Subject: [PATCHv3 2/5] arm64: factor out spin-table boot method

From: [email-masked] (Santosh Shilimkar)
Date: Wed, 14 Aug 2013 15:17:38 -0400

On Wednesday 14 August 2013 02:21 PM, Nicolas Pitre wrote:
> On Wed, 14 Aug 2013, Santosh Shilimkar wrote:
> 
>> On Wednesday 14 August 2013 12:20 PM, Mark Rutland wrote:
>>> The arm64 kernel has an internal holding pen, which is necessary for
>>> some systems where we can't bring CPUs online individually and must hold
>>> multiple CPUs in a safe area until the kernel is able to handle them.
>>> The current SMP infrastructure for arm64 is closely coupled to this
>>> holding pen, and alternative boot methods must launch CPUs into the pen,
>>> where they sit before they are launched into the kernel proper.
>>>
>>> With PSCI (and possibly other future boot methods), we can bring CPUs
>>> online individually, and need not perform the secondary_holding_pen
>>> dance. Instead, this patch factors the holding pen management code out
>>> to the spin-table boot method code, as it is the only boot method
>>> requiring the pen.
>>>
>>> A new entry point for secondaries, secondary_entry is added for other
>>> boot methods to use, which bypasses the holding pen and its associated
>>> overhead when bringing CPUs online. The smp.pen.text section is also
>>> removed, as the pen can live in head.text without problem.
>>>
>>> The smp_operations structure is extended with two new functions,
>>> cpu_boot and cpu_postboot, for bringing a cpu into the kernel and
>>> performing any post-boot cleanup required by a bootmethod (e.g.
>>> resetting the secondary_holding_pen_release to INVALID_HWID).
>>> Documentation is added for smp_operations.
>>>
>>> Signed-off-by: Mark Rutland <[email-masked]>
>>> Cc: Santosh Shilimkar <[email-masked]>
>>> ---
>>
>> [..]
>>> diff --git a/arch/arm64/kernel/smp_spin_table.c b/arch/arm64/kernel/smp_spin_table.c
>>> index 5fecffc..87af6bb 100644
>>> --- a/arch/arm64/kernel/smp_spin_table.c
>>> +++ b/arch/arm64/kernel/smp_spin_table.c

>>> @@ -59,8 +82,60 @@ static int smp_spin_table_cpu_prepare(unsigned int cpu)
>>>  	return 0;
>>>  }
>>>  
>>> +static int smp_spin_table_cpu_boot(unsigned int cpu)
>>> +{
>>> +	unsigned long timeout;
>>> +
>>> +	/*
>>> +	 * Set synchronisation state between this boot processor
>>> +	 * and the secondary one
>>> +	 */
>>> +	raw_spin_lock(&boot_lock);
>>> +
>>> +	/*
>>> +	 * Update the pen release flag.
>>> +	 */
>>> +	write_pen_release(cpu_logical_map(cpu));
>>> +
>>> +	/*
>>> +	 * Send an event, causing the secondaries to read pen_release.
>>> +	 */
>>> +	sev();
>>> +
>>> +	timeout = jiffies + (1 * HZ);
>>> +	while (time_before(jiffies, timeout)) {
>>> +		if (secondary_holding_pen_release == INVALID_HWID)
>>> +			break;
>>> +		udelay(10);
>>> +	}
>>> +
>>> +	/*
>>> +	 * Now the secondary core is starting up let it run its
>>> +	 * calibrations, then wait for it to finish
>>> +	 */
>>> +	raw_spin_unlock(&boot_lock);
>>> +
>>> +	return secondary_holding_pen_release != INVALID_HWID ? -ENOSYS : 0;
>>> +}
>>> +
>>> +void smp_spin_table_cpu_postboot(void)
>>> +{
>>> +	/*
>>> +	 * Let the primary processor know we're out of the pen.
>>> +	 */
>>> +	write_pen_release(INVALID_HWID);
>>> +
>>> +	/*
>>> +	 * Synchronise with the boot thread.
>>> +	 */
>>> +	raw_spin_lock(&boot_lock);
>>> +	raw_spin_unlock(&boot_lock);
>>> +}
>>> +
>> I was just wonderring whether we can absrtact the synchronization
>> further out of spin_table and psci method. At least the lock
>> synchronization is common and needed in both cases.
> 
> Why?  This synchronization looks rather useless to me.
> 
I thought it was needed one always. If it is useless then
no concerns.

Regards,
Santosh