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SubjectRe: [RFC PATCH 2/7] ARM: SMP: generic SMP spin-table method routines
On 03/31/2014 10:21 AM, Mark Rutland wrote:
> Hi Alex,
>
> On Fri, Mar 28, 2014 at 09:12:55PM +0000, Alex Elder wrote:
>> Implement a centralized version of the spin table (a.k.a. "holding
>> pen") method of secondary CPU initialization. This is the first
>> step in removing a number of duplicate implementations of this code.
>>
>> The eventual goal is to allow "enable-method" properties in device
>> tree nodes for CPUs to select and use this common code. As such,
>> some of the names are selected to match the names used in the SMP
>> spin-table code for ARM64.

Thanks for reviewing this Mark. I'll respond below, but given
that Russell King is not interested in incorporating my changes
into the core code I think it may be moot. Russell believes
centralizing this code encourages people who don't have a clue
what the hell they're doing to cargo-cult program.

> Given that there is a fundamental difference to the spin-table protocol
> in use on arm64 (in that here we are required to poke an arbitrary
> interrupt controller to send an SGI rather than just issuing a SEV), I
> would prefer that this had a name other than "spin-table" to
> disambiguate the two protocols.

It's possible (but I have no way of knowing) that a SEV is
sufficient to wake up the processor as well. It is on the
platform I'm working on. But in any case I would happily
use a different name, maybe "holding-pen" or something.

>> Note:
>> Most implementations examine only the bottom 4 bits of the MPIDR in
>> order to determine a CPU's id. This version looks at the bottom 24
>> bits instead, based on MPIDR_HWID_BITMASK. If using only 4 bits is
>> a requirement for most of the platforms that might use it I'll
>> switch this use 4 bits instead.
>
> Given that we require people to describe all of the MPIDR Aff* fields in
> the DT, and can update any board files as necessary, is this a problem?

You're right, it should not be a problem. I'm just a little
nervous about changing *any* behavior when I don't have the
hardware available to test the result. So I wanted to be
sure to call attention to this.

> IMO Using all the Aff bits is preferable.

I agree, absolutely.

Thanks again.

-Alex

> Cheers,
> Mark.
>
>>
>> Signed-off-by: Alex Elder <elder@linaro.org>
>> ---
>> arch/arm/include/asm/smp.h | 5 +++
>> arch/arm/kernel/head.S | 33 +++++++++++++++++++
>> arch/arm/kernel/smp.c | 77 ++++++++++++++++++++++++++++++++++++++++++++
>> 3 files changed, 115 insertions(+)
>>
>> diff --git a/arch/arm/include/asm/smp.h b/arch/arm/include/asm/smp.h
>> index 22a3b9b..83064d1 100644
>> --- a/arch/arm/include/asm/smp.h
>> +++ b/arch/arm/include/asm/smp.h
>> @@ -75,6 +75,11 @@ struct secondary_data {
>> extern struct secondary_data secondary_data;
>> extern volatile int pen_release;
>>
>> +extern volatile u32 secondary_holding_pen_release;
>> +extern void secondary_holding_pen(void);
>> +extern int smp_boot_secondary(unsigned int cpu, struct task_struct *idle);
>> +extern void smp_secondary_init(unsigned int cpu);
>> +
>> extern int __cpu_disable(void);
>>
>> extern void __cpu_die(unsigned int cpu);
>> diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
>> index f5f381d..3340f94 100644
>> --- a/arch/arm/kernel/head.S
>> +++ b/arch/arm/kernel/head.S
>> @@ -22,6 +22,7 @@
>> #include <asm/memory.h>
>> #include <asm/thread_info.h>
>> #include <asm/pgtable.h>
>> +#include <asm/cputype.h>
>>
>> #if defined(CONFIG_DEBUG_LL) && !defined(CONFIG_DEBUG_SEMIHOSTING)
>> #include CONFIG_DEBUG_LL_INCLUDE
>> @@ -402,6 +403,38 @@ __secondary_data:
>> .long .
>> .long secondary_data
>> .long __secondary_switched
>> +
>> +
>> + /*
>> + * Secondary cores spin in this "holding pen" until they are
>> + * signaled to proceed by jumping to secondary_startup
>> + * (above). A core knows to proceed when it finds that the
>> + * value of the secondary_holding_pen_release global matches
>> + * its (hardware) CPU ID. The secondary core acknowledges
>> + * it has begun executing by writing an invalid value (-1)
>> + * back into secondary_holding_pen_release (in
>> + * smp_operations->smp_secondary_init).
>> + */
>> +ENTRY(secondary_holding_pen)
>> + ARM_BE8(setend be)
>> + mrc p15, 0, r0, c0, c0, 5 @ Get MPIDR and extract CPU id from it
>> + and r0, r0, #MPIDR_HWID_BITMASK
>> + adr r4, 1f @ Get secondary_holding_pen_release
>> + ldmia r4, {r5, r6} @ and compute its physical address
>> + sub r4, r4, r5
>> + add r6, r6, r4
>> +pen: ldr r7, [r6] @ while secondary_holding_pen_release
>> + cmp r7, r0 @ doesn't hold our CPU id, spin
>> + bne pen
>> + /*
>> + * At this point we have been released from the holding pen;
>> + * secondary_stack now contains the SVC stack for this core.
>> + */
>> + b secondary_startup
>> +ENDPROC(secondary_holding_pen)
>> + .align
>> +1: .long .
>> + .long secondary_holding_pen_release
>> #endif /* defined(CONFIG_SMP) */
>>
>>
>> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
>> index b7b4c86..e18151a 100644
>> --- a/arch/arm/kernel/smp.c
>> +++ b/arch/arm/kernel/smp.c
>> @@ -59,6 +59,8 @@ struct secondary_data secondary_data;
>> * boot "holding pen"
>> */
>> volatile int pen_release = -1;
>> +volatile u32 secondary_holding_pen_release = -1;
>> +static DEFINE_RAW_SPINLOCK(boot_lock);
>>
>> enum ipi_msg_type {
>> IPI_WAKEUP,
>> @@ -386,6 +388,81 @@ asmlinkage void secondary_start_kernel(void)
>> cpu_startup_entry(CPUHP_ONLINE);
>> }
>>
>> +static void write_pen_release(int val)
>> +{
>> + secondary_holding_pen_release = val;
>> + smp_wmb();
>> + sync_cache_w(&secondary_holding_pen_release);
>> +}
>> +
>> +/*
>> + * This is a smp_operations->smp_boot_secondary function, called by
>> + * boot_secondary() to signal a secondary core spinning in
>> + * secondary_holding_pen() that it should proceed. The current
>> + * (boot) core writes the secondary's (hardware) CPU ID into
>> + * secondary_holding_pen_release. The secondary core signals it has
>> + * started running by rewriting an invalid value (-1) back
>> + * into secondary_holding_pen_release.
>> + */
>> +int smp_boot_secondary(unsigned int cpu, struct task_struct *idle)
>> +{
>> + unsigned long timeout;
>> +
>> + /*
>> + * The secondary core will wait for this lock after
>> + * signaling it has started. That way we know it won't
>> + * proceed until we've recognized the acknowledgement.
>> + */
>> + raw_spin_lock(&boot_lock);
>> +
>> + /*
>> + * Release the secondary core from its holding pen by
>> + * writing its CPU ID into secondary_holding_pen_release.
>> + */
>> + write_pen_release(cpu_logical_map(cpu));
>> +
>> + /*
>> + * Send the secondary CPU a soft interrupt, thereby causing
>> + * it to jump to its secondary entry point.
>> + */
>> + arch_send_wakeup_ipi_mask(cpumask_of(cpu));
>> +
>> + /* Give it some time to start running. */
>> + timeout = jiffies + (1 * HZ);
>> + while (time_before(jiffies, timeout)) {
>> + smp_rmb();
>> + if (secondary_holding_pen_release == -1)
>> + break;
>> +
>> + udelay(10);
>> + }
>> +
>> + /*
>> + * We now know that the secondary core is running (or it
>> + * timed out). Release the lock so it can proceed.
>> + */
>> + raw_spin_unlock(&boot_lock);
>> +
>> + return secondary_holding_pen_release == -1 ? 0 : -ENOSYS;
>> +}
>> +
>> +/*
>> + * This is a smp_operations->secondary_init function, called by
>> + * secondary_start_kernel() on a newly-booted secondary cpu to do
>> + * some initialization activity. All we need to do is write
>> + * secondary_holding_pen_release with an invalid value to signal
>> + * we've started executing. We synchronize with the boot core by
>> + * waiting to acquire the boot lock before continuing.
>> + */
>> +void smp_secondary_init(unsigned int cpu)
>> +{
>> + /* Let the primary processor know we're out of the pen. */
>> + write_pen_release(-1);
>> +
>> + raw_spin_lock(&boot_lock);
>> + raw_spin_unlock(&boot_lock);
>> +}
>> +
>> void __init smp_cpus_done(unsigned int max_cpus)
>> {
>> printk(KERN_INFO "SMP: Total of %d processors activated.\n",
>> --
>> 1.7.9.5
>>
>> --
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