linux-kernel - Re: Handshaking on USB serial devices

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Message-ID: <20080214121026.16a9c510@core>
Date:	Thu, 14 Feb 2008 12:10:26 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	David Newall <davidn@...idnewall.com>
Cc:	Greg KH <greg@...ah.com>, linux-usb@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Handshaking on USB serial devices

> To make it clear: Even aside from the buffer in 2.6's pl2303.c, there's
> a race: An in-flight write URB can fill all hardware buffers, making
> unsafe what previously appeared to be a safe write.  I think it's
> essential to delay submission of the URB on a stop-transmit condition.

Hardware flow control *is* a race, and always will be. The remote end has
a delay in signalling 'stop' there is a propogation delay and a response
delay. This is why most implementations assert stop a bit *before* they
run out.

Given the size of transfers and the internal buffering one would hope the
USB devices do their own flow control if told to properly.

Alan
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