On 10/05/17 10:05, Phil Elwell wrote:
> On 10/05/2017 09:55, Marc Zyngier wrote:
>> On Wed, May 10 2017 at 9:27:10 am BST, Phil Elwell <p...@raspberrypi.org>
>> wrote:
>>> On 10/05/2017 08:42, Marc Zyngier wrote:
>>>> On 09/05/17 20:02, Phil Elwell wrote:
>>>>> On 09/05/2017 19:53, Marc Zyngier wrote:
>>>>>> On 09/05/17 19:52, Phil Elwell wrote:
>>>>>>> On 09/05/2017 19:14, Marc Zyngier wrote:
>>>>>>>> On 09/05/17 19:08, Eric Anholt wrote:
>>>>>>>>> Marc Zyngier <marc.zyng...@arm.com> writes:
>>>>>>>>>
>>>>>>>>>> On 09/05/17 17:59, Eric Anholt wrote:
>>>>>>>>>>> Phil Elwell <p...@raspberrypi.org> writes:
>>>>>>>>>>>
>>>>>>>>>>>> In order to reduce power consumption and bus traffic, it is
>>>>>>>>>>>> sensible
>>>>>>>>>>>> for secondary cores to enter a low-power idle state when waiting to
>>>>>>>>>>>> be started. The wfe instruction causes a core to wait until an
>>>>>>>>>>>> event
>>>>>>>>>>>> or interrupt arrives before continuing to the next instruction.
>>>>>>>>>>>> The sev instruction sends a wakeup event to the other cores, so
>>>>>>>>>>>> call
>>>>>>>>>>>> it from bcm2836_smp_boot_secondary, the function that wakes up the
>>>>>>>>>>>> waiting cores during booting.
>>>>>>>>>>>>
>>>>>>>>>>>> It is harmless to use this patch without the corresponding change
>>>>>>>>>>>> adding wfe to the ARMv7/ARMv8-32 stubs, but if the stubs are
>>>>>>>>>>>> updated
>>>>>>>>>>>> and this patch is not applied then the other cores will sleep
>>>>>>>>>>>> forever.
>>>>>>>>>>>>
>>>>>>>>>>>> See: https://github.com/raspberrypi/linux/issues/1989
>>>>>>>>>>>>
>>>>>>>>>>>> Signed-off-by: Phil Elwell <p...@raspberrypi.org>
>>>>>>>>>>>> ---
>>>>>>>>>>>> drivers/irqchip/irq-bcm2836.c | 3 +++
>>>>>>>>>>>> 1 file changed, 3 insertions(+)
>>>>>>>>>>>>
>>>>>>>>>>>> diff --git a/drivers/irqchip/irq-bcm2836.c
>>>>>>>>>>>> b/drivers/irqchip/irq-bcm2836.c
>>>>>>>>>>>> index e10597c..6dccdf9 100644
>>>>>>>>>>>> --- a/drivers/irqchip/irq-bcm2836.c
>>>>>>>>>>>> +++ b/drivers/irqchip/irq-bcm2836.c
>>>>>>>>>>>> @@ -248,6 +248,9 @@ static int __init
>>>>>>>>>>>> bcm2836_smp_boot_secondary(unsigned int cpu,
>>>>>>>>>>>> writel(secondary_startup_phys,
>>>>>>>>>>>> intc.base + LOCAL_MAILBOX3_SET0 + 16 * cpu);
>>>>>>>>>>>>
>>>>>>>>>>>> + dsb(sy); /* Ensure write has completed before waking the other
>>>>>>>>>>>> CPUs */
>>>>>>>>>>>> + sev();
>>>>>>>>>>>> +
>>>>>>>>>>>> return 0;
>>>>>>>>>>>> }
>>>>>>>>>>>
>>>>>>>>>>> This is also the behavior that the standard arm64 spin-table
>>>>>>>>>>> method has,
>>>>>>>>>>> which we unfortunately can't quite use.
>>>>>>>>>>
>>>>>>>>>> And why is that so? Why do you have to reinvent the wheel (and hide
>>>>>>>>>> the
>>>>>>>>>> cloned wheel in an interrupt controller driver)?
>>>>>>>>>>
>>>>>>>>>> That doesn't seem right to me.
>>>>>>>>>
>>>>>>>>> The armv8 stubs (firmware-supplied code in the low page that do the
>>>>>>>>> spinning) do actually implement arm64's spin-table method. It's the
>>>>>>>>> armv7 stubs that use these registers in the irqchip instead of plain
>>>>>>>>> addresses in system memory.
>>>>>>>>
>>>>>>>> Let's put ARMv7 aside for the time being. If your firmware already
>>>>>>>> implements spin-tables, why don't you simply use that at least on
>>>>>>>> arm64?
>>>>>>>
>>>>>>> We do.
>>>>>>
>>>>>> Obviously not the way it is intended if you have to duplicate the core
>>>>>> architectural code in the interrupt controller driver, which couldn't
>>>>>> care less.
>>>>>
>>>>> If we were using this method on arm64 then the other cores would not
>>>>> start up
>>>>> because armstub8.S has always included a wfe. Nothing in the commit
>>>>> mentions
>>>>> arm64 - this is an ARCH=arm fix.
>>>>
>>>> Thanks for the clarification, which you could have added to the commit
>>>> message.
>>>>
>>>> The question still remains: why do we have CPU bring-up code in an
>>>> interrupt controller, instead of having it in the architecture code?
>>>>
>>>> The RPi-2 is the *only* platform to have its SMP bringup code outside of
>>>> arch/arm, so the first course of action would be to move that code where
>>>> it belongs.
>>>
>>> You were CC'd on the commit (41f4988cc287e5f836d3f6620c9f900bc9b560e9) that
>>> introduced bcm2836_smp_boot_secondary - it seems strange to start objecting
>>> now.
>>
>> Well, I'm far from being perfect. If I had noticed it, I'd have NACKed
>> it.
>>
>>> Yes, I think it is odd that it didn't go into arch/arm/mach-bcm, but in
>>> the interests of making changes in small, independent steps, do you have a
>>> problem with this commit?
>>
>> On its own, no. I'm just not keen on adding more unrelated stuff to this
>> file, so let's start with dealing with the original bug, and you can
>> then add this fix on top.
>
> That's an interesting use of the word "bug". From Wikipedia:
>
> "A software bug is an error, flaw, failure or fault in a computer program or
> system that causes it to produce an incorrect or unexpected result, or to
> behave in unintended ways."
Whatever. Should I call it "pile of crap dumped in unsuitable locations"
instead? What does Wikipedia says about it?
> Although your concerns are valid, the faults you are objecting to are not
> causing
> a malfunction of any kind. If we were to update the RPi firmware before this
> patch was merged then upstream users would be left with one wheel on their
> wagon.
And that'd be your problem, not mine. Look, you can argue around this
all day, or you can fix this mess. Your choice.
M.
--
Jazz is not dead. It just smells funny...
