On 12/12/2015 12:43 AM, Segher Boessenkool wrote:
> 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").
Thanks for the correct description. I was told XER[S0] is floating overflow
from FPU.
>
> 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.
Do you mean reset this before calling NX? I believe NX coprocessor should not
set CR bit as XER[S0] nothing to do with NX request and it is no use. NX is
copying this CR bit with XER. But reset XER[S0] has to be done before NX
request. We can not do this in icswx since this instruction can be used by
other coprocessors in future. But I am not comfortable clearing as we are not
touching this XER in the driver or result of NX operation. So I am proposing
this patch to fix this not proper NX behaviour - ignores CR bit.
If you are OK, I can repost the patch with proper description.
Thanks
Haren
>
>
> Segher
>
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