linux-kernel - Re: reading errors on JMicron JM20337 USB-SATA

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Date:	Mon, 03 Aug 2009 17:33:48 +0200
From:	Artur Skawina <art.08.09@...il.com>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	"Lev A. Melnikovsky" <melnikovsky@...l.ru>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-usb@...r.kernel.org
Subject: Re: reading errors on JMicron JM20337 USB-SATA

Alan Stern wrote:
> On Mon, 3 Aug 2009, Lev A. Melnikovsky wrote:
>> AS> yes, jmicron bridges do not report errors properly and just stall pretty
>> AS> much indefinitely; found out the hard way, when a disk started to develop
>> My interpretation was different - the bridge firmware does not crash but 
>> remains alive (it does not report the error properly but "zis iz probably 
>> perfectly normal behaviour for a Vogon"). This is the Linux kernel that 
>> indefinitely tries to re-read. Am I wrong?

No, but that's arguably the right thing to do -- the device didn't
report an error, so why should the kernel fail?..

> You are correct except for the term "indefinitely".  The retries _will_
> stop if you wait long enough.  Unfortunately, because of all the nested
> retry loops in the SCSI drivers and at the application level, you may
> have to wait as long as half an hour.

iirc, i had stalls _way_ longer than that, probably because the reads
eventually succeeded, only to stall on the next ones.

> I agree that this should be fixed.  But it is a SCSI issue, not a USB 
> issue.  You could try bringing it up on the linux-scsi mailing list.

actually, the number of retries should probably be configurable, but i
wouldn't lower them by default; losing data because of recoverable errors
is bad. In this case the bridge may be at fault (by not passing along the
error), but to make a significant difference you'd have to reduce the number
of retries to something like zero, maybe one at most, and that's just too
low for a default. 

artur
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