Re: [PATCH v3 1/4] dt-bindings: add SMP enable-method for Broadcom NSP

[Kapil Hali]

Hi Russel,

On 11/8/2015 11:01 PM, Russell King - ARM Linux wrote:
> On Sat, Nov 07, 2015 at 01:40:23PM -0800, Florian Fainelli wrote:
>> Le 06/11/2015 13:11, Kapil Hali a écrit :
>>> Add a compatible string "brcm,bcm-nsp-smp" for Broadcom's
>>> Northstar Plus CPU to the 32-bit ARM CPU device tree binding
>>> documentation file and create a new binding documentation for
>>> Northstar Plus CPU.
>>>
>>> Signed-off-by: Kapil Hali <kapilh@xxxxxxxxxxxx>
>>> ---
>>>  .../bindings/arm/bcm/brcm,nsp-cpu-method.txt       | 36 ++++++++++++++++++++++
>>>  Documentation/devicetree/bindings/arm/cpus.txt     |  1 +
>>>  2 files changed, 37 insertions(+)
>>>  create mode 100644 Documentation/devicetree/bindings/arm/bcm/brcm,nsp-cpu-method.txt
>>>
>>> diff --git a/Documentation/devicetree/bindings/arm/bcm/brcm,nsp-cpu-method.txt b/Documentation/devicetree/bindings/arm/bcm/brcm,nsp-cpu-method.txt
>>> new file mode 100644
>>> index 0000000..8506da7
>>> --- /dev/null
>>> +++ b/Documentation/devicetree/bindings/arm/bcm/brcm,nsp-cpu-method.txt
>>> @@ -0,0 +1,36 @@
>>> +Broadcom Northstar Plus SoC CPU Enable Method
>>> +---------------------------------------------
>>> +This binding defines the enable method used for starting secondary
>>> +CPUs in the following Broadcom SoCs:
>>> +  BCM58522, BCM58525, BCM58535, BCM58622, BCM58623, BCM58625, BCM88312
>>> +
>>> +The enable method is specified by defining the following required
>>> +properties in the "cpus" device tree node:
>>> +  - enable-method = "brcm,bcm-nsp-smp";
>>> +  - secondary-boot-reg = <...>;
>>> +
>>> +The secondary-boot-reg property is a u32 value that specifies the
>>> +physical address of the register used to request the ROM holding pen
>>> +code release a secondary CPU.
>>
>> Is it really how the ROM code is implemented, as a pen holding/release
>> mechanism (which sounds like how this was implemented previously in the
>> kernel actually) or should this be described in a more generic way as
>> the physical address of the register where the secondary CPUs reset
>> vector address must be written to? Or something along these lines.
> 
> Why do people insist on using holding pens to bring their secondary CPUs
> into existence?  I hope the hardware people aren't being dumb and have no
> way to hold in reset or power down their secondary CPUs, either of which
> is a vital feature for things like kexec and the like.  If they do have
> a way to hold secondary CPUs in reset or powered down, why aren't they
> using that at boot instead of implementing the stupid Versatile scheme,
> which exists because Versatile _can't_ hold its CPUs in reset or power
> them down...
> 
> It's times like this that I wonder what kind of drugs the hardware SoC
> people are on, but I'm well aware that people contributing SMP bringup
> solutions are also dumb idiots who copy the Versatile scheme with very
> little thought... (as you can see, I'm not mincing my words here - if
> people want to be lazy in this regard despite this having been brought
> up multiple times, and the lead developers having said that the versatile
> pen_release stuff should not be used, they earn themselves the right to
> be called dumb idiots.  Simple solution to avoid that title: don't be a
> dumb idiot by copy the Versatile SMP bring up code!  It's not a sane
> model for any SoC sane SoC to follow.)
> 
> Is this clear enough?
> 
It was clear the very first time itself as pointed out by you and the 
lead developers and hence the change was readily sent in the very next
patch set. I didn't change a comment in this patch, which is misleading 
about the SMP enable-method used in the patch set, it is my mistake and   
I apologies for the same. I will change it and send the next patch set.

Also, before sending out the patch set, I had asked for a clarification 
about the method: https://lkml.org/lkml/2015/11/6/234
For my understanding, I am repeating my query- In case of simple method of 
waking up secondary core, smp_boot_secondary() will always return success 
indicating secondary core successfully started. I understand that in 
__cpu_up(), primary core waits for completion till secondary core comes 
online or time outs. However, is it appropriate to return successful start 
of secondary core without knowing if it really did?

Thanks,
Kapil Hali
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