Re: [PATCH v3 04/15] pwm: lpss: Add range limit check for the base_unit register value

Hans de Goede
2020-07-06T20:53:18Z

Hi,

Thank you for your review and sorry for the slow reply.

I would like to get this series upstream this cycle, so
I will do my best to be a lot faster with responding from
now on.

On 6/22/20 9:35 AM, Uwe Kleine-König wrote:
> On Sat, Jun 20, 2020 at 02:17:47PM +0200, Hans de Goede wrote:
>> When the user requests a high enough period ns value, then the
>> calculations in pwm_lpss_prepare() might result in a base_unit value of 0.
>>
>> But according to the data-sheet the way the PWM controller works is that
>> each input clock-cycle the base_unit gets added to a N bit counter and
>> that counter overflowing determines the PWM output frequency. Adding 0
>> to the counter is a no-op. The data-sheet even explicitly states that
>> writing 0 to the base_unit bits will result in the PWM outputting a
>> continuous 0 signal.
>>
>> When the user requestes a low enough period ns value, then the
>> calculations in pwm_lpss_prepare() might result in a base_unit value
>> which is bigger then base_unit_range - 1. Currently the codes for this
>> deals with this by applying a mask:
>>
>> base_unit &= (base_unit_range - 1);
>>
>> But this means that we let the value overflow the range, we throw away the
>> higher bits and store whatever value is left in the lower bits into the
>> register leading to a random output frequency, rather then clamping the
>> output frequency to the highest frequency which the hardware can do.
>>
>> This commit fixes both issues by clamping the base_unit value to be
>> between 1 and (base_unit_range - 1).
>>
>> Fixes: 684309e5043e ("pwm: lpss: Avoid potential overflow of base_unit")
>> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
>> ---
>> Changes in v3:
>> - Change upper limit of clamp to (base_unit_range - 1)
>> - Add Fixes tag
>> ---
>> drivers/pwm/pwm-lpss.c | 4 +++-
>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/pwm/pwm-lpss.c b/drivers/pwm/pwm-lpss.c
>> index 43b1fc634af1..80d0f9c64f9d 100644
>> --- a/drivers/pwm/pwm-lpss.c
>> +++ b/drivers/pwm/pwm-lpss.c
>> @@ -97,6 +97,9 @@ static void pwm_lpss_prepare(struct pwm_lpss_chip *lpwm, struct pwm_device *pwm,
>> freq *= base_unit_range;
>>
>> base_unit = DIV_ROUND_CLOSEST_ULL(freq, c);
>
> DIV_ROUND_CLOSEST_ULL is most probably wrong, too. But I didn't spend
> the time to actually confirm that.

Yes I saw your comment elsewhere that the PWM API defines rounding
in a certain direction, but fixing that falls outside of this patch.

>> + /* base_unit must not be 0 and we also want to avoid overflowing it */
>> + base_unit = clamp_t(unsigned long long, base_unit, 1,
>> + base_unit_range - 1);
>
> .get_state seems to handle base_unit == 0 just fine?!

It tries to do something with it to avoid a divide by 0, back when
I wrote the get_state code I wasn't fully aware of how the PWM
controller works. I did have access to the same datasheets as today
(the datasheets for this one are public) but the datasheet needs to
be read and then left to sync in for a couple of months and then read
again, or iow the datasheet does not explain things all that well.

As I tried to explain in the commit msg the way this PWM controller
works is it takes its input clock and then each input clock-cycle the
"base_unit" gets added to a n-bit register lets say a 16-bit register
at that is the case for the HW on which I've done all my testing.

The 8 most significant bits of the 16 bit register are compared with
a 8 bit value programmed by the PWM driver / coming from a ctrl
register and the output of that comparator is the PWM output.

The problem with a base_unit value of '0' is that adding 0 to the
16 bit register is a no-op, so the register never increments
(iow is always 0) and thus can never become bigger then the comparator
input and thus the PWM output is always 0.

The datasheet does helpfully contain a note explicitly warning of
this behavior.

So when we are programming the base_unit value, it seems best to
clamp the lower end to 1, which gives an PWM output frequency of
e.g. 19200000 / 65536 = 293 Hz

If the user has request an even lower output frequency, which
would result in our base_unit calculation outputting 0, then we
can either output always low, which is an infinite low output
frequency, or give the user 293 Hz and a working PWM.

This is the low end of the clamp. The high end clamp simply is
there because base_unit itself is e.g. a 16 bit value too.

The looks a bid weird because instead of 65536 (for the divides)
/ 65535 (for the clamp / masking) it uses base_unit_range and
(base_unit_range - 1). This is because different versions of
the SoCs using this driver have a different register size for the
base_unit value.

I hope this helps to explain what is going on a bit.

###

As for the behavior on base_unit==0 in the get_state method,
as mentioned above I wrote that when I did not fully understood
how the controller works.

We really should never encounter this.

But if we do then I think closest to the truth would be:

state->period = UINT_MAX;
state->duty_cycle = 0;

I can submit a separate patch for that, if you agree
that this is the best way to describe the "output always low"
state in which a base_unit value of 0 will result.

Regards,

Hans