[PATCH v2] pwm: bcm2835: Support apply function for atomic configuration

[Sean Young]

On Fri, Dec 04, 2020 at 08:44:17AM +0000, Sean Young wrote:
> On Fri, Dec 04, 2020 at 12:42:15AM +0100, Lino Sanfilippo wrote:
> > 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.

Let me rephrase that.

Changing the division to rounding down will exactly revert the fix I made
in commit 11fc4edc483bea8bf0efa0cc726886d2342f6fa6.

So in the case described in that commit, the requested frequency was 455kHz,
but rounding down resulted in a frequency of 476kHz.

That's totally broken and a bad idea.


Sean