[PATCH] ARM: zynq: wfi exit on same cpu is valid

[Michal Simek]

On 06/05/2013 01:28 PM, Steve.Zhan at spreadtrum.com wrote:
> linaro-kernel-bounces at lists.linaro.org wrote on 2013-06-05 18:47:46:
> 
>> From: Michal Simek <monstr at monstr.eu>
>> To: Russell King - ARM Linux <linux at arm.linux.org.uk>, 
>> Cc: Lists linaro-kernel <linaro-kernel at lists.linaro.org>, Patch 
>> Tracking <patches at linaro.org>, michal.simek at xilinx.com, Lists LAKML 
>> <linux-arm-kernel at lists.infradead.org>
>> Date: 2013-06-05 18:48
>> Subject: Re: [PATCH] ARM: zynq: wfi exit on same cpu is valid
>> Sent by: linaro-kernel-bounces at lists.linaro.org
>>
>> On 06/04/2013 04:17 PM, Russell King - ARM Linux wrote:
>>> On Tue, Jun 04, 2013 at 03:10:17PM +0100, Russell King - ARM Linux 
> wrote:
>>>> On Tue, Jun 04, 2013 at 01:58:31PM +0200, Daniel Lezcano wrote:
>>>>> On 06/04/2013 01:39 PM, Amit Kucheria wrote:
>>>>>> I'm curious why it is called pen_release. :) Is there some 
> historical
>>>>>> link to some HW lines?
>>>>>
>>>>> I tried to figure out the same but I did not found any information 
> on
>>>>> that. I assumed the name could be referring to a simplified mutual
>>>>> exclusion algorithm from the 'Dining philosophers problem' [1] where 
> the
>>>>> fork is a pen.
>>>>
>>>> Where it comes from is the original ARM SMP patches from early 2000, 
> which
>>>> everyone has blindly copied with no thought about what they're doing. 
>  This
>>>> is why I'm totally against any consolidation of this code, because 
> I'm of
>>>> the opinion that _no one_ other than the ARM Ltd development 
> platforms
>>>> should be using it.
>>>>
>>>> "pen" means "holding pen".  It comes about because early on in the 
> SMP
>>>> development, ARM SMP platforms had four CPUs, and it was only 
> possible to
>>>> release all three secondary CPUs from the boot loader simultaneously 
> to
>>>> a common piece of code.
>>>>
>>>> As the kernel was not able to serialize the release of each CPU, ARM 
> Ltd
>>>> worked around this problem by having all the CPUs jump to assembly 
> code
>>>> which "holds" the CPUs which we didn't want to boot yet, and the CPUs
>>>> are released one at a time by setting pen_release to the hardware CPU
>>>> number.
>>>>
>>>> Modern platforms either have just one secondary CPU, or they have a 
> way
>>>> to control the reset/power to the secondary CPU.  This makes the 
> holding
>>>> pen entirely redundant, and such platforms should _not_ make use of 
> any
>>>> kind of holding pen.
>>>
>>> And yes, indeed, zynq can control the secondary CPU:
>>>
>>> void zynq_slcr_cpu_start(int cpu)
>>> {
>>>         /* enable CPUn */
>>>         writel(SLCR_A9_CPU_CLKSTOP << cpu,
>>>                zynq_slcr_base + SLCR_A9_CPU_RST_CTRL);
>>>         /* enable CLK for CPUn */
>>>         writel(0x0 << cpu, zynq_slcr_base + SLCR_A9_CPU_RST_CTRL);
>>> }
>>>
>>> void zynq_slcr_cpu_stop(int cpu)
>>> {
>>>         /* stop CLK and reset CPUn */
>>>         writel((SLCR_A9_CPU_CLKSTOP | SLCR_A9_CPU_RST) << cpu,
>>>                zynq_slcr_base + SLCR_A9_CPU_RST_CTRL);
>>> }
>>>
>>> So there's no need for the pen.  There's no need for the low power 
> crap
>>> in hotplug.c, there's no need for the pen in hotplug.c.  You just 
> arrange
>>> for the secondary CPU to have its clock stopped and reset when it is
>>> taken offline.
>>>
>>> Hotplugging a CPU back in _should_ be no different from its initial
>>> bringup into the kernel.
>>
>> I have tested that and cpu_die code is performed on cpu which
>> should die.
>> And simple calling zynq_slcr_cpu_stop() on cpu which should die
>> just doesn't work.
>> There is probably any expectation which I can't see.
>>
>> Feel free to suggest me proper solution.
>>
>> Thanks,
>> Michal
>>
> 
> Hi Michal,
>         Because most SOC design is that secondary cpu can not poweecho 0 > /sys/devices/system/cpu/cpu1/onlinerdown by 
> itself.
> These are some instructions is running in the bus.
> 
>         We can only put the core in lowpower mode using wfi/wfe by itself.
> If wakeup or boot this core from die/deepsleep again, we don't need to use 
> holding pen.

pen usage is quite clear right now.

Let me comment Russel's comment.

Thanks,
Michal

-- 
Michal Simek, Ing. (M.Eng), OpenPGP -> KeyID: FE3D1F91
w: www.monstr.eu p: +42-0-721842854
Maintainer of Linux kernel - Microblaze cpu - http://www.monstr.eu/fdt/
Maintainer of Linux kernel - Xilinx Zynq ARM architecture
Microblaze U-BOOT custodian and responsible for u-boot arm zynq platform


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