Re: [PATCH v4 1/5] devicetree: bindings: document Broadcom CPU enable method

[Alex Elder]

On 05/28/2014 05:36 AM, Lorenzo Pieralisi wrote:
> On Wed, May 28, 2014 at 04:30:47AM +0100, Alex Elder wrote:
>> On 05/27/2014 06:49 AM, Lorenzo Pieralisi wrote:
>>> On Tue, May 20, 2014 at 06:43:46PM +0100, Alex Elder wrote:
>>>> Broadcom mobile SoCs use a ROM-implemented holding pen for
>>>> controlled boot of secondary cores.  A special register is
>>>> used to communicate to the ROM that a secondary core should
>>>> start executing kernel code.  This enable method is currently
>>>> used for members of the bcm281xx and bcm21664 SoC families.
>>>>
>>>> The use of an enable method also allows the SMP operation vector to
>>>> be assigned as a result of device tree content for these SoCs.
>>>>
>>>> Signed-off-by: Alex Elder <elder@xxxxxxxxxx>
>>>
>>> This is getting out of control, it is absolutely ghastly. I wonder how
>>> I can manage to keep cpus.txt updated if anyone with a boot method
>>> du jour adds into cpus.txt, and honestly in this specific case it is even
>>> hard to understand why.
>>
>> OK, in this message I'll focus on the particulars of this
>> proposed binding.
>>
>>> Can't it be done with bindings for the relative register address space
>>> (regmap ?) and platform code just calls the registers driver to set-up the
>>> jump address ? It is platform specific code anyway there is no way you
>>> can make this generic.
>>
>> I want to clarify what you're after here.
>>
>> My aim is to add SMP support for a class of Broadcom SMP
>> machines.  To do so, I'm told I need to use the technique
>> of assigning the SMP operations vector as a result of
>> identifying an enable method in the DT.
>>
>> For 32-bit ARM, there are no generic "enable-method" values.
>> (I did attempt to create one for "spin-table" but that was
>> rejected by Russell King.)  For the machines I'm trying to
>> enable, secondary CPUS start out spinning in a ROM-based
>> holding pen, and there is no need for a kernel-based one.
>>
>> However, like a spin-table/holding pen enable method, a
>> memory location is required for coordination between the
>> boot CPU running kernel code and secondary CPUs running ROM
>> code.  My proposal specifies it using a special numeric
>> property value named "secondary-boot-reg" in the "cpus"
>> node in the DT.
>>
>> And as I understand it, the issue you have relates to how
>> this memory location is specified.
> 
> The issue I have relates to cluttering cpus.txt with all
> sorts of platform specific SMP boot hacks.

OK, as I mentioned in my other message, I totally
agree with you here.  It's a total (and building)
mess.  I discussed this with Mark Rutland at ELC
last month and suggested splitting that stuff out
of "cpus.txt", which I have now proposed with a
patch.
    https://lkml.org/lkml/2014/5/8/545

>> You suggest regmap.  I'm using a single 32-bit register,
>> only at very early boot time, and thereafter access to
>> it is meaningless.  It seems like overkill if it's only
>> used for this purpose.  I could hide the register values
>> in the code, but with the exception of that, the code I'm
>> using is generic (in the context of this class of Broadcom
>> machine).  I could specify the register differently somehow,
>> in a different node, or with a different property.
> 
> Is that register part of a larger registers block ? What I wanted
> to say is that you can use a driver "API" (we wish) to write that
> register, something like eg vexpress does with sysflags:
> 
> drivers/mfd/vexpress-sysreg.c
> 
> vexpress_flags_set()
> 
> instead of grabbing the reg address from a platform specific boot
> method DT entry.

I had this exact thought when I was working on this.  Yes,
this register *is* part of a register block, "ChipRegs", and
I wanted to have a separate driver/library/API to manage access
to registers in that block.  But the block is sort of a dumping
ground for many unrelated things, and lots of them aren't going
to be used for normal operation (lots of debug and diagnostic
functionality, for example).  After some discussion with others
on my team I made the explicit decision to *not* have any sort
of API in this case, and just specify the register.  We decided
that if and when we had a need for multiple users of addresses
in this range we'd figure out how to do best what was required.

