On Thu, Jan 14, 2021 at 10:28:02PM +0000, Russell King - ARM Linux admin wrote:

On Thu, Jan 14, 2021 at 09:25:45PM +0100, Uwe Kleine-König wrote:

quoted

Hello Baruch,

On Thu, Jan 14, 2021 at 08:57:37PM +0200, Baruch Siach wrote:

quoted

Add a comment on why the code never sets on/off registers to zero.

Reported-by: Uwe Kleine-König <redacted>
Analyzed-by: Russell King [off-list ref]
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
---
 drivers/gpio/gpio-mvebu.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpio/gpio-mvebu.c b/drivers/gpio/gpio-mvebu.c
index 6b017854ce61..09780944bef9 100644
--- a/drivers/gpio/gpio-mvebu.c
+++ b/drivers/gpio/gpio-mvebu.c

@@ -706,6 +706,10 @@ static int mvebu_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 	do_div(val, NSEC_PER_SEC);
 	if (val > UINT_MAX)
 		return -EINVAL;
+	/*
+	 * Zero on/off values don't work as expected. Experimentation shows
+	 * that zero value is treated as 2^32. This behavior is not documented.
+	 */

This is too easy. The right thing to do is to adapt .apply and
.get_state to use this new information.

What exactly do you expect the changes to be?

What I expect is:

 - let .apply() write 0 if the intention is to configure 2^32 clock
   steps for the on or off register; and symmetrically
 - let .get_state report 2^32 * NSEC_PER_SEC / clk_rate if the register
   value is 0.
 

Bear in mind that the hardware is not capable of atomically updating
e.g. the duty cycle without affecting the period, because any change
in duty cycle needs the "on" and "off" durations to be separately
programmed, and there's a chance that the hardware could start using
either value mid-update.

Also, disabling "blink" mode to achieve a steady output (for 0% or 100%
duty cycle) would require further investigation to find out how the
hardware behaves at the moment where blink mode is disabled: does the
output retain its current state (does the bit in the output register
toggle with the blink) or does it revert to the value in the output
register that was programmed before blink mode was enabled.

I have some plans here about what is the right behaviour, but this needs
some preparatory work that I didn't do yet. I'll come back to this
eventually.

Best regards
Uwe

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