[Skiboot] [PATCH 1/2] debug_descriptor: Claim reserved field for host kernel log buffer

[Vasant Hegde]

On 05/28/2018 09:10 AM, Joel Stanley wrote:
> On 25 May 2018 at 15:24, Vasant Hegde <hegdevasant at linux.vnet.ibm.com> wrote:
>> On 05/24/2018 07:07 AM, Joel Stanley wrote:
>>>
>>> Signed-off-by: Joel Stanley <joel at jms.id.au>
>>> ---
>>> We could bump the version instead, but I don't think there's any call
>>> for that.
>>>
>>> Another option would be to re-use some of the memcons fields that are
>>> currently only used by FSP.
>>>
>>> A third option would be to use this field to link to yet another debug
>>> structure to allow for future flexibility.
>>> ---
>>>    include/skiboot.h | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/include/skiboot.h b/include/skiboot.h
>>> index b4bdf37795dd..7f3740965412 100644
>>> --- a/include/skiboot.h
>>> +++ b/include/skiboot.h
>>> @@ -75,7 +75,7 @@ struct debug_descriptor {
>>>                                           * low 4 bits driver (e.g. uart).
>>> */
>>>          u8      state_flags; /* various state flags - OPAL_BOOT_COMPLETE
>>> etc */
>>>          u16     reserved2;
>>> -       u32     reserved[2];
>>> +       u64     log_buf_phys;           /* Pointer to kernel log buffer */
>>
>>
>> May be increment DEBUG_DESC_VERSION as well? So that user can identify
>> whether this field
>> is supported or not?
> 
> I considered this. Users running skiboot before this patch will see
> zeros for the reserved[2] field. Users running skiboot after this
> patch will either see zeroes, or see the field populated with the
> memory address. Given that there's no change in behaviour, I don't
> think there's any value in bumping the field.

That's correct. Only issue is its difficult to differentiate between old OPAL vs 
kernel failed to register.. Not a big deal.. upto you.

-Vasant