Re: [PATCH anybus v1 2/4] dt-bindings: anybus-bridge: document devicetree binding.

From: thesven73
Date: Thu Oct 25 2018 - 09:55:24 EST

Hi Linus, thank you for the patch review !!

>> + - pwms : the pwm connected to the bridge's 'pwm input'.
> That is really unintuitive and needs a detailed explanation. What
> is a bridge doing with a PWM? Is it 100% certain this is a PWM,
> it's not just a .... clock? A pwm is a pule WIDTH modulator and
> I can't for my life understand why a bus bridge needs a signal
> with variable pulse width, but surprise me! :D

You are 100% correct, this is a clock !
The hardware designers attached the bridge's clock input to an iMX pwm output,
and instructed us to provide a clock with 50% duty cycle and a certain freq.

The only way I know to activate a pwm is to connect it to a driver in the fdt,
then inside the driver enable the pwm, like so:

+ /* PWM */
+ pwm = devm_pwm_get(dev, NULL);
+ if (IS_ERR(pwm)) {
+ dev_err(dev, "pwm not found\n");
+ return -EINVAL;
+ }
+ pwm_get_args(pwm, &pargs);
+ period = pargs.period;
+ err = pwm_config(pwm, period/2, period);
+ if (err)
+ return err;
+ err = pwm_enable(pwm);
+ if (err)
+ return err;

This is why the bridge driver has a dependency on a pwm.
If the pwm could be enabled individually, I could drop this dependency.
Can you think of a way?

>> + fsl,weim-cs-timing = <0x024400b1 0x00001010 0x20081100
>> + 0x00000000 0xa0000240 0x00000000>;
> Is it just a copy/paste from
> Documentation/devicetree/bindings/bus/imx-weim.txt
> leftover?

No. We attach the bridge to the i.MX WEIM bus. Every fdt WEIM child node
requires a fsl,weim-cs-timing property, which provides the bus timing for
that particular chip select. It's the weim driver that requires this,
I'm only following its instructions.

Should I just leave this out in the example? In theory, the bridge can be
connected to any parallel bus, so fsl,weim-cs-timing is Too Much Information
for the example?