> I doubt that register exists on its own, even though I have to say this
> would force you to write yet another platform specific driver to control
> a bunch of registers, I do not see any other solution.

Yes, exactly, and I was prepared to do that.  But doing a whole
driver for that one address seemed to make less sense.

> One thing is for certain: I really do not see the point in adding a boot
> method per-SoC, and I do not want to end up having a cpus.txt file with a

I'd love to see common, "standard" boot methods.  For ARM32
it seems each one was done separately, and in many cases,
slightly differently.  The one I saw the most was "spin table,"
but making that "standard" was deemed unacceptable.

> gazillion entries just because every given platform reinvents the wheel when
> it comes to booting an SMP system, cpus.txt would become a document that
> describes platform quirks, not a proper binding anymore.

Agreed.

> At least all platform specific quirks must be moved out of cpus.txt and
> in platform documentation, I understand it is just a cosmetic change but
> I want to prevent cpus.txt to become an abomination.

Agreed.

>> The bottom line here is I'm not sure whether I understand
>> what you're suggesting, or perhaps why what you suggest is
>> preferable.  I'm very open to suggestions, I just need it
>> laid out a bit more detail in order to respond directly.
> 
> See above.

I really appreciate your responses.  One problem with this
process is you don't get the benefit of the discussion and
design process that went on prior to posting this stuff
publicly.

If you still feel strongly we should have a custom driver
to manage this "ChipRegs" address space, say so, and I can
put one together.  If my explanations above mitigate your
concerns, please say that as well.  Either way I just want
to make progress toward getting this support upstream.

Thanks a lot.

					-Alex

> 
> Thanks !
> Lorenzo
> 
>>
>> Thanks.
>>
>> 					-Alex
>>
>>> I really do not see the point in cluttering cpus.txt with this stuff, it
>>> is a platform specific hack, and do not belong in generic bindings in my
>>> opinion.
>>>
>>> Thanks,
>>> Lorenzo
>>>
>>>> ---
>>>>  Documentation/devicetree/bindings/arm/cpus.txt | 12 ++++++++++++
>>>>  1 file changed, 12 insertions(+)
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/arm/cpus.txt b/Documentation/devicetree/bindings/arm/cpus.txt
>>>> index 333f4ae..c6a2411 100644
>>>> --- a/Documentation/devicetree/bindings/arm/cpus.txt
>>>> +++ b/Documentation/devicetree/bindings/arm/cpus.txt
>>>> @@ -185,6 +185,7 @@ nodes to be present and contain the properties described below.
>>>>  			    "qcom,gcc-msm8660"
>>>>  			    "qcom,kpss-acc-v1"
>>>>  			    "qcom,kpss-acc-v2"
>>>> +			    "brcm,bcm11351-cpu-method"
>>>>  
>>>>  	- cpu-release-addr
>>>>  		Usage: required for systems that have an "enable-method"
>>>> @@ -209,6 +210,17 @@ nodes to be present and contain the properties described below.
>>>>  		Value type: <phandle>
>>>>  		Definition: Specifies the ACC[2] node associated with this CPU.
>>>>  
>>>> +	- secondary-boot-reg
>>>> +		Usage:
>>>> +			Required for systems that have an "enable-method"
>>>> +			property value of "brcm,bcm11351-cpu-method".
>>>> +		Value type: <u32>
>>>> +		Definition:
>>>> +			Specifies the physical address of the register used to
>>>> +			request the ROM holding pen code release a secondary
>>>> +			CPU.  The value written to the register is formed by
>>>> +			encoding the target CPU id into the low bits of the
>>>> +			physical start address it should jump to.
>>>>  
>>>>  Example 1 (dual-cluster big.LITTLE system 32-bit):
>>>>  
>>>> -- 
>>>> 1.9.1
>>>>
>>>>
>>>
>>
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> 

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