netdev - Re: [PATCH net-next v4 4/6] ptp: support event queue reader channel masks

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Date: Fri, 6 Oct 2023 20:53:01 -0700
From: Richard Cochran <richardcochran@...il.com>
To: Vinicius Costa Gomes <vinicius.gomes@...el.com>
Cc: Xabier Marquiegui <reibax@...il.com>, netdev@...r.kernel.org,
	tglx@...utronix.de, jstultz@...gle.com, horms@...nel.org,
	chrony-dev@...ony.tuxfamily.org, mlichvar@...hat.com,
	ntp-lists@...tcorallo.com, alex.maftei@....com, davem@...emloft.net,
	rrameshbabu@...dia.com, shuah@...nel.org
Subject: Re: [PATCH net-next v4 4/6] ptp: support event queue reader channel
 masks

On Fri, Oct 06, 2023 at 03:05:12PM -0700, Vinicius Costa Gomes wrote:

> Sorry that I only noticed a (possible) change in behavior now.
> 
> Before this series, when there was a single queue, events where
> accumulated until the application reads the fd associated with the PTP
> device. i.e. it doesn't matter when the application calls open().
> 
> AFter this series events, are only accumulated after the queue
> associated with that fd is created, i.e. after open(). Events that
> happened before open() are lost (is this true? are we leaking them?).
> 
> Is this a desired/wanted change? Is it possible that we have
> applications that depend on the "old" behavior?

So the existing behavior is not very nice to user space.  The is
forced to clear the fifo after open, like this

ts2phc_pps_sink.c:

 117 static int ts2phc_pps_sink_clear_fifo(struct ts2phc_pps_sink *sink)
 118 {
 119         struct pollfd pfd = {
 120                 .events = POLLIN | POLLPRI,
 121                 .fd = sink->clock->fd,
 122         };
 123         struct ptp_extts_event event;
 124         int cnt, size;
 125 
 126         while (1) {
 127                 cnt = poll(&pfd, 1, 0);
 128                 if (cnt < 0) {
 129                         if (EINTR == errno) {
 130                                 continue;
 131                         } else {
 132                                 pr_emerg("poll failed");
 133                                 return -1;
 134                         }
 135                 } else if (!cnt) {
 136                         break;
 137                 }
 138                 size = read(pfd.fd, &event, sizeof(event));

So, no one will miss the old behavior!

Thanks,
Richard