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Date:   Wed, 09 Nov 2011 15:26:32 +0400
From:   "Gleb O. Raiko" <raiko@niisi.msk.ru>
Organization: NIISI RAN
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To:     Ralf Baechle <ralf@linux-mips.org>
CC:     Al Cooper <alcooperx@gmail.com>, linux-mips@linux-mips.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] MIPS: Kernel hangs occasionally during boot.
References: <y> <1320764341-4275-1-git-send-email-alcooperx@gmail.com> <20111108175532.GA15493@linux-mips.org> <4EBA2E65.3010009@niisi.msk.ru> <20111109103432.GA27378@linux-mips.org>
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On 09.11.2011 14:34, Ralf Baechle wrote:
> Hmm...  Looking at the R4000 manual which generall has the longest
> pipeline hazards, mtc0 gets executed at stage 7, interrupts get sampled
> at stage 3 meaning there is a (7 - 3 - 1) = 3 cycles hazard.  Does
> that one statisfy your constraints?  Or are additional cycles needed
> for a hazard that's generated outside of the CPU's pipeline?
In fact, current back_to_back_hazard is more than enough for cpus I deal 
with. I guess, required time to wait equals number of stages between EX 
(or RD) and WB stages for modern cpus, because CP0 CAUSE is updated 
during WB nowadays.

I suspect, the time required to update internal counter logic for 
original r4k might be bigger though. At least old code waited 12 cycles 
(4*irq_disable_hazard which is 3 for r4k). Perhaps, we should keep this 
code and insert the same amount of nops for old cpus at least.

Regards,
Gleb.