linux-kernel - Re: [RFC 1/2] n_tty: fix dropped output characters

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Date:	Mon, 14 Apr 2014 15:27:34 +0200
From:	Johan Hovold <jhovold@...il.com>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	Johan Hovold <jhovold@...il.com>, Oliver Neukum <oneukum@...e.de>,
	Jiri Slaby <jslaby@...e.cz>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
	Peter Hurley <peter@...leysoftware.com>,
	Xiao Jin <jin.xiao@...el.com>, david.a.cohen@...ux.intel.com,
	yanmin.zhang@...el.com
Subject: Re: [RFC 1/2] n_tty: fix dropped output characters

On Mon, Apr 14, 2014 at 01:53:33PM +0100, One Thousand Gnomes wrote:
> On Fri, 11 Apr 2014 11:41:24 +0200
> Johan Hovold <jhovold@...il.com> wrote:
> 
> > Fix characters being dropped by n_tty_write() due to a failure to
> > check the return value of tty_put_char() in do_output_char().
> > 
> > Characters are currently being dropped by write if a tty driver claims
> > to have write room available, but still fails to buffer any data
> 
> Your driver is buggy. If you advertise a buffer you must honour it and
> neither shrink nor revoke it. 

Very well, that settles the question.
 
> For an URB based device you almost certainly want internal buffering so
> you can do proper packetisation. USB serial these days gets it right - see
> drivers/usb/serial/generic.c for a fairly simple kfifo based approach.

I'm quite aware of the usb-serial approach as I implemented it. ;)

I have considered "porting" it to the ACM driver, and unless anyone is
against the extra copy the fifo would imply, I'll go ahead and do just
that (although, I know of at least one buggy ACM device that violates
the ACM specification and cannot handle merged writes...).

> Whether applying it to cdc_acm would make sense I don't know but it looks
> like it might be simpler over-all than the current arrangement.

With the current fifo-less implementation, the ACM driver could use the
approach taken by the usb-wwan and sierra usb-serial drivers, which
queue up all there write urbs while suspended. That way the available
buffer space never shrinks.

Thanks,
Johan
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