pwm: bcm2835: Improve period and duty cycle calculation

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With an input clk rate bigger than 2000000000, scaler would have been
zero which then would have resulted in a division by zero.

Also the originally implemented algorithm divided by the result of a
division. This nearly always looses precision. Consider a requested period
of 1000000 ns. With an input clock frequency of 32786885 Hz the hardware
was configured with an actual period of 983869.007 ns (PERIOD = 32258)
while the hardware can provide 1000003.508 ns (PERIOD = 32787).
And note if the input clock frequency was 32786886 Hz instead, the hardware
was configured to 1016656.477 ns (PERIOD = 33333) while the optimal
setting results in 1000003.477 ns (PERIOD = 32787).

This patch implements proper range checking and only divides once for
the calculation of period (and similar for duty_cycle).

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Tested-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

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Uwe Kleine-König 2021-01-14 21:48:04 +01:00 • committed by Thierry Reding

parent a38fd87484

commit ca0d2fb790

1 changed files with 27 additions and 8 deletions

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37

drivers/pwm/pwm-bcm2835.c

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