linux-kernel - Re: [PATCH tty-next 5/7] n_tty: Refactor input_available

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Message-ID: <20131124002254.265c2d28@alan.etchedpixels.co.uk>
Date:	Sun, 24 Nov 2013 00:22:54 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH tty-next 5/7] n_tty: Refactor input_available_p() by
 call site

On Fri, 22 Nov 2013 10:59:23 -0500
Peter Hurley <peter@...leysoftware.com> wrote:

> Distinguish if caller is n_tty_poll() or n_tty_read(), and
> set the read/wakeup threshold accordingly.

That looks very wrong to me. The poll() and read() wakeup should always
match. In addition if MIN_CHAR is set without a timeout then it should not
wake up until MIN_CHAR characters are present. The MIN_CHAR feature is
used by various (these days obscure) protocols to optimise block transfer
rates.

Changing this is also a userspace visible API change.

So NAK.

The underlying intention of the system (and SYS5.4) is that you can do

	MIN_CHAR = 64

	alarm(some_timeout)
	while(1) {
		poll();
		events
		read(chunksized blocks)
	}

while it's hardly as critical with modern fast hardware its still the API
and necessary to avoid SWS type behaviours in the queue turning it from
block/syscall to byte I/O.

Alan
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