linux-kernel - Re: AMR: sun7i: CPU hotplug support?

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Date:	Mon, 10 Nov 2014 09:52:28 +0000
From:	Marc Zyngier <marc.zyngier@....com>
To:	Jan Kiszka <jan.kiszka@....de>
CC:	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-sunxi@...glegroups.com" <linux-sunxi@...glegroups.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: AMR: sun7i: CPU hotplug support?

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On 10/11/14 09:36, Jan Kiszka wrote:
> On 2014-11-10 10:17, Marc Zyngier wrote:
>> On 10/11/14 08:25, Jan Kiszka wrote:
>>> On 2014-11-10 07:03, Jan Kiszka wrote:
>>>> On 2014-11-10 00:17, Maxime Ripard wrote:
>>>>> Hi Jan,
>>>>> 
>>>>> On Sun, Nov 09, 2014 at 08:35:49PM +0100, Jan Kiszka
>>>>> wrote:
>>>>>> did anyone already happen to look into enabling CPU
>>>>>> hotplug for the Allwinner A20 in upstream? I'm currently
>>>>>> running the sunxi-next branch on Banana Pi, and echo 0 >
>>>>>> .../cpu1/online just hangs the system. The old 3.4
>>>>>> LeMaker kernel works fine in this regard. I can try to
>>>>>> look into details and port things over, just want to
>>>>>> avoid duplicate efforts.
>>>>> 
>>>>> Having hotplug support would indeed be very welcome.
>>>>> 
>>>>> However, it should be done in u-boot, through PSCI, and not
>>>>> in the kernel itself.
>>>>> 
>>>>> As far as I'm aware, no one worked actively on it, beside
>>>>> some WIP commit from Marc a while ago: 
>>>>> https://git.kernel.org/cgit/linux/kernel/git/maz/u-boot.git/commit/?h=wip/psci&id=45379c0f9cf812f0f62722f4015ec907fa5dc144
>>>>
>>>>
>>>>>
>>
>>>>> 
OK - I guess I will need a little guidance in then: Is there a good
>>>> reference board to study and to derive from? And maybe also:
>>>> What is missing or not working in that u-boot branch? If I
>>>> get this interface right, I just takes some device tree bits
>>>> to enable this for the kernel afterward, correct?
>>> 
>>> Started to play with that patch in naive ways: CPU0 locks up
>>> when offlining CPU1 - unless I disable the FIQ signal from
>>> CPU1. Then it "works", both offlining and onlining again.
>>> However, I suspect that this only parks CPU1 in wfi and does
>>> not do anything interesting to it.
>> 
>> Here's how this is supposed to work: - CPU1 sends a FIQ to CPU0,
>> bringing it into secure mode. - CPU0 then kills CPU1 by doing the
>> magic incantations on the power controller
>> 
>> What is missing here is all the cache cleaning before signalling
>> CPU0. If you add that, things should look a lot better (patches
>> welcome).
> 
> Unsure about this, or maybe this was too simplistic: I added calls
> to u-boot's flush_dcache_all and invalidate_icache_all (right
> after disabling the cache, just like the vendor kernel does), but
> CPU0 still locks up. I suspect there is still a bug in the FIQ
> handling. There is also a suspicious single "@" printed on the
> console. I'll play with the FIQ handler a bit.

The '@' is just my own debug stuff, and might be causing issues too.

Now, you have to realise that by the time you call into this code,
u-boot itself is long gone. Only the tiny bit of code dealing with
PSCI still lives in a bank of static, secure memory. So calling into
u-boot for anything is doomed. You need to actually put the code
inside the PSCI backend.

Thanks,

	M.
- -- 
Jazz is not dead. It just smells funny...
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