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

From: Uwe Kleine-König
Date: Mon Mar 25 2019 - 04:48:39 EST


On Sun, Mar 24, 2019 at 11:02:17PM +0100, 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@xxxxxxxxxxxxxx>

Looks right.

Reviewed-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>

Bonus points for adding a comment to struct meson_pwm::lock. Something

* Protects register access to the MISC_AB register that is
* shared between the two PWMs.

Best regards

Pengutronix e.K. | Uwe Kleine-König |
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