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

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Message-ID: <20210222101133.GB14587@amd>
Date:   Mon, 22 Feb 2021 11:11:33 +0100
From:   Pavel Machek <pavel@....cz>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     William Breathitt Gray <vilhelm.gray@...il.com>,
        kernel@...gutronix.de, linux-stm32@...md-mailman.stormreply.com,
        a.fatoum@...gutronix.de, kamel.bouhara@...tlin.com,
        gwendal@...omium.org, alexandre.belloni@...tlin.com,
        david@...hnology.com, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        syednwaris@...il.com, patrick.havelange@...ensium.com,
        fabrice.gasnier@...com, mcoquelin.stm32@...il.com,
        alexandre.torgue@...com, o.rempel@...gutronix.de
Subject: Re: [PATCH v8 18/22] docs: counter: Document character device
 interface

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?

> > +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

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