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Date:   Fri, 26 Feb 2021 09:27:19 +0900
From:   William Breathitt Gray <vilhelm.gray@gmail.com>
To:     Pavel Machek <pavel@ucw.cz>
Cc:     Jonathan Cameron <jic23@kernel.org>, kernel@pengutronix.de,
        linux-stm32@st-md-mailman.stormreply.com, a.fatoum@pengutronix.de,
        kamel.bouhara@bootlin.com, gwendal@chromium.org,
        alexandre.belloni@bootlin.com, david@lechnology.com,
        linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org,
        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,
        o.rempel@pengutronix.de
Subject: Re: [PATCH v8 18/22] docs: counter: Document character device
 interface
Message-ID: <YDhAZ5bQ9fqd3FWB@shinobu>
References: <cover.1613131238.git.vilhelm.gray@gmail.com>
 <350cafba81d3220b64efdb019bd76c08eb1e5d10.1613131238.git.vilhelm.gray@gmail.com>
 <20210214174819.6757e2b0@archlinux>
 <20210222101133.GB14587@amd>
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On Mon, Feb 22, 2021 at 11:11:33AM +0100, Pavel Machek wrote:
> Hi!
>=20
> > > +* COUNTER_ENABLE_EVENTS_IOCTL:
> > > +  Enables monitoring the events specified by the Counter watches that
> > > +  were queued by ``COUNTER_ADD_WATCH_IOCTL``. If events are already
> > > +  enabled, the new set of watches replaces the old one. Calling this
> > > +  ioctl also has the effect of clearing the queue of watches added by
> > > +  ``COUNTER_ADD_WATCH_IOCTL``.
> > > +
> > > +* COUNTER_DISABLE_EVENTS_IOCTL:
> > > +  Stops monitoring the previously enabled events.
> >=20
> > Is there a way to remove a watch?=20
>=20
> Is there a way to eat all kernel memory and crash the system by adding
> too many watches?

There can only ever be as many watches as there are Counter components
for the respective Counter device. This is enforced by
counter_set_event_node() which checks whether a particular watch has
been created before and returns an EINVAL if it so has.

> > > +For example, the following userspace code opens ``/dev/counter0``,
> > > +configures the ``COUNTER_EVENT_INDEX`` event channel 0 to gather Cou=
nt 0
> > > +and Count 1, and prints out the data as it becomes available on the
> > > +character device node::
> > > +
> >=20
> > Consider adding an example program under tools/
> >=20
> > > +        #include <fcntl.h>
> > > +        #include <linux/counter.h>
> > > +        #include <stdio.h>
> > > +        #include <string.h>
> > > +        #include <sys/ioctl.h>
> > > +        #include <unistd.h>
> > > +
> > > +        struct counter_watch watches[2] =3D {
> > > +                {
> > > +                        .component.type =3D COUNTER_COMPONENT_COUNT,
> > > +                        .component.scope =3D COUNTER_SCOPE_COUNT,
> > > +                        .component.parent =3D 0,
> >=20
> > Good to add comments on what these elements actually are?
>=20
> > > +                fd =3D open("/dev/counter0", O_RDWR);
> > > +
> > > +                ioctl(fd, COUNTER_ADD_WATCH_IOCTL, watches);
> > > +                ioctl(fd, COUNTER_ADD_WATCH_IOCTL, watches + 1);
> > > +                ioctl(fd, COUNTER_ENABLE_EVENTS_IOCTL);
>=20
> > > +                for (;;) {
> > > +                        read(fd, event_data, sizeof(event_data));
>=20
> If this goes to tools it really should have error handling and
> handling of short read.
>=20
> Best regards,
> 							Pavel
>=20
> --=20
> http://www.livejournal.com/~pavelmachek

Ack. I'll improve the error handling for this.

William Breathitt Gray

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