[U-Boot] driver model is not smp safe

[Bin Meng]

Hi Andrew,

On Fri, Jul 31, 2015 at 8:30 PM, Andrew Bradford
<andrew at bradfordembedded.com> wrote:
> Hi Bin,
>
> On 07/30 12:12, Bin Meng wrote:
>> Hi Simon,
>>
>> When adding x86 multi-cpu initialization on a board with 4 cores, I found:
>>
>> => cpu list
>>   0: cpu at 0               Genuine Intel(R) CPU         @ 1.58GHz
>>   1: cpu at 1               Genuine Intel(R) CPU         @ 1.58GHz
>>   2: cpu at 2               Genuine Intel(R) CPU         @ 1.58GHz
>>   2: cpu at 3               Genuine Intel(R) CPU         @ 1.58GHz
>>
>> cpu at 2 and cpu at 3 have the same sequence number, which indicates they
>> are running parallelly to get the same sequence number. The call chain
>> on an ap is: mp_init_cpu() -> device_probe() -> uclass_resolve_seq().
>> Apparently ap2 and ap3 are running at the same time to get the same
>> number.
>>
>> Note so far all x86 boards that we have enabled x86 multi-cpu
>> initialization on only have 2 cores, which will not expose such issue
>> as there is no parallel execution among aps.
>>
>> What does this mean?
>>
>> -  Driver model is not smp safe. But given U-Boot is a single-threaded
>> environment, I don't think we want to add such support to driver
>> model.
>>
>> OR:
>>
>> - We are using driver model incorrectly on x86 mp initialization codes.
>>
>> What do you think?
>
> I'm not sure what to do about this (if anything) but I also see this on
> an E3845 based board.  I don't think it has affected me in any way.
>

So far I only see the seq number is not correct. Booting Linux with MP
table seems to be OK as Linux can start all 4 cores correctly.

> Does this also affect non-x86 processors?
>

I believe currently the cpu uclass is only implemented on x86, so only
x86 is affected for now.

> => cpu list
>   0: cpu at 0              Intel(R) Atom(TM) CPU  E3845  @ 1.91GHz
>   2: cpu at 1              Intel(R) Atom(TM) CPU  E3845  @ 1.91GHz
>   2: cpu at 2              Intel(R) Atom(TM) CPU  E3845  @ 1.91GHz
>   1: cpu at 3              Intel(R) Atom(TM) CPU  E3845  @ 1.91GHz
>


Regards,
Bin