linux-kernel - Re: [PATCH 1/2] pwm: sunxi: allow the pwm to finish its pulse before disable

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Message-ID: <20160927201628.GB9084@lukather>
Date:   Tue, 27 Sep 2016 22:16:28 +0200
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Olliver Schinagl <oliver@...inagl.nl>
Cc:     Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Chen-Yu Tsai <wens@...e.org>, linux-pwm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] pwm: sunxi: allow the pwm to finish its pulse before
 disable

Hi,

On Mon, Sep 26, 2016 at 10:46:25AM +0200, Olliver Schinagl wrote:
> > For the spin_lock part, I was just comparing it to a
> > spin_lock_irqsave, which is pretty expensive since it masks all the
> > interrupts in the system, introducing latencies.
>
> so spin_lock is very expensive and we should avoid if we can?

spin_lock_irqsave, if possible, yes.

> > > but I think we need the ndelay for the else where we do not
> > > have the ready flag (A10 or A13 iirc?)
> > 
> > Hmmmm, good point. But that would also apply to your second patch
> > then, wouldn't it?
> yeah, you would have an if/else for the case of !hasready.
> 
> this is what i've been dabbling in the train last week, but haven't
> thought it through yet, let alone tested it:
> 
> 
> +       if (!(sun4i_pwm->data->has_rdy))
> +               ndelay(pwm_get_period(pwm));
> +       else
> +               do {
> +                       spin_lock(&sun4i_pwm->ctrl_lock);
> +                       val = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
> +                       spin_unlock(&sun4i_pwm->ctrl_lock);
> +               } while (!(val & PWM_RDY(pwm->hwpwm)))
> 
> Here I assumed the spin_lock is cheap to make, expensive to hold for
> long, e.g. reducing the length the spin-lock is active for. the
> alternative was to remove the spin_lock here, and remove unlock-lock
> before-after this block where you basically get a very long lasting
> spin_lock, the alternative.

If you're only reading, why do you need to take the lock?

You probbaly want to have a timeout too.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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