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

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Message-ID: <87sik2czm3.fsf@spindle.srvr.nix>
Date:	Mon, 08 Sep 2014 08:58:44 +0100
From:	Nix <nix@...eri.org.uk>
To:	Oliver Neukum <oliver@...kum.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [3.16.1 REGRESSION]: Simtec Entropy Key (cdc-acm) broken in 3.16

On 8 Sep 2014, Oliver Neukum said:

> On Fri, 2014-09-05 at 16:17 +0100, Nix wrote:
>> On 5 Sep 2014, Oliver Neukum verbalised:
>> 
>> > On Fri, 2014-09-05 at 00:40 +0100, Nix wrote:
>> >> I'm working around this confusing morass by rebooting into each test
>> >> kernel, unplugging and replugging the entropy key if it was fubared,
>> >> then rebooting into the same kernel again and seeing if it was still
>> >> fubared. But this is not terribly fast, particularly not on a headless
>> >> compact-flash-based Geode box which doesn't even complete booting
>> >> without the entropy source which this bug cuts off :) so it'll be
>> >> sometime tomorrow before I can get this bisection done, I'm afraid.
>> >
>> > Ugh. My sympathies. I cannot suggest a better method, I am afraid.
>> 
>> Well, that method doesn't work. I've found pairs of kernels (e.g.
>> 59a3d4c3631e553357b7305dc09db1990aa6757c and
>> b05d59dfceaea72565b1648af929b037b0f96d7f) where each kernel works on its
>> own (rebooting from that kernel into the same kernel keeps a working
>> key, so I would normally assume that each kernel is OK) but rebooting
>> from the first into the second yields a broken one if it was working
>> before (so one of them must, in fact, be broken, but I have no clue
>> which one).
>> 
>> So I can't figure out how to bisect this.
>> 
>> Any suggestions as to what failure-test I might use, or what other
>> methods I might use to figure out what's going wrong? Not knowing
>> anything about USB doesn't help here. I don't know for sure that this is
>> a cdc-acm problem -- bisecting just the cdc-acm driver was fruitless --
>> so it might be something more generally USBish.
>
> Do your kernels work if you start with a known good kernel e.g.
> 3.15 and then reboot?

That case works -- aha, so I could orchestrate it by going

3.15 -> test -> test -> reset to 3.15 -> test -> test ...

i.e. a triple reboot cycle. Should have thought of that.

God, what a pain :)

I'll give it a try tonight.

-- 
NULL && (void)
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