linux-kernel - Re: Handshaking on USB serial devices

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Date:	Thu, 14 Feb 2008 10:53:55 -0800
From:	David Brownell <david-b@...bell.net>
To:	David Newall <davidn@...idnewall.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, 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

On Thursday 14 February 2008, David Newall wrote:
> RS232 is (normally) so much slower than USB that, on an extended
> transmission, the buffer internal to the local hardware can fill well
> before the remote device has demanded that transmission stop.  In fact,
> now that you've mentioned it, I can't see that anything to stop the
> driver from overflowing the internal buffer, which is very perplexing. 
> Would that be right? 

Only for stupidly designed hardware ... which you might well
be using, though I happen to never have seen anything that's
quite *that* stupid.  (There's always a first time though.)

USB has enough control flow to prevent that from happening.
If the host sends data that the peripheral isn't ready to
accept, the peripheral just refuses to accept it, and the
host will retry later.

- Dave

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