linux-kernel - Re: [3.16.1 BISECTED REGRESSION]: Simtec Entropy Key (cdc-acm) broken in 3.16

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Date:	Sun, 12 Oct 2014 12:14:05 +0100
From:	Paul Martin <pm@...ian.org>
To:	Nix <nix@...eri.org.uk>
Cc:	Oliver Neukum <oliver@...kum.org>,
	Johan Hovold <jhovold@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org,
	Daniel Silverstone <dsilvers@...ian.org>
Subject: Re: [3.16.1 BISECTED REGRESSION]: Simtec Entropy Key (cdc-acm)
 broken in 3.16

On Sat, Oct 11, 2014 at 11:24:59PM +0100, Nix wrote:
> On 11 Oct 2014, Paul Martin spake thusly:
> 
> > Having been privy to the firmware of the eKey, it is very simplistic,
> > with no implementation whatsoever of any flow control.
> 
> That's what I thought. (Why would something that just provides data at a
> constant rate way below that of even the slowest USB bus *need* flow
> control?)
> 
> One presumes therefore that the kernel suddenly trying to do flow
> control on shutdown would fubar the firmware's internal state, leading
> to the symptoms I see.
> 
> So, the question becomes, is there a way to spot this general 'no flow
> control on this device' thing from the kernel side, or do we need a
> blacklist? Or, perhaps, if this is commonplace for cdc-acm devices, a
> whitelist? I can't imagine it's *that* commonplace or someone would have
> spotted this already in the months and months it took me to do the
> bisection.
> 
> Maybe all non-modem cdc-acm devices should eschew flow control, or
> something? (This is a genuine guess and is almost certainly wrong.)
> 

I'm going to pass this on to Daniel Silverstone, who did the actual
coding of the firmware.

-- 
Paul Martin <pm@...ian.org>
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