Hi,
On 6/8/20 2:51 PM, Andy Shevchenko wrote:
On Mon, Jun 08, 2020 at 01:07:12PM +0200, Hans de Goede wrote:
On 6/8/20 5:50 AM, Andy Shevchenko wrote:
On Sun, Jun 07, 2020 at 08:18:28PM +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.
So, and why it's a problem?
Lets sya the user requests a PWM output frequency of 100Hz on Cherry Trail
which has a 19200000 Hz clock this will result in 100 * 65536 / 19200000 =
0.3 -> 0 as base-unit value. So instead of getting 100 Hz the user will
now get a pin which is always outputting low.
OTOH if we clamp to 1 as lowest value, the user will get 192000000 / 65536
= 292 Hz as output frequency which is as close to the requested value as
we can get while actually still working as a PWM controller.
So, we should basically divide and round up, no?
Yes, that will work for the low limit of base_unit but it will make
all the other requested period values less accurate.
At least for 0 we will get 0.
We're dealing with frequency here, but the API is dealing with period,
so to get 0 HZ the API user would have to request a period of > 1s e.g.
request 2s / 0.5 Hz but then the user is still not really requesting 0Hz
(that would correspond with a period of infinity which integers cannot
represent.
base_unit values > (base_unit_range / 256), or iow base_unit values using
the 8 most significant bits, cause loss of resolution of the duty-cycle.
E.g. assuming a base_unit_range of 65536 steps, then a base_unit value of
768 (256 * 3), limits the duty-cycle resolution to 65536 / 768 = 85 steps.
Clamp the max base_unit value to base_unit_range / 32 to ensure a
duty-cycle resolution of at least 32 steps. This limits the maximum
output frequency to 600 KHz / 780 KHz depending on the base clock.
This part I don't understand. Why we limiting base unit? I seems like a
deliberate regression.
The way the PWM controller works is that the base-unit gets added to
say a 16 bit (on CHT) counter each input clock and then the highest 8
bits of that counter get compared to the value programmed into the
ON_TIME_DIV bits.
Lets say we do not clamp and allow any value and lets say the user
selects an output frequency of half the input clock, so base-unit
value is 32768, then the counter will only have 2 values:
0 and 32768 after that it will wrap around again. So any on time-div
value < 128 will result in the output being always high and any
value > 128 will result in the output being high/low 50% of the time
and a value of 255 will make the output always low.
So in essence we now only have 3 duty cycle levels, which seems like
a bad idea to me / not what a pwm controller is supposed to do.
It's exactly what is written in the documentation. I can't buy base unit clamp.
Though, I can buy, perhaps, on time divisor granularity, i.e.
1/ 0% - 25%-1 (0%)
2/ 25% - 50% - 75% (50%)
3/ 75%+1 - 100% (100%)
And so on till we got a maximum resolution (8 bits).
Note that the PWM API does not expose the granularity to the API user,
which is why I went with just putting a minimum on it of 32 steps.
Anyways I don't have a strong opinion on this, so I'm fine with not clamping
the base-unit to preserve granularity. We should still clamp it to avoid
overflow if the user us requesting a really high frequency though!
The old code had:
base_unit &= base_unit_range;
Which means that if the user requests a too high value, then we first
overflow base_unit and then truncate it to fit leading to a random
frequency.
So if we forget my minimal granularity argument, then at a minimum
we need to replace the above line with:
base_unit = clamp_t(unsigned long long, base_unit, 1, base_unit_range - 1);
And since we need the clamp anyways we can then keep the current round-closest
behavior.
Regards,
Hans
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