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SubjectRe: [PATCH] n_tty: wake up poll(POLLRDNORM) on receiving data
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Hi, thank you for the reply.

> On Mon, Jan 24, 2022 at 08:17:22AM +0000, TATSUKAWA KOSUKE(立川 江介) wrote:
> > Event POLLRDNORM should be equivalent to POLLIN when used as event in
> > poll().
>
> Where is that documented?

It's in "man poll". Below is an excerpt from the man page.
POLLRDNORM
Equivalent to POLLIN.

I'll send a v2 patch in a separate message.

Many other calls to wake_up_interruptible_poll() in the kernel source
code already specify "POLLIN | POLLRDNORM", but there are places under
drivers/tty which only passes POLLIN (i.e. pty.c, tty_io.c,
tty_ldisc.c). I haven't yet made test programs to hit the other
locations and I wasn't sure if they should also be fixed just for
consistency.

Best regards.


> > However, in n_tty driver, POLLRDNORM does not return until
> > timeout even if there is terminal input, whereas POLLIN returns.
> >
> > The following test program works until kernel-3.17, but the test stops
> > in poll() after commit 57087d515441 ("tty: Fix spurious poll() wakeups").
> >
> > [Steps to run test program]
> > $ cc -o test-pollrdnorm test-pollrdnorm.c
> > $ ./test-pollrdnorm
> > foo <-- Type in something from the terminal followed by [RET].
> > The string should be echoed back.
> >
> > ------------------------< test-pollrdnorm.c >------------------------
> > #include <stdio.h>
> > #include <errno.h>
> > #include <poll.h>
> > #include <unistd.h>
> >
> > void main(void)
> > {
> > int n;
> > unsigned char buf[8];
> > struct pollfd fds[1] = {{ 0, POLLRDNORM, 0 }};
> >
> > n = poll(fds, 1, -1);
> > if (n < 0)
> > perror("poll");
> > n = read(0, buf, 8);
> > if (n < 0)
> > perror("read");
> > if (n > 0)
> > write(1, buf, n);
> > }
> > ------------------------------------------------------------------------
> >
> > The attached patch fixes this problem.
> >
> > Signed-off-by: Kosuke Tatsukawa <tatsu-ab1@nec.com>
> > Fixes: commit 57087d515441 ("tty: Fix spurious poll() wakeups")
>
> No need for "commit" here, please remove as the documentation asks you
> to.
>
> Can you resend this with this fixed up, and a pointer to where the
> documentation is for this functionality.
>
> thanks,
>
> greg k-h
---
Kosuke TATSUKAWA | 1st Platform Software Division
| NEC Solution Innovators
| tatsu-ab1@nec.com
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