linux-kernel - Re: [Linux-usb-users] [SOLVED] Serial buffer corruption [was Re: FTDI usb-serial possible bug]

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Message-ID: <Pine.LNX.4.44L0.0705061744540.21407-100000@netrider.rowland.org>
Date:	Sun, 6 May 2007 17:51:11 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	Antonino Ingargiola <tritemio@...il.com>,
	Paul Fulghum <paulkf@...rogate.com>,
	Oliver Neukum <oliver@...kum.org>,
	<linux-kernel@...r.kernel.org>,
	<linux-usb-users@...ts.sourceforge.net>
Subject: Re: [Linux-usb-users] [SOLVED] Serial buffer corruption [was Re:
 FTDI usb-serial possible bug]

On Sun, 6 May 2007, Alan Cox wrote:

> > However, whatever policy the buffer uses, the fundamental point it's that
> > when I flush the input buffer I should be sure that each byte read
> > after the flush is *new* (current) data and not old one. This because
> 
> Define "new" and "old" in this case. I don't believe you can give a
> precise definition or that such a thing is physically possible.

One can come close.  It would make sense to say that after a flush,
subsequent reads should retrieve _contiguous_ bytes from the input stream.  
In other words, rule out the possibility that the read would get bytes
1-10 (from some buffer somewhere) followed by bytes 30-60 (because bytes
11-29 were dropped by the flush).  By contrast, it would be permissible
for the read to obtain bytes 20-60, even though 20-29 may have been
entered the input stream before the flush occurred.

> The hardware itself has buffers at both ends of the link, there may be
> buffers in modems, muxes and the like as well. We can certainly flush
> input buffers in the kernel but it isn't clear we can always do so at the
> hardware level, let alone at the remote end or buffers on devices on the
> link.

This is of course the fly in the ointment.

Alan Stern

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