linux-kernel - Re: [PATCH v5 4/5] docs: counter: Document character device interface

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Message-ID: <20201008122845.GA3314@shinobu>
Date:   Thu, 8 Oct 2020 08:28:45 -0400
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     Pavel Machek <pavel@....cz>
Cc:     jic23@...nel.org, 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-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, syednwaris@...il.com,
        patrick.havelange@...ensium.com, fabrice.gasnier@...com,
        mcoquelin.stm32@...il.com, alexandre.torgue@...com
Subject: Re: [PATCH v5 4/5] docs: counter: Document character device interface

On Thu, Oct 08, 2020 at 10:09:09AM +0200, Pavel Machek wrote:
> Hi!
> 
> > +        int main(void)
> > +        {
> > +                struct pollfd pfd = { .events = POLLIN };
> > +                struct counter_event event_data[2];
> > +
> > +                pfd.fd = open("/dev/counter0", O_RDWR);
> > +
> > +                ioctl(pfd.fd, COUNTER_SET_WATCH_IOCTL, watches);
> > +                ioctl(pfd.fd, COUNTER_SET_WATCH_IOCTL, watches + 1);
> > +                ioctl(pfd.fd, COUNTER_LOAD_WATCHES_IOCTL);
> > +
> > +                for (;;) {
> > +                        poll(&pfd, 1, -1);
> 
> Why do poll, when you are doing blocking read?
> 
> > +                        read(pfd.fd, event_data,  sizeof(event_data));
> 
> Does your new chrdev always guarantee returning complete buffer?
> 
> If so, should it behave like that?
> 
> Best regards,
> 									Pavel
> -- 
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

I suppose you're right: a poll() should be redundant now with this
version of the character device implementation because buffers will
always return complete; so a blocking read() should achieve the same
behavior that a poll() with read() would.

I'll give some more time for additional feedback to come in for this
version of the patchset, and then likely remove support for poll() in
the v6 submission.

William Breathitt Gray

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