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    SubjectRe: [PATCH] irq_bcm2836: Send event when onlining sleeping cores
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    On 05/10/2017 03:31 AM, Phil Elwell wrote:
    > On 10/05/2017 11:09, Marc Zyngier wrote:
    >> 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 <phil@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.zyngier@arm.com> writes:
    >>>>>>>>>>>
    >>>>>>>>>>>> On 09/05/17 17:59, Eric Anholt wrote:
    >>>>>>>>>>>>> Phil Elwell <phil@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 <phil@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.
    >
    > Is that the opinion of all here?

    The choice of word here got largely out of the original topic and I
    surely did eat a ton of popcorn here. There are two things that need
    fixing, and the time line and process for fixing these is clear:

    - your bugfix (Phil) is something that should be applied now, and
    backported to -stable trees once the fix hits the irqchip tree (or Linus')

    - relocating the code that does the secondary boot out of
    drivers/irqchip/ into arch/arm/mach-bcm/ needs to happen (Marc), and
    this is 4.13 material, there is no urgency in doing this *right now*,
    but it needs to happen

    Does that work for everyone?
    --
    Florian

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