Re: [PATCH 1/2] pwm: sunxi: allow the pwm to finish its pulse before disable

[Olliver Schinagl <o.schinagl@ultimaker.com>]

Hey Maxime,

Happy new year! I'm sorry that I missed your previous mail! I completely 
looked over it. Sorry!

On 12-12-16 13:24, Maxime Ripard wrote:
> On Thu, Dec 08, 2016 at 02:23:39PM +0100, Olliver Schinagl wrote:
>> Hey Maxime,
>>
>> first off, also sorry for the slow delay :) (pun not intended)
>>
>> On 27-08-16 00:19, Maxime Ripard wrote:
>>> On Thu, Aug 25, 2016 at 07:50:10PM +0200, Olliver Schinagl wrote:
>>>> When we inform the PWM block to stop toggeling the output, we may end up
>>>> in a state where the output is not what we would expect (e.g. not the
>>>> low-pulse) but whatever the output was at when the clock got disabled.
>>>>
>>>> To counter this we have to wait for maximally the time of one whole
>>>> period to ensure the pwm hardware was able to finish. Since we already
>>>> told the PWM hardware to disable it self, it will not continue toggling
>>>> but merly finish its current pulse.
>>>>
>>>> If a whole period is considered to much, it may be contemplated to use a
>>>> half period + a little bit to ensure we get passed the transition.
>>>>
>>>> Signed-off-by: Olliver Schinagl<oliver@schinagl.nl>
>>>> ---
>>>>   drivers/pwm/pwm-sun4i.c | 11 +++++++++++
>>>>   1 file changed, 11 insertions(+)
>>>>
>>>> diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c
>>>> index 03a99a5..5e97c8a 100644
>>>> --- a/drivers/pwm/pwm-sun4i.c
>>>> +++ b/drivers/pwm/pwm-sun4i.c
>>>> @@ -8,6 +8,7 @@
>>>>   #include <linux/bitops.h>
>>>>   #include <linux/clk.h>
>>>> +#include <linux/delay.h>
>>>>   #include <linux/err.h>
>>>>   #include <linux/io.h>
>>>>   #include <linux/module.h>
>>>> @@ -245,6 +246,16 @@ static void sun4i_pwm_disable(struct pwm_chip *chip, struct pwm_device *pwm)
>>>>   	spin_lock(&sun4i_pwm->ctrl_lock);
>>>>   	val = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
>>>>   	val &= ~BIT_CH(PWM_EN, pwm->hwpwm);
>>>> +	sun4i_pwm_writel(sun4i_pwm, val, PWM_CTRL_REG);
>>>> +	spin_unlock(&sun4i_pwm->ctrl_lock);
>>>> +
>>>> +	/* Allow for the PWM hardware to finish its last toggle. The pulse
>>>> +	 * may have just started and thus we should wait a full period.
>>>> +	 */
>>>> +	ndelay(pwm_get_period(pwm));
>>> Can't that use the ready bit as well?
>>
>> I started to implement our earlier discussed suggestions, but I do not think
>> they will work. The read bit is not to let the user know it is ready with
>> all of its commands, but only if the period registers are ready. I think it
>> is some write lock while it copies the data into its internal control loop.
>> From the manual:
>> PWM0 period register ready.
>> 0: PWM0 period register is ready to write,
>> 1: PWM0 period register is busy.
>>
>>
>> So no, I don't think i can use the ready bit here at all. The only thing we
>> can do here, but I doubt it's worth it, is to read the period register,
>> caluclate a time from it, and then ndelay(pwm_get_period(pwm) - ran_time)
>>
>> The only 'win' then is that we could are potentially not waiting the full
>> pwm period, but only a fraction of it. Since we are disabling the hardware
>> (for power reasons) anyway, I don't think this is any significant win,
>> except for extreme situations. E.g. we have a pwm period of 10 seconds, we
>> disable it after 9.9 second, and now we have to wait for 10 seconds before
>> the pwm_disable is finally done. So this could in that case be reduced to
>> then only wait for 0.2 seconds since it is 'done' sooner.
>>
>> However that optimization is also not 'free'. We have to read the period
>> register and calculate back the time. I suggest to do that when reworking
>> this driver to work with atomic mode, and merge this patch 'as is' to
>> atleast fix te bug where simply not finish properly.
>
> That whole discussion made me realise something that is really
> bad. AFAIK, pwm_get_period returns a 32 bits register, which means a
> theorical period of 4s. Busy looping during 4 seconds is already very
> bad, as you basically kill one CPU during that time, but doing so in a
> (potentially) atomic context is even worse.
Well technically, isn't it a 16 bit register? (half for the period, 
other half for the duty cycle?) Anyway, I think the delay can be far 
exceeding 4 seconds (though I haven't checked what the PWM delay max 
option is).

Anyway, you are right, we should absolutely not do this!

>
> NACK.
Absolutely! But what do you suggest? Would usleep (or msleep) instead of 
the ndelay work properly?

>
> Maxime
>