Add a software PWM which toggles a GPIO from a high-resolution timer.

Recent discussions in the Raspberry Pi community revealt that a lot
of users still use MMIO userspace tools for GPIO access. One argument
for this approach is the lack of a GPIO PWM kernel driver. So this
series tries to fill this gap.

This continues the work of Vincent Whitchurch [1], which is easier
to read and more consequent by rejecting sleeping GPIOs than Nicola's
approach [2].

The work has been tested on a Raspberry Pi 3 B+ and a cheap logic
analyzer.

V3:
 - rebase on top of v6.8-pwm-next
 - cherry-pick improvements from Nicola's series
 - try to address Uwe's, Linus' and Andy's comments
 - try to avoid GPIO glitches during probe
 - fix pwm_gpio_remove()
 - some code clean up's and comments

V2:
 - Rename gpio to gpios in binding
 - Calculate next expiry from expected current expiry rather than "now"
 - Only change configuration after current period ends
 - Implement get_state()
 - Add error message for probe failures
 - Stop PWM before unregister

[1] - https://lore.kernel.org/all/20200915135445.al75xmjxudj2rgcp@axis.com/T/ (local)
[2] - https://lore.kernel.org/all/20201205214353.xapax46tt5snzd2v@einstein.dilieto.eu/ (local)

Nicola Di Lieto (1):
  dt-bindings: pwm: Add pwm-gpio

Vincent Whitchurch (1):
  pwm: Add GPIO PWM driver

 .../devicetree/bindings/pwm/pwm-gpio.yaml     |  42 ++++
 drivers/pwm/Kconfig                           |  11 +
 drivers/pwm/Makefile                          |   1 +
 drivers/pwm/pwm-gpio.c                        | 221 ++++++++++++++++++
 4 files changed, 275 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/pwm/pwm-gpio.yaml
 create mode 100644 drivers/pwm/pwm-gpio.c

--
2.34.1