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

From: Nix <nix@esperi.org.uk>
To: Paul Martin <pm@debian.org>
Cc: Oliver Neukum <oliver@neukum.org>,
	Johan Hovold <jhovold@gmail.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [3.16.1 BISECTED REGRESSION]: Simtec Entropy Key (cdc-acm) broken in 3.16
Date: Sat, 11 Oct 2014 23:24:59 +0100	[thread overview]
Message-ID: <87y4smz1l0.fsf@spindle.srvr.nix> (raw)
In-Reply-To: <20141011195108.GA8275@thinkpad.nowster.org.uk> (Paul Martin's message of "Sat, 11 Oct 2014 20:51:08 +0100")

On 11 Oct 2014, Paul Martin spake thusly:

> Having been privy to the firmware of the eKey, it is very simplisting,
> 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.)

-- 
NULL && (void)