[PATCH V5 0/2] pwm: Add GPIO PWM driver

[Stefan Wahren <wahrenst@xxxxxxx>]

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 latest version has just been boot tested on a Raspberry Pi 3 B+.

Since i'm not sure that i have the time to finish the driver soon, i'm
happy if someone finish this job. From my POV, V5 of this series only
lacks a solution for possible interrupt storm in case the period/duty cycle
is set too short [3].

V5:
 - add Reviewed-by's for dt-binding patch
 - rebase on top of v6.10-rc1
 - print hr resolution at driver probe
 - fix grammar in Kconfig
 - fix return type of pwm_gpio_toggle
 - implement hrtimer resolution rounding as noted by Uwe
 - use firmware node path instead of GPIO numbers as suggested by Andy
 - adjust some header includes and style issues as noted by Andy

V4:
 - address DT bindings comments from Conor Dooley
 - drop unnecessary MODULE_ALIAS() as suggested by Krzysztof Kozlowski
 - add range checks to apply which consider the hrtimer resolution
   (idea comes from Sean Young)
 - mark driver as atomic as suggested by Sean Young

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@xxxxxxxx/T/
[2] - https://lore.kernel.org/all/20201205214353.xapax46tt5snzd2v@xxxxxxxxxxxxxxxxxxx/
[3] - https://lore.kernel.org/linux-pwm/CACRpkdary+kDrTJ=u4VbSTv7wXGLQj9_fy7mv0w-Zg+eDvGXVQ@xxxxxxxxxxxxxx/T/#m513f255daa9f30c325d11999cacd086411591bf9

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                        | 249 ++++++++++++++++++
 4 files changed, 303 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/pwm/pwm-gpio.yaml
 create mode 100644 drivers/pwm/pwm-gpio.c

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2.34.1