long (i.e. 64 bits) in an u32. If
> > you are sure that this won't discard relevant bits, please explain
> > this in a comment for the cursory reader.
>
> What about an extra check then to make sure that the period has not been
> truncated,
> e.g:
>
> value = DIV_ROUND_CLOSEST_ULL(state->period, scaler);
>
> /* dont accept a period that is too small or has been truncated */
> if ((value < PERIOD_MIN) ||
> (value != DIV_ROUND_CLOSEST_ULL(state->period, scaler)))
> return -EINVAL;
Rather than doing another 64 bit division which is expensive (esp on 32 bit
kernels), you could assign to u64 and check:
if (value < PERIOD || value > U32_MAX)
return -EINVAL;
> > Also note that round_closed is probably wrong, as .apply() is
> > supposed to round down the period to the next achievable period. (But
> > fixing this has to do done in a separate patch.)
>
> According to commit 11fc4edc4 rounding to the closest integer has been
> introduced
> to improve precision in case that the pwm controller is used by the pwm-ir-tx
> driver.
> I dont know how strong the requirement is to round down the period in
> apply(), but I
> can imagine that this may be a good reason to deviate from this rule.
> (CCing Sean Young who introduced DIV_ROUND_CLOSEST)
There was a problem where the carrier is incorrect for some IR hardware
which uses a carrier of 455kHz. With periods that small, rounding errors
do really matter and rounding down might cause problems.
A policy of rounding down the carrier is not the right thing to do
for pwm-ir-tx, and such a change will probably break pwm-ir-tx in some
edge cases.
Thanks
Sean
