linux-kernel - Re: [PATCH v2 0/2] pwm: sysfs: fix exporting PWM channel

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Date:   Mon, 1 Oct 2018 18:24:52 +0200
From:   Michal Vokáč <michal.vokac@...ft.com>
To:     Fabrice Gasnier <fabrice.gasnier@...com>, thierry.reding@...il.com,
        gottfried.haider@...il.com
Cc:     stefan.wahren@...e.com, hsweeten@...ionengravers.com,
        loic.pallardy@...com, broonie@...nel.org, gohai@...zessiv.net,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org,
        linux-rpi-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-pwm@...r.kernel.org
Subject: Re: [PATCH v2 0/2] pwm: sysfs: fix exporting PWM channel

On 1.10.2018 15:23, Fabrice Gasnier wrote:
> 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

The [2] report came from me. I tested both patches on my i.MX6 boards and
it works just fine. Thanks for the fix Fabrice!

Michal

> 
> ---
> 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(-)
>