[U-Boot] [PATCH 2/2] arm64: add better spin-table support

[Andre Przywara]

Hi Masahiro,

On 25/06/16 05:08, Masahiro Yamada wrote:
> Hi Andre,
> 
> 
> 2016-06-19 19:33 GMT+09:00 André Przywara <andre.przywara at arm.com>:
>> Hi,
>>
>> On 19/06/16 09:57, Masahiro Yamada wrote:
>>> 2016-06-18 18:40 GMT+09:00 Linus Walleij <linus.walleij at linaro.org>:
>>>> On Fri, Jun 17, 2016 at 2:51 PM, Masahiro Yamada
>>>> <yamada.masahiro at socionext.com> wrote:
>>>>
>>>>> There are two enable methods supported by ARM64 Linux; psci and
>>>>> spin-table.  The latter is simpler and easier to use for quick SoC
>>>>> bring-up.
>>>>>
>>>>> So, I used the spin-table for my first ARMv8 SoC porting, but I
>>>>> found its support in U-Boot was poor.  It is true there exists a
>>>>> code fragment for the spin code in arch/arm/cpu/armv8/start.S,
>>>>> but I see some problems:
>>>>
>>>> Is part of the motivation for this approach to boot an ARMv8 system
>>>> without using the ARM Trusted Firmware?
>>>>
>>>> Yours,
>>>> Linus Walleij
>>>
>>> Yes, exactly.
>>>
>>> It would be the best choice
>>> to switch over to PSCI with ATF in a long run,
>>> but, I decided to use spin-table for the initial SoC bring-up
>>> because of tight schedule.
>>
>> So if you don't have an ATF port ready, why not use U-Boot's PSCI
>> implementation meanwhile? I think there are efforts underway to make
>> PSCI enablement for random new boards a walk in the park (by making the
>> PSCI support as generic as possible, CCing Chen-Yu for this).
> 
> 
> So, you mean U-Boot can serve PSCI for ARMv8 SoCs without ATF, right?
> (and efforts underway for further improvement)

Yes, if I am not mistaken then there are patches floating around to
achieve this.

> I know PSCI support is available for ARMv7
> (arch/arm/cpu/armv7/psci.S),
> but I could not find PSCI implementation
> in arch/arm/cpu/armv8/ directory in the mainline U-Boot.

I think that's work in progress (as in "on the ML").

> I checked Chen-Yu's patches on Patchwork,
> but I think they are mostly for improvement of ARMv7 PSCI support.
> 
> If I am missing something,
> could you point me to the reference, please?

I think you found this yourself already in that other mail of yours?

>> IIRC the spin-table boot method was just introduced to cope with cores
>> that don't have EL3 and thus cannot provide PSCI services the normal way
>> (and that don't want to or cannot sacrifice EL2 for that).
>> So I am a bit wary of proliferating this SMP method.
> 
> Proliferating?
> 
> Many of ARM 32bit SoCs have vendor-specific SMP methods.

Yes, and this is a massive PITA, but hard to change now.

> On the other hand, ARM64 only has two methods; psci and spin-table.

For a reason: Actually we just wanted to have PSCI only, but it turns
out that it's not easily possible on cores that don't implement EL3
(which is architecturally allowed and also implemented).
So spin-table support was added to provide a fall-back for those cases.
But if you have the choice, then PSCI is your friend.
Please note that any support for another SMP method is very unlikely to
ever get merged into upstream Linux.

> Is this a problem?

As mentioned, if you have a proper PSCI framework (as we are about to
get for U-Boot and which ATF already provides), adding SMP support is
not more involved than implementing spin tables. After all it's about
specifying the pen address and possibly providing per-core power hooks.

Cheers,
Andre.

>> Wouldn't it be better to help making U-Boot's PSCI stack as easy to use
>> as possible? I don't see technical reasons that adding PSCI support for
>> a board should be harder or more involved than adding spin-table support
>> - in the end you need to tell it about the SMP pen, maybe providing (or
>> faking?) reset and shutdown for 0.2 compliance.
> 
> 
> Right.
> 
> My motivation is to bring up Linux quickly
> before ATF becomes ready.
> 
> From your statement, I felt
> efforts for the ARMv8 PSCI implementation in U-Boot are underway.
> If so, I am definitely interested in it.
> 
>