linux-kernel - Re: Crypto/nx842: Ignore invalid XER[S0] return error

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Date:	Sat, 12 Dec 2015 02:43:48 -0600
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Haren Myneni <haren@...ux.vnet.ibm.com>
Cc:	herbert@...dor.apana.org.au, ddstreet@...e.org,
	davem@...emloft.net, mpe@...erman.id.au, pair@...ibm.com,
	linuxppc-dev@...ts.ozlabs.org, linux-crypto@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Crypto/nx842: Ignore invalid XER[S0] return error

On Fri, Dec 11, 2015 at 07:30:29PM -0800, Haren Myneni wrote:
> NX842 coprocessor sets 3rd bit in CR register with XER[S0] which is
> nothing to do with NX request. On powerpc, XER[S0] will be set if
> overflow in FPU and stays until another floating point operation is
> executed. Since this bit can be set with other valuable return status,
> ignore this XER[S0] value.

XER[SO] is the *integer* summary overflow bit.  It is set by OE=1
instructions ("addo" and the like), and can only be cleared explicitly
(using "mtxer").

The floating point overflow bit is FPSCR[OX].

> +	/*
> +	 * NX842 coprocessor sets 3rd bit in CR register with XER[S0].
> +	 * Setting XER[S0] happens if overflow in FPU and stays until
> +	 * other floating operation is executed. XER[S0] value is nothing
> +	 * to NX and no use to user. Since this bit can be set with other
> +	 * return values, ignore this error.
> +	 */
> +	if (ret & ICSWX_XERS0)
> +		ret &= ~ICSWX_XERS0;

You can just always clear it, there is no need to check if it is set first.


Segher
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