linux-kernel - Re: [PATCH 3/3] ARM: Work around faulty ISAR0 register in some Krait CPUs

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Message-ID: <20130317142854.GC19071@mudshark.cambridge.arm.com>
Date:	Sun, 17 Mar 2013 14:28:54 +0000
From:	Will Deacon <will.deacon@....com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	Stepan Moskovchenko <stepanm@...eaurora.org>
Subject: Re: [PATCH 3/3] ARM: Work around faulty ISAR0 register in some Krait
 CPUs

On Wed, Mar 13, 2013 at 01:32:01AM +0000, Stephen Boyd wrote:
> Some early versions of the Krait CPU design incorrectly indicate
> that they only support the UDIV and SDIV instructions in Thumb
> mode when they actually support them in ARM and Thumb mode. It
> seems that these CPUs follow the DDI0406B ARM ARM which has two
> possible values for the divide instructions field, instead of the
> DDI0406C document which has three possible values.
> 
> Work around this problem by checking the MIDR against Krait CPUs
> with this faulty ISAR0 register and force the detection code
> to indicate support in both modes.
> 
> Cc: Will Deacon <will.deacon@....com>
> Cc: Stepan Moskovchenko <stepanm@...eaurora.org>
> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
> ---
>  arch/arm/kernel/setup.c | 8 ++++++++
>  1 file changed, 8 insertions(+)

After all this, you might as well just pass the relevant HWCAPs for your
krait entry in proc-v7.S rather than have an exception in the CPU-agnostic
code.

Thanks for adding the detection code though -- we can use that for A7/A15.

Will
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