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

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Message-ID: <m1pr7hdlp3.fsf@fess.ebiederm.org>
Date:	Tue, 17 Nov 2009 10:56:40 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oliver Neukum <oliver@...kum.org>
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.

Oliver Neukum <oliver@...kum.org> writes:

> 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?

I will have to give that a try.  The problem isn't that easy to reproduce.
But I do have a 100% correlation between urb retry counts and hangs
in tty_wait_until_sent.

Eric
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