On Wed, Jun 06, 2018 at 01:42:10PM -0400, Thomas Hebb wrote: > The Berlin PWM driver is currently broken on at least BG2CD. The > symptoms manifest as a very non-linear and erratic mapping from the duty > cycle configured in software to the duty cycle produced by hardware. > > The cause of the bug is software's configuration of the prescaler, and > in particular its usage of the six prescaler values between the minimum > value of 1 and the maximum value of 4096. As it turns out, these six > values do not actually slow down the PWM clock; rather, they emulate > slowing down the clock by internally multiplying the value of TCNT. > > This would be a fine trick, if not for the fact that the internal, > scaled TCNT value has no extra bits beyond the 16 already exposed to > software in the register. What this means is that, for a prescaler of 4, > the software must ensure that the top two bits of TCNT are not set, > because hardware will chop them off; for a prescaler of 8, the top three > bits must not be set, and so forth. Software does not currently ensure > this, resulting in a TCNT several orders of magnitude lower than > intended any time one of those six prescalers are selected. > > Because hardware chops off the high bits in its internal shift, the > middle six prescalers don't actually allow *anything* that the first > doesn't. In fact, they are strictly worse than the first, since the > internal shift of TCNT prevents software from setting the low bits, > decreasing the resolution, without providing any extra high bits. > > By skipping the useless prescalers entirely, this patch both fixes the > driver's behavior and increases its performance (since, when the 4096 > prescaler is selected, it now does only a single shift rather than the > seven successive divisions it did before). > > Tested on BG2CD. > > Signed-off-by: Thomas Hebb <tommyh...@gmail.com> > --- > drivers/pwm/pwm-berlin.c | 45 ++++++++++++++++++++++------------------ > 1 file changed, 25 insertions(+), 20 deletions(-)
Applied, thanks. Thierry
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