[PATCH v2] powerpc/powernv: Use darn instr for random_seed on p9

[Matt Brown]

On Tue, Aug 1, 2017 at 10:57 PM, Segher Boessenkool
<segher at kernel.crashing.org> wrote:
> On Mon, Jul 31, 2017 at 07:10:15PM +1000, Michael Ellerman wrote:
>> And ___PPC_RA() is not quite right. The L field is only 2 bits wide, not
>> the 5 that ___PPC_RA() allows.
>>
>> We don't have a __PPC_L() macro, because L fields vary in size and
>> location. So I think you're best of open coding it, eg:
>>
>> +#define PPC_DARN(t, l)               stringify_in_c(.long PPC_INST_DARN |  \
>> +                                             __PPC_RT(t)        |  \
>> +                                             (((l) & 0x3) << 16))
>
> It would be better if you could do a compile-time error if the L value
> is out of range.  Hrm, nothing else does such checking either?
>

Yeah currently the only checks are whether the register value is
valid, using the __PPC_R{A,B,S,T} macros.
However, we can't use these macros for inline asm because we're
passing a variable into it
so the pre-processor attempts to look for register %0 which breaks it.
(Have to use triple underscore versions)

We could add more checking to validate the L value, but I don't know
how much of an issue it currently is.
A question for mpe I guess.

Thanks,
Matt


>
> Segher