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

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Message-ID: <d06d5e47-5776-85ee-0dc5-8b624e36d83d@lechnology.com>
Date:   Mon, 12 Oct 2020 12:04:10 -0500
From:   David Lechner <david@...hnology.com>
To:     William Breathitt Gray <vilhelm.gray@...il.com>,
        Pavel Machek <pavel@....cz>
Cc:     jic23@...nel.org, kamel.bouhara@...tlin.com, gwendal@...omium.org,
        alexandre.belloni@...tlin.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 10/8/20 7:28 AM, William Breathitt Gray wrote:
> 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
> 

I hope that you mean that you will just remove it from the example
and not from the chardev. Otherwise it won't be possible to
integrate this with an event loop.