Re: [PATCH v8 18/22] docs: counter: Document character device interface

From: William Breathitt Gray <vilhelm.gray@gmail.com>
To: Pavel Machek <pavel@ucw.cz>
Cc: kamel.bouhara@bootlin.com, gwendal@chromium.org,
	a.fatoum@pengutronix.de, david@lechnology.com,
	linux-iio@vger.kernel.org, patrick.havelange@essensium.com,
	alexandre.belloni@bootlin.com, mcoquelin.stm32@gmail.com,
	linux-kernel@vger.kernel.org, o.rempel@pengutronix.de,
	linux-arm-kernel@lists.infradead.org, kernel@pengutronix.de,
	fabrice.gasnier@st.com, syednwaris@gmail.com,
	linux-stm32@st-md-mailman.stormreply.com,
	Jonathan Cameron <jic23@kernel.org>,
	alexandre.torgue@st.com
Subject: Re: [PATCH v8 18/22] docs: counter: Document character device interface
Date: Fri, 26 Feb 2021 09:27:19 +0900	[thread overview]
Message-ID: <YDhAZ5bQ9fqd3FWB@shinobu> (raw)
In-Reply-To: <20210222101133.GB14587@amd>

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On Mon, Feb 22, 2021 at 11:11:33AM +0100, Pavel Machek wrote:
> Hi!
> 
> > > +* 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.
> > 
> > Is there a way to remove a watch? 
> 
> 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 Count 0
> > > +and Count 1, and prints out the data as it becomes available on the
> > > +character device node::
> > > +
> > 
> > Consider adding an example program under tools/
> > 
> > > +        #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] = {
> > > +                {
> > > +                        .component.type = COUNTER_COMPONENT_COUNT,
> > > +                        .component.scope = COUNTER_SCOPE_COUNT,
> > > +                        .component.parent = 0,
> > 
> > Good to add comments on what these elements actually are?
> 
> > > +                fd = 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);
> 
> > > +                for (;;) {
> > > +                        read(fd, event_data, sizeof(event_data));
> 
> If this goes to tools it really should have error handling and
> handling of short read.
> 
> Best regards,
> 							Pavel
> 
> -- 
> http://www.livejournal.com/~pavelmachek

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

William Breathitt Gray

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