linux-kernel - Re: [PATCH v2 1/1] pwm: meson: use the spin-lock only to protect register modifications

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Date:   Thu, 9 May 2019 16:52:42 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     linux-pwm@...r.kernel.org, linux-amlogic@...ts.infradead.org,
        narmstrong@...libre.com, jbrunet@...libre.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        u.kleine-koenig@...gutronix.de
Subject: Re: [PATCH v2 1/1] pwm: meson: use the spin-lock only to protect
 register modifications

On Mon, Apr 01, 2019 at 07:57:48PM +0200, Martin Blumenstingl wrote:
> Holding the spin-lock for all of the code in meson_pwm_apply() can
> result in a "BUG: scheduling while atomic". This can happen because
> clk_get_rate() (which is called from meson_pwm_calc()) may sleep.
> Only hold the spin-lock when modifying registers to solve this.
> 
> The reason why we need a spin-lock in the driver is because the
> REG_MISC_AB register is shared between the two channels provided by one
> PWM controller. The only functions where REG_MISC_AB is modified are
> meson_pwm_enable() and meson_pwm_disable() so the register reads/writes
> in there need to be protected by the spin-lock.
> 
> The original code also used the spin-lock to protect the values in
> struct meson_pwm_channel. This could be necessary if two consumers can
> use the same PWM channel. However, PWM core doesn't allow this so we
> don't need to protect the values in struct meson_pwm_channel with a
> lock.
> 
> Fixes: 211ed630753d2f ("pwm: Add support for Meson PWM Controller")
> Signed-off-by: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
> Reviewed-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> ---
>  drivers/pwm/pwm-meson.c | 25 +++++++++++++++++--------
>  1 file changed, 17 insertions(+), 8 deletions(-)

Applied, thanks.

Thierry

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