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

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Date:   Fri, 12 Oct 2018 13:55:00 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Fabrice Gasnier <fabrice.gasnier@...com>
Cc:     gottfried.haider@...il.com, stefan.wahren@...e.com,
        hsweeten@...ionengravers.com, loic.pallardy@...com,
        broonie@...nel.org, gohai@...zessiv.net, michal.vokac@...ft.com,
        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 Mon, Oct 01, 2018 at 03:23:55PM +0200, 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
> 
> ---
> 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(-)

Both patches applied, thanks. What do you think would be the importance
of getting this into stable kernels? We can't get one of the patches in
without the other, so they'd both have to be backported. The second one
is still fairly small, so would qualify for stable, I think.

However, given that it's taken a long time for somebody to notice this,
I'm not sure if this is something that people care about too much in the
stable kernels.

Thierry

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