linux-kernel - Re: [PATCH AUTOSEL 5.4 053/205] pwm: sun4i: Fix incorrect calculation of duty

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Message-ID: <20200116165614.a3u5x7g4qxxrm6s4@pengutronix.de>
Date:   Thu, 16 Jan 2020 17:56:14 +0100
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Ondrej Jirman <megous@...ous.com>,
        Thierry Reding <thierry.reding@...il.com>,
        linux-pwm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH AUTOSEL 5.4 053/205] pwm: sun4i: Fix incorrect
 calculation of duty_cycle/period

On Thu, Jan 16, 2020 at 11:40:28AM -0500, Sasha Levin wrote:
> From: Ondrej Jirman <megous@...ous.com>
> 
> [ Upstream commit 50cc7e3e4f26e3bf5ed74a8d061195c4d2161b8b ]
> 
> Since 5.4-rc1, pwm_apply_state calls ->get_state after ->apply
> if available, and this revealed an issue with integer precision
> when calculating duty_cycle and period for the currently set
> state in ->get_state callback.
> 
> This issue manifested in broken backlight on several Allwinner
> based devices.
> 
> Previously this worked, because ->apply updated the passed state
> directly.
> 
> Fixes: deb9c462f4e53 ("pwm: sun4i: Don't update the state for the caller of pwm_apply_state")
> Signed-off-by: Ondrej Jirman <megous@...ous.com>
> Acked-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> Signed-off-by: Thierry Reding <thierry.reding@...il.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>

Note that while the patch is still correct, the problem isn't that bad
any more since commit 01ccf903edd65f6421612321648fa5a7f4b7cb10 was
reverted.

So .get_state is only called once during boot where the breakage doesn't
hurt that much.

I let you decide if you still want to apply this patch.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |