Re: [PATCH v2,5/5] PowerPC/mpc85xx: Add CPU hotplug support for E6500

From: Scott Wood
Date: Fri Aug 28 2015 - 01:13:53 EST

On Fri, 2015-08-28 at 09:42 +0800, Chenhui Zhao wrote:
> On Thu, Aug 27, 2015 at 6:42 AM, Scott Wood <scottwood@xxxxxxxxxxxxx>
> wrote:
> > On Wed, Aug 26, 2015 at 08:09:48PM +0800, Chenhui Zhao wrote:
> > > + .globl booting_thread_hwid
> > > +booting_thread_hwid:
> > > + .long INVALID_THREAD_HWID
> > > + .align 3
> >
> > The commit message goes into no detail about the changes you're
> > making to
> > thread handling, nor are there relevant comments.
>
> OK. Will add some comments.
>
> >
> > > +/*
> > > + * r3 = the thread physical id
> > > + */
> > > +_GLOBAL(book3e_stop_thread)
> > > + li r4, 1
> > > + sld r4, r4, r3
> > > + mtspr SPRN_TENC, r4
> > > + isync
> > > + blr
> >
> > Why did the C code not have an isync, if it's required here?
>
> Just make sure "mtspr" has completed before the routine returns.
>
> >
> >
> > > _GLOBAL(fsl_secondary_thread_init)
> > > /* Enable branch prediction */
> > > lis r3,BUCSR_INIT@h
> > > @@ -197,8 +236,10 @@ _GLOBAL(fsl_secondary_thread_init)
> > > * but the low bit right by two bits so that the cpu numbering is
> > > * continuous.
> > > */
> > > - mfspr r3, SPRN_PIR
> > > - rlwimi r3, r3, 30, 2, 30
> > > + bl 10f
> > > +10: mflr r5
> > > + addi r5,r5,(booting_thread_hwid - 10b)
> > > + lwz r3,0(r5)
> > > mtspr SPRN_PIR, r3
> > > #endif
> >
> > I assume the reason for this is that, unlike the kexec case, the cpu
> > has
> > been reset so PIR has been reset? Don't make me guess -- document.
>
> We can not rely on the value saved in SPRN_PIR. Every time running
> fsl_secondary_thread_init, SPRN_PIR may not always has a reset value.
> Using booting_thread_hwid to ensure SPRN_PIR has a correct value.

But when will the cpu ever be in a state other than "reset PIR value and
reset BUCSR value" or "Software-desired PIR value and BUCSR value"?

> > > @@ -245,6 +286,30 @@ _GLOBAL(generic_secondary_smp_init)
> > > mr r3,r24
> > > mr r4,r25
> > > bl book3e_secondary_core_init
> > > +
> > > +/*
> > > + * If we want to boot Thread1, start Thread1 and stop Thread0.
> > > + * Note that only Thread0 will run the piece of code.
> > > + */
> >
> > What ensures that only thread 0 runs this? Especially if we're
> > entering
> > via kdump on thread 1?
>
> This piece of code will be executed only when core resets (Thead0 will
> start first).

This is not true with kexec/kdump.

> Thead1 will run fsl_secondary_thread_init() to start.
>
> How can kdump run this on Thread1? I know little about kexec.

kexec/kdump involves booting a new kernel image without resetting the
hardware.

+ /* start Thread1 */
> > > + LOAD_REG_ADDR(r5, fsl_secondary_thread_init)
> > > + ld r4, 0(r5)
> > > + li r3, 1
> > > + bl book3e_start_thread
> > > +
> > > + /* stop Thread0 */
> > > + li r3, 0
> > > + bl book3e_stop_thread
> > > +10:
> > > + b 10b
> > > +20:
> > > #endif
> > >
> > > generic_secondary_common_init:
> > > diff --git a/arch/powerpc/platforms/85xx/smp.c
> > > b/arch/powerpc/platforms/85xx/smp.c
> > > index 73eb994..61f68ad 100644
> > > --- a/arch/powerpc/platforms/85xx/smp.c
> > > +++ b/arch/powerpc/platforms/85xx/smp.c
> > > @@ -181,17 +181,11 @@ static inline u32 read_spin_table_addr_l(void
> > > *spin_table)
> > > static void wake_hw_thread(void *info)
> > > {
> > > void fsl_secondary_thread_init(void);
> > > - unsigned long imsr1, inia1;
> > > - int nr = *(const int *)info;
> > > + unsigned long inia;
> > > + int hw_cpu = get_hard_smp_processor_id(*(const int *)info);
> > >
> > > - imsr1 = MSR_KERNEL;
> > > - inia1 = *(unsigned long *)fsl_secondary_thread_init;
> > > -
> > > - mttmr(TMRN_IMSR1, imsr1);
> > > - mttmr(TMRN_INIA1, inia1);
> > > - mtspr(SPRN_TENS, TEN_THREAD(1));
> > > -
> > > - smp_generic_kick_cpu(nr);
> > > + inia = *(unsigned long *)fsl_secondary_thread_init;
> > > + book3e_start_thread(cpu_thread_in_core(hw_cpu), inia);
> > > }
> > > #endif
> > >
> > > @@ -279,7 +273,6 @@ static int smp_85xx_kick_cpu(int nr)
> > > int ret = 0;
> > > #ifdef CONFIG_PPC64
> > > int primary = nr;
> > > - int primary_hw = get_hard_smp_processor_id(primary);
> > > #endif
> > >
> > > WARN_ON(nr < 0 || nr >= num_possible_cpus());
> > > @@ -287,33 +280,43 @@ static int smp_85xx_kick_cpu(int nr)
> > > pr_debug("kick CPU #%d\n", nr);
> > >
> > > #ifdef CONFIG_PPC64
> > > + booting_thread_hwid = INVALID_THREAD_HWID;
> > > /* Threads don't use the spin table */
> > > - if (cpu_thread_in_core(nr) != 0) {
> > > - int primary = cpu_first_thread_sibling(nr);
> > > + if (threads_per_core == 2) {
> > > + booting_thread_hwid = get_hard_smp_processor_id(nr);
> >
> > What does setting booting_thread_hwid to INVALID_THREAD_HWID here
> > accomplish? If threads_per_core != 2 it would never have been set to
> > anything else, and if threads_per_core == 2 you immediately overwrite
> > it.
>
> booting_thread_hwid is valid only for the case that one core has two
> threads (e6500). For e5500 and e500mc, one core one thread,
> "booting_thread_hwid" is invalid.
>
> "booting_thread_hwid" will determine starting which thread in
> generic_secondary_smp_init().

You didn't answer my question.

> > > + /*
> > > + * If either one of threads in the same core is online,
> > > + * use the online one to start the other.
> > > + */
> > > + if (qoriq_pm_ops)
> > > + qoriq_pm_ops->cpu_up_prepare(nr);
> >
> > cpu_up_prepare does rcpm_v2_cpu_exit_state(cpu, E500_PM_PH20). How do
> > you know the cpu is already in PH20? What if this is initial boot?
> > Are
> > you relying on it being a no-op in that case?
>
> Yes, if the cpu is in PH20, it will exit; if not, cpu_up_prepare() is
> equal to a no-op.

This warrants a comment.

-Scott

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