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Date:   Fri, 16 Mar 2018 13:46:38 +0100
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Jacek Anaszewski <jacek.anaszewski@gmail.com>
Cc:     Pavel Machek <pavel@ucw.cz>,
        Matthias Schiffer <mschiffer@universe-factory.net>,
        Sasha Levin <Alexander.Levin@microsoft.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        Matthieu CASTET <matthieu.castet@parrot.com>,
        "linux-leds@vger.kernel.org" <linux-leds@vger.kernel.org>
Subject: Re: [PATCH AUTOSEL for 4.14 065/110] led: core: Fix brightness
 setting when setting delay_off=0
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On Tue, Mar 13, 2018 at 08:44:49PM +0100, Jacek Anaszewski wrote:
> On 03/13/2018 02:27 PM, Pavel Machek wrote:
> > Hi!
> > 
> >>>> At least 7b6af2c531 ("leds: core: Fix regression caused by commit
> >>>> 2b83ff96f51d") is missing, causing visible regressions (LEDs not working at
> >>>> all) on some OpenWrt devices. This was fixed in 4.4.121 by reverting the
> >>>> offending commit, but if I followed the discussion correctly, 4.9 should
> >>>> get the follow-up commit 7b6af2c531 instead (like 4.14 already did).
> >>>>
> >>>> Jacek's mail I replied to mentions that eb1610b4c273 ("led: core: Fix
> >>>> blink_brightness setting race") should be included in 4.9 as well, but I
> >>>> don't know the impact of the issue it fixes.
> >>>
> >>> It doesn't fix any reported issue, but is just an improvement
> >>> aiming at preventing potential races while changing blink brightness.
> >>>
> >>> After taking closer look it turns out that for the patches in question
> >>> to apply cleanly we need in 4.9 also a patch which introduces atomic
> >>> bit fields for blink flags.
> >>>
> >>> Effectively, here is the list of patches required in 4.9 stable:
> >>>
> >>> Revert "led: core: Fix brightness setting when setting delay_off=0"
> >>>
> >>> followed by:
> >>>
> >>> a9c6ce57ec ("led: core: Use atomic bit-field for the blink-flags")
> >>> eb1610b4c2 ("led: core: Fix blink_brightness setting race")
> >>> 2b83ff96f5 ("led: core: Fix brightness setting when setting delay_off=0")
> >>> 7b6af2c531 ("leds: core: Fix regression caused by commit 2b83ff96f51d")
> >>
> >> Odd, I just got another report that the 4.9.87 release fixed some
> >> reported LED issues, so why do I need all of these?
> 
> Because 2b83ff96f5 introduces another bug, fixed in 7b6af2c531.
> 7b6af2c531 in turn uses atomic blink flags introduced in a9c6ce57ec.
> 
> eb1610b4c2 fixes theoretical races, actually we can do without it
> in stable.
> 
> In order to avoid applying patch a9c6ce57ec, we could come up with the
> below change which does exactly what 7b6af2c531 intended, but without
> atomic blink flags, which are irrelevant for this bug.
> 
> 
> diff --git a/drivers/leds/led-core.c b/drivers/leds/led-core.c
> index 3bce448..454ed4d 100644
> --- a/drivers/leds/led-core.c
> +++ b/drivers/leds/led-core.c
> @@ -188,6 +188,7 @@ void led_blink_set(struct led_classdev *led_cdev,
>  {
>         del_timer_sync(&led_cdev->blink_timer);
> 
> +       led_cdev->flags &= ~LED_BLINK_SW;
>         led_cdev->flags &= ~LED_BLINK_ONESHOT;
>         led_cdev->flags &= ~LED_BLINK_ONESHOT_STOP;
> 
> 
> I can submit it to stable if it is preferred.
> 
> In every case tha patch 2b83ff96f5 needs to be reverted beforehand,
> since otherwise none of the discussed patches will apply cleanly
> (besides the aforementioned reasoning it has a truncated commit
> message).

Yes, please submit it to stable, along with a very simple "please
add/revert these patches" so I know what in the world to do as this
thread is really confusing at the moment :)

thanks,

greg k-h