Re: [PATCH v9 14/27] pwm: jz4740: Improve algorithm of clock calculation

From: Paul Cercueil
Date: Fri Feb 22 2019 - 20:18:27 EST

Hi,

Le jeu. 10 janv. 2019 à 11:04, Paul Cercueil <paul@xxxxxxxxxxxxxxx> a écrit :
Adding Stephen to the discussion.
Adding Stephen to the discussion.

On Sat, Jan 5, 2019 at 6:27 PM, Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
Hello Paul,

On Sat, Jan 05, 2019 at 06:05:38PM -0300, Paul Cercueil wrote:
On Sat, Jan 5, 2019 at 4:57 PM, Uwe Kleine-König
<u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
> You are assuming stuff here about the parent clk which isn't guaranteed
> (AFAICT) by the clk framework: If you call clk_round_rate(clk, rate - 1)
> this might well return rate even if the clock could run slower than
> rate.

It may not be guaranteed by the clock framework itself, but it is guaranteed
to behave like that on this family of SoCs.

You shouldn't rely on that. Experience shows that people will start
copying code to machines where this is not guaranteed. Even if they
don't copy and only learn from reading this is bad. Also how do you
guarantee that this won't change in the future making the pwm code break
without noticing?

If you use an API better don't assume more things given than are
guaranteed by the API.

Having said that I would consider it sensible to introduce something
like clk_roundup_rate() and clk_rounddown_rate() which would allow
calculations like that.

@Stephen:
Some context: my algorithm makes use of clk_round_rate(clk, rate - 1) to get the
next (smaller) clock rate that a clock support.

Is it something safe to assume? If not is there a better way?

Bump.

What should I do here?

> Wouldn't it make sense to start iterating with rate = 0xffff * 1e9 /
> period? Otherwise you get bad configurations if rate is considerable
> slower than necessary.

The algorithm will start with 'rate' being the parent clock's rate, which
will always be the highest rate that the child clock will support.

Ah right, I missed that bit.

Thanks,
-Paul