[PATCH v15 1/2] pwm: add microchip soft ip corePWM driver
Conor Dooley
conor at kernel.org
Sat Apr 1 13:50:47 PDT 2023
On Thu, Mar 30, 2023 at 08:12:03AM +0100, Conor Dooley wrote:
> + /*
> + * Because 0xff is not a permitted value some error will seep into the
> + * calculation of prescale as prescale grows. Specifically, this error
> + * occurs where the remainder of the prescale calculation is less than
> + * prescale.
> + * For small values of prescale, only a handful of values will need
> + * correction, but overall this applies to almost half of the valid
> + * values for tmp.
> + *
> + * To keep the algorithm's decision making consistent, this case is
> + * checked for and the simple solution is to, in these cases,
> + * decrement prescale and check that the resulting value of period_steps
> + * is valid.
> + *
> + * period_steps can be computed from prescale:
> + * period * clk_rate
> + * period_steps = ----------------------------- - 1
> + * NSEC_PER_SEC * (prescale + 1)
> + *
> + */
> + if (tmp % (MCHPCOREPWM_PERIOD_STEPS_MAX + 1) < *prescale) {
Hmm, looks like 32-bit doesn't like this modulus.
I pushed things out for LKP to test before sending as I felt I'd not be
allowed to do that operation, but got a build success email from it.
I'm not sure why the mail wasn't sent as a reply to this, but
<202304020410.A86IBNES-lkp at intel.com> complains:
pwm-microchip-core.c:(.text+0x20a): undefined reference to `__aeabi_uldivmod'
I know that tmp < 65536 at this point, so if the general approach is
fine, I can always cast it to a non 64-bit type without losing any
information.
> + u16 smaller_prescale = *prescale - 1;
> +
> + *period_steps = div_u64(tmp, smaller_prescale + 1) - 1;
> + if (*period_steps < 255) {
> + *prescale = smaller_prescale;
> +
> + return 0;
> + }
> + }
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