linux-kernel - Re: [PATCH] USB: ftdi_sio: Remove tty->low

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Message-Id: <200911171941.34781.oliver@neukum.org>
Date:	Tue, 17 Nov 2009 19:41:34 +0100
From:	Oliver Neukum <oliver@...kum.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Johan Hovold <jhovold@...il.com>,
	"Greg Kroah-Hartman" <gregkh@...e.de>,
	Michael Trimarchi <trimarchi@...dalf.sssup.it>,
	linux-usb@...r.kernel.org, Alan Cox <alan@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH] USB: ftdi_sio: Remove tty->low_latency.

Am Dienstag, 17. November 2009 19:35:07 schrieb Eric W. Biederman:
> Boiled down.  ftdi_chars_in_buffer is essentially
>  priv->tx_outstanding_bytes. tx_outstanding_bytes is incremented when an
>  urb request is sent and tx_outstanding_bytes is decremented when an urb
>  request completes.
> 
> It turns out I have a moderately noisy usb line, and so I occasionally
> get messages like:
> 
> ehci_hcd 0000:00:0a.1: detected XactErr len 0/7 retry 31
> 
> Which as best as I can tell result in the urb getting abandoned and
> neither completed nor canceled (because we have hit the maximum
> retry count and they still don't succeed).

The URB should be finished with an error code in urb->status.
ftdi_write_bulk_callback() does decrement the counter even in
the error case.

> Which appears to result in tx_outstanding_bytes getting stuck at
> some positive number.

Do you see this message

 	if (status) {
		dbg("nonzero write bulk status received: %d", status);
		return;
	}

if you enable debugging output?

	Regards
		Oliver

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