Re: [PATCH v2 0/3] Add PWM clock support for bcm2835

From: Stefan Wahren
Date: Sun Nov 29 2015 - 16:23:40 EST


Hi Remi,

Am 29.11.2015 um 01:31 schrieb Remi Pommarel:
Hi Stefan,

On Sat, Nov 28, 2015 at 09:52:07PM +0100, Stefan Wahren wrote:
i applied the series including the devicetree modification, but it
doesn't work for me.

First of all i get an ugly division by zero warning from the pwm
driver. The pwm driver still assume a fixed clock and doesn't handle
the error cases of clk_get_rate(). I attached a patch at the end.

Yes the devicetree patch from patchset version one does not work with
this version.

thanks. I successfully tested the pwm with the led pwm driver.

I haven't sent the modified devicetree because Eric said
it is better to send it in a separate patchset. If you want to test it I
attached the working devicetree patch at the end.

I don't think that he said that. He wanted you to send the devicetree changes as a separate patch. So it should be okay if it's part of the same patchset.


But, yes, that would be nice if pwm driver was protected from this
division by zero.

I will create a proper patch.



The reason in my case why clk_get_rate() returns zero is that the
pwm clock is orphan ( pwm is listed under
/sys/kernel/debug/clk_orphan_summary ).

My suspicion is it has something to do with the clock manager driver.
The bcm2835_clock_per_parents contains only 8 entries. But according to
BCM2835-ARM-Peripherals.pdf [1] CM_GP0CTL SRC page 107 has 16
entries. The upper 8 entries are all mapped to GND. It looks to me
that the driver doesn't take care of this and so the pwm clock isn't
able to determine it's parent.


In fact, default parent for pwm after boot up is GND (CM_GP0CTL SRC ==
0). Which means that the default pwm clock rate is 0. The clock appears
to be orphan because in the bcm2835 clock driver, the GND clock is not
registered.

Thanks for the explanation.

Best regards
Stefan


So, IMHO, we have to set the default pwm rate from the devicetree, using
assigned-clock-rates. That what does the following dts patch.

This patch also set the gpio pin 18 to proper alternate function in order
to be able to get pwm output from this gpio.

Thanks


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/