Re: [PATCH] ARM: shmobile: Shared APMU SMP support code

[Sudeep KarkadaNagesha <Sudeep.KarkadaNagesha@arm.com>]

On 28/08/13 07:04, Magnus Damm wrote:
> Hi Sudeep,
> 
> On Tue, Aug 13, 2013 at 2:07 AM, Sudeep KarkadaNagesha
> <Sudeep.KarkadaNagesha@arm.com> wrote:
>> On 07/08/13 23:45, Magnus Damm wrote:
>>> From: Magnus Damm <damm@opensource.se>
>>>
>>> Introduce shared APMU SMP code for mach-shmobile. Both SMP boot up
>>> and CPU Hotplug is supported. DT is used for configuration of the
>>> APMU hardware block, as the following r8a73a4 example shows:
>>>
>>>        apmu@e6152000 {
>>>                compatible = "renesas,r8a73a4-apmu", "renesas,apmu";
>>>                reg = <0 0xe6152000 0 0x88>;
>>>                cpus = <&cpu0 &cpu1 &cpu2 &cpu3>;
>>>        };
>>>
>> This is introducing a new DT binding which needs to be documented. Also
>> you need to cc devicetree mailing list in case you need to add new
>> bindings. But I think you should not require this new binding.
> 
> Good idea, I have no objections against DT binding documentation. So
> if future versions end up using DT then those bindings surely need to
> be documented.
> 
> The reason behind using DT here is that it was recommended to me
> during the review of an earlier SMP prototype version. The base
> addresses for the Renesas-specific APMU hardware needs to be
> configured somehow and using DT may not be such a bad idea.
> 
That's fine, I am referring specifically to 'cpus' property in the binding:
	cpus = <&cpu0 &cpu1 &cpu2 &cpu3>;
It's difficult to understand what it means without a binding document.
I am not convinced if this 'cpus' property is needed.

>>> The code is designed around CONFIG_NR_CPUS and should in theory support
>>> any number of APMUs. At this point only the APMU that includes the
>>> boot CPU is enabled - this to prevent non-deterministic scheduling on
>>> upstream in case of multi-cluster hardware with varying performance.
>>>
>> I couldn't understand this patch completely but I believe you are
>> trying to solve multi-cluster power management and in your own custom
>> way.
>>
>> But there are 2 ways to handle this in a generic way:
>> 1. If Linux runs in non-secure mode, you need to use PSCI.
>>    You can refer Calxeda platform for reference[1]
>> 2. If Linux runs in secure mode, you need to use MPCM
>>    You can refer Vexpress CA15_CA7/TC2 platform for reference[2]
> 
> Thanks for this information. I'm not really trying to do any custom
> multi-cluster power management here, only provide software support for
> our APMU hardware block. The APMU hardware block is used in several
> SoCs from Renesas - for instance in single-cluster CA15-only
> configurations or multi-cluster CA15 and CA7 configurations.
> 
That's fine. I understand APMU is needed. But you may need access it as
part of MPCM platform_ops. You can refer [1] on how to use APMU code
from MCPM backend. SPC referred in there has similar functionality as APMU.

Especially for multi-cluster(infact even for single cluster), you may
you need to coordinate CPU/Cluster setup/powerdown which needs to be
abstracted out using MCPM or secure firmware supporting PSCI to avoid
duplication. Documentation/arm/cluster-pm-race-avoidance.txt describes
in detail about the same.

> So regardless of PSCI or MPCM it seems to me that the APMU hardware
> needs to be supported somewhere. I would like to have the APMU
> software support in Linux to follow the same style as our other SoCs.
> Having dependencies on binary blobs is something that I would like to
> avoid unless it is absolutely necessary. Regarding secure vs
> non-secure mode, as you may have guessed by now - the hardware on my
> desk runs in non-secure mode.
> 
Yes as I said using DT for APMU and supporting it is fine.
But I assume no hardware access restriction from power management
perspective in non-secure world. Otherwise you need to have some
firmware running in secure mode to handle power management. In that case
PSCI is the solution if Linux runs in non-secure world.

> As for PSCI, I wonder how that is supposed to work when power domains
> are shared between I/O devices and CPUs? I can understand the benefits
> of using PSCI to share independent CPU core PM support code outside of
> Linux if multiple OS would be supported perhaps together with TOS. But
> if there is no TOS and only a single OS and/or the hardware power
> domains include CPU cores and I/O devices driven by the OS then the
> merits of PSCI become less clear to me.
> 
OK if your platform doesn't plan to use PSCI, you need to use MCPM to
avoid code duplication as I mentioned above.

Regards,
Sudeep

[1]
http://lists.infradead.org/pipermail/linux-arm-kernel/2013-July/184372.html