[PATCH v3] powerpc/powernv: Use darn instr for random_seed on p9

[Michael Ellerman]

Michael Ellerman <mpe at ellerman.id.au> writes:
> Matt Brown <matthew.brown.dev at gmail.com> writes:
>> On Sat, Aug 5, 2017 at 3:06 AM, Tyrel Datwyler <tyreld at linux.vnet.ibm.com> wrote:
>>> On 08/03/2017 06:12 PM, Matt Brown wrote:
>>>> @@ -135,8 +152,9 @@ static __init int rng_create(struct device_node *dn)
>>>> @@ -150,6 +168,21 @@ static __init int rng_init(void)
>>>>               of_platform_device_create(dn, NULL, NULL);
>>>>       }
>>>>
>>>> +     if (cpu_has_feature(CPU_FTR_ARCH_300)) {
>>>> +             for (i = 0; i < 10; i++) {
>>>> +                     if (powernv_get_random_darn(&darn_test)) {
>>>> +                             ppc_md.get_random_seed =
>>>> +                                     powernv_get_random_darn;
>>>> +                             break;
>>>
>>> If you return directly here you can avoid the (i == 9) conditional every iteration of the
>>> loop by moving the pr_warn to just outside the loop.
>>
>> That's true, although it is very unlikely for the
>> powernv_get_random_darn to fail. So in practice we should never reach
>> the (i == 9) conditional.
>> The loop is more of a backup in the rare case that it does fail.
>
> All true, the runtime overhead is really not an issue. It's more that
> testing for loop almost-termination in the loop is a bit gross :)
>
> I think the best solution is to just pull it out into a separate
> function, it also avoids adding sometimes unused variables to the outer
> function, eg:
>
> static int initialise_darn(void)
> ...

I decided this patch had reached my bike-shedding threshold, so I just
unilaterally changed it to the way I liked it and applied it :)

All the bugs are mine.

cheers