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Date:   Mon, 25 Mar 2019 09:48:22 +0100
From:   Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= 
        <u.kleine-koenig@pengutronix.de>
To:     Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Cc:     thierry.reding@gmail.com, narmstrong@baylibre.com,
        jbrunet@baylibre.com, linux-pwm@vger.kernel.org,
        linux-amlogic@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] pwm: meson: use the spin-lock only to protect
 register modifications
Message-ID: <20190325084822.ozixel6bzi6xiw67@pengutronix.de>
References: <20190324220217.15813-1-martin.blumenstingl@googlemail.com>
 <20190324220217.15813-2-martin.blumenstingl@googlemail.com>
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Hello,

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@googlemail.com>

Looks right.

Reviewed-by: Uwe Kleine-K?nig <u.kleine-koenig@pengutronix.de>

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

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

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-K?nig            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |