Re: [PATCH v4] media: rc: pwm-ir-tx: Switch to atomic PWM API

[Sean Young <sean@mess.org>]

Hi Uwe,

On Thu, Oct 28, 2021 at 01:15:35PM +0200, Uwe Kleine-König wrote:
> On Thu, Oct 28, 2021 at 10:14:42AM +0100, Sean Young wrote:
> > We still have the problem that the pwm drivers calculate the period
> > incorrectly by rounding down (except pwm-bcm2835). So the period is not
> > as good as it could be in most cases, but this driver can't do anything
> > about that.
> 
> Yeah, some time ago I started coding a round_state function
> (wip at
> https://git.pengutronix.de/cgit/ukl/linux/commit/?h=pwm-wip&id=ae348eb6a55d6526f30ef4a49819197d9616391e)
> but this was pushed down on my todo-list by more important stuff.
> 
> If you want to experiment with that ...

I was thinking about this problem this morning. 

- The pwm-ir-tx driver gets a carrier set in Hz, which it has to convert to
  a period (1e9 / carrier). There is loss of accuracy there.
- When it gets to the pwm driver, the period is converted into the format
  the pwm hardware expects. For example the pwm-bcm2835 driver converts
  it into clock cycles (1e9 / 8e8).

Both calculations involve loss of accuracy because of integer representation.

Would it make more sense for the pwm interface to use numer/denom rational
numbers?

struct rational {
	u64 numer;
	u64 denom;
};

If pwm-ir-tx would like to set the carrier, it could it like so:

	struct rational period = {
		.numer = NUSEC_PER_SEC,
		.denom = carrier,
	};

	pwm_set_period(&period);

Now pwm-bcm2835 could do it like so:

	int bcm2835_set_period(struct rational *period)
	{
		struct rational rate = {
			.numer = NUSEC_PER_SEC,
			.denum = clk_get_rate(clk),
		};

		rational_div(&rate, period);

		int step = rational_to_u64(&rate);
	}

Alternatively, since most of the pwm hardware is doing scaling based on the
clock (I think), would not make more sense for the pwm driver interface to
take a frequency rather than a period? Then the integer calculations can be
simpler: just divide the clock rate by the required frequency and you have
the period.

Just some thoughts.


Sean