[PATCH v2 0/2] pwm: sysfs: fix exporting PWM channel

[Fabrice Gasnier]

Since commit 7e5d1fd75c3d ("pwm: Set class for exported channels in sysfs")
- it's not possible to export more than one PWM channel
- ABI has changed, as a side effect. It may cause bad behavior as pwmchip
  attributes are wrongly added to pwm channels and report wrong values.
See [1] and [2].

One purpose of the original patch is to send uevents to udev, when exporting a
PWM channel through the sysfs. This series:
- Reverts the original patch.
- Proposes a new way to send notifications to be used by udev rules.

- With this series:
$ echo 0 > /sys/class/pwm/pwmchip0/export
$ ls /sys/class/pwm
pwmchip0 pwmchip4

$ ls /sys/class/pwm/pwmchip0/pwm0/
capture     enable      polarity    uevent
duty_cycle  period      power

- Without this series:
$ echo 0 > /sys/class/pwm/pwmchip0/export
$ ls /sys/class/pwm
pwm0 pwmchip0 pwmchip4

$ ls /sys/class/pwm/pwmchip0/pwm0/
capture     duty_cycle  export      period      power       uevent
device      enable      npwm        polarity    subsystem   unexport

- Backtrace when exporting a 2nd channel (0) on a separate pwmchip device:
$ echo 0 > /sys/class/pwm/pwmchip4/export
[   95.286558] sysfs: cannot create duplicate filename '/class/pwm/pwm0'
[   95.293630] CPU: 0 PID: 54 Comm: sh Not tainted 4.19.0-rc6-00013-g00b49b0 #151
[   95.301344] Hardware name: STM32 (Device Tree Support)
[   95.306833] [<0000c155>] (unwind_backtrace) from [<0000b273>] (show_stack+0xb/0xc)
[   95.315136] [<0000b273>] (show_stack) from [<00092455>] (sysfs_warn_dup+0x31/0x48)
[   95.323247] [<00092455>] (sysfs_warn_dup) from [<00092635>] (sysfs_do_create_link_sd+0x75/0x88)
[   95.332539] [<00092635>] (sysfs_do_create_link_sd) from [<00125823>] (device_add+0x133/0x3b0)
[   95.341694] [<00125823>] (device_add) from [<001059ed>] (export_store+0xb5/0x12c)
[   95.349761] [<001059ed>] (export_store) from [<00091911>] (kernfs_fop_write+0x87/0xda)
[   95.358150] [<00091911>] (kernfs_fop_write) from [<0005beb1>] (__vfs_write+0x1d/0xe0)
[   95.366295] [<0005beb1>] (__vfs_write) from [<0005bfe7>] (vfs_write+0x4f/0x7c)
[   95.374053] [<0005bfe7>] (vfs_write) from [<0005c0bf>] (ksys_write+0x33/0x70)
[   95.381708] [<0005c0bf>] (ksys_write) from [<00009001>] (ret_fast_syscall+0x1/0x58)
[   95.389682] Exception stack(0x01bcffa8 to 0x01bcfff0)
[   95.394946] ffa0:                   00000000 00c4883c 00000001 00c4e590 00000002 00000004
[   95.403639] ffc0: 00000000 00c4883c 00c4cbe8 00000004 00000002 00000020 00000000 00c4d008
[   95.412223] ffe0: 00c29151 00c4cbe8 00c17833 00c13c0c
-sh: write error: File exists

[1] https://lkml.org/lkml/2018/9/25/713
[2] https://lkml.org/lkml/2018/9/25/447

---
Changes in v2:
- update revert commit message
- new patch 2/2 to propose uevent notification (change) on pwmchip

Fabrice Gasnier (2):
  Revert "pwm: Set class for exported channels in sysfs"
  pwm: send a uevent on the pwmchip device upon channel sysfs (un)export

 drivers/pwm/sysfs.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

-- 
1.9.1