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Date:   Sun, 18 Oct 2020 12:58:22 -0400
From:   William Breathitt Gray <vilhelm.gray@gmail.com>
To:     David Lechner <david@lechnology.com>
Cc:     jic23@kernel.org, kamel.bouhara@bootlin.com, gwendal@chromium.org,
        alexandre.belloni@bootlin.com, linux-iio@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        linux-stm32@st-md-mailman.stormreply.com,
        linux-arm-kernel@lists.infradead.org, syednwaris@gmail.com,
        patrick.havelange@essensium.com, fabrice.gasnier@st.com,
        mcoquelin.stm32@gmail.com, alexandre.torgue@st.com
Subject: Re: [PATCH v5 3/5] counter: Add character device interface
Message-ID: <20201018165822.GE231549@shinobu>
References: <cover.1601170670.git.vilhelm.gray@gmail.com>
 <00be1fccc672c5207f3b04fe4cc09c29e22641f4.1601170670.git.vilhelm.gray@gmail.com>
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On Wed, Oct 14, 2020 at 05:40:44PM -0500, David Lechner wrote:
> On 9/26/20 9:18 PM, William Breathitt Gray wrote:
> > +static ssize_t counter_chrdev_read(struct file *filp, char __user *buf,
> > +				   size_t len, loff_t *f_ps)
> > +{
> > +	struct counter_device *const counter =3D filp->private_data;
> > +	int err;
> > +	unsigned long flags;
> > +	unsigned int copied;
> > +
> > +	if (len < sizeof(struct counter_event))
> > +		return -EINVAL;
> > +
> > +	do {
> > +		if (kfifo_is_empty(&counter->events)) {
> > +			if (filp->f_flags & O_NONBLOCK)
> > +				return -EAGAIN;
> > +
> > +			err =3D wait_event_interruptible(counter->events_wait,
> > +					!kfifo_is_empty(&counter->events));
> > +			if (err)
> > +				return err;
> > +		}
> > +
> > +		raw_spin_lock_irqsave(&counter->events_lock, flags);
> > +		err =3D kfifo_to_user(&counter->events, buf, len, &copied);
> > +		raw_spin_unlock_irqrestore(&counter->events_lock, flags);
> > +		if (err)
> > +			return err;
> > +	} while (!copied);
> > +
> > +	return copied;
> > +}
>=20
> All other uses of kfifo_to_user() I saw use a mutex instead of spin
> lock. I don't see a reason for disabling interrupts here.

The Counter character device interface is special in this case because
counter->events could be accessed from an interrupt context. This is
possible if counter_push_event() is called for an interrupt (as is the
case for the 104_quad_8 driver). In this case, we can't use mutex
because we can't sleep in an interrupt context, so our only option is to
use spin lock.

William Breathitt Gray

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