Re: [PATCH 2/4] ARC: smp-boot: run-on-reset: add callback to allow non masters to wait

2017-01-17 Thread Vineet Gupta 
On 01/17/2017 12:58 PM, Alexey Brodkin wrote:
>>
>> +static void arc_default_smp_wait_to_boot(int cpu) {
>> +while (wake_flag != cpu)
>> +;
>> +
>> +wake_flag = 0;
> 
> Why don't we convert "wake_flag" into bit-field so each core uses its special 
> bit.
> It is IMHO beneficial for 2 reasons:
>  1. If we ever decide to have master core with ARCID != 0 implementation of 
> that procedure won't change,
> because "wake_flag" for core with ARCID=0 will be 1 but not 0, see for 
> example http://patchwork.ozlabs.org/patch/703645/

That's not a real use case - it is just a debug exercise ...

> 
>  2. There's no need in resetting "wake_flag" to 0 at all as well because each 
> core has its own bit and they not affect anybody else.
> And in that case ...


True, but you need to do a read-modify-write. More importantly, the cores are
setup one at a time by the master - so there just was no need to do this to 
begin
with - one at a time was just sufficient. If you really want to do this in right
way - it will not a bit filed either, it needs to be a strictly per cpu 
variable.

>> +}
>> +
>>  void arc_platform_smp_wait_to_boot(int cpu)  {
>>  /* for halt-on-reset, we've waited already */
>>  if (IS_ENABLED(CONFIG_ARC_SMP_HALT_ON_RESET))
>>  return;
> 
> ...we may just remove above part. Master core by that time has already set 
> our bit in "wake_flag" so we
> will effectively fall through the following "while".

No. They way this works is, same routine arc_platform_smp_wait_to_boot() is 
called
from early boot code for both halt-on-reset and run-on-reset. For latter we need
to actually wait. For former, they were already halted and when they land here,
they've waited enough so we need to return !

This is same as what was before, I've just moved the #ifdef from head.s (where 
it
looked ugly) to here.

-Vineet

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RE: [PATCH 2/4] ARC: smp-boot: run-on-reset: add callback to allow non masters to wait

2017-01-17 Thread Alexey Brodkin 
Hi Vineet,

> -Original Message-
> From: Vineet Gupta
> Sent: Monday, January 16, 2017 11:58 PM
> To: linux-snps-arc@lists.infradead.org; Alexey Brodkin
> 
> Cc: linux-ker...@vger.kernel.org; Vineet Gupta 
> Subject: [PATCH 2/4] ARC: smp-boot: run-on-reset: add callback to allow non
> masters to wait
> 
> Currently for halt-on-reset, non masters spin on a shared memory which is
> wiggled by Master at right time. This is not efficient when hardware support
> exists to stop and resume them from Master (using an external IP block).
> More importantly Master might be setting up SLC or IO-Coherency aperutes
> in early boot duting which other cores need NOT perturb the coherency unit,
> thus need a mechanism that doesn't rely on polling memory.
> 
> This just adds infrastructure for next patch which will add the hardware
> support (MCIP Inter Core Debug Unit).
> 
> Signed-off-by: Vineet Gupta 
> ---
>  arch/arc/include/asm/smp.h |  3 +++
>  arch/arc/kernel/smp.c  | 17 +
>  2 files changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arc/include/asm/smp.h b/arch/arc/include/asm/smp.h index
> 0861007d9ef3..6187b9d14b03 100644
> --- a/arch/arc/include/asm/smp.h
> +++ b/arch/arc/include/asm/smp.h
> @@ -50,6 +50,8 @@ extern int smp_ipi_irq_setup(int cpu,
> irq_hw_number_t hwirq);
>   *   mach_desc->init_early()
>   * @init_per_cpu:Called for each core so SMP h/w block driver can do
>   *   any needed setup per cpu (e.g. IPI request)
> + * @cpu_wait:Non masters wait to be resumed later by
> master (to avoid
> + *   perturbing SLC / cache coherency in early boot)
>   * @cpu_kick:For Master to kickstart a cpu (optionally at a 
> PC)
>   * @ipi_send:To send IPI to a @cpu
>   * @ips_clear:   To clear IPI received at @irq
> @@ -58,6 +60,7 @@ struct plat_smp_ops {
>   const char  *info;
>   void(*init_early_smp)(void);
>   void(*init_per_cpu)(int cpu);
> + void(*cpu_wait)(int cpu);
>   void(*cpu_kick)(int cpu, unsigned long pc);
>   void(*ipi_send)(int cpu);
>   void(*ipi_clear)(int irq);
> diff --git a/arch/arc/kernel/smp.c b/arch/arc/kernel/smp.c index
> 44a0d21ed342..e60c11b8f4b9 100644
> --- a/arch/arc/kernel/smp.c
> +++ b/arch/arc/kernel