[PATCH 0/4] pwm: omap-dmtimer: fix period/duty_cycle calculation

[David Rivshin (Allworx)]

From: David Rivshin <drivshin at allworx.com>

When using a short PWM period (approaching the min of 2/clk_rate),
pwm-omap-dmtimer does not produce accurate results. In the worst case a
requested period of 2/clk_rate would result in a real period of 4/clk_rate
instead. This is a series includes a fix for that problem, as well as
other related improvements, and is based on the current linux-pwm/for-next
tip.

I have tested on a Sitara AM335x platform, using a scope to verify the
output with a variety of periods and duty cycles. This includes a PWM
rate up clk_rate/2 with 50% duty cycle (e.g. generating fclk/2) with
both 32768Hz and 24MHz fclks. I do not have an OMAP4 board to test with,
although appropriate sections in the the reference manuals appear
substantially the same, so I believe the changes are equally correct
there.

Note that the OMAP4 TRMs do effectively state that the maximum PWM
rate is clk_rate/4, so at very fast PWM rates the behavior may not be
as reliable as I observed with Sitara. Although I suspect that it's
the same module and will also work, at least under some circumstances.
If anyone with OMAP4 hardware and a scope is so inclined, I would be
curious to know the results.

David Rivshin (4):
  pwm: omap-dmtimer: fix inaccurate period/duty_cycle calculation
  pwm: omap-dmtimer: add sanity checking for load and match values
  pwm: omap-dmtimer: round load and match values rather than truncate
  pwm: omap-dmtimer: add dev_dbg() message for effective period and duty
    cycle

 drivers/pwm/pwm-omap-dmtimer.c | 71 ++++++++++++++++++++++++++++++++----------
 1 file changed, 55 insertions(+), 16 deletions(-)

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2.5.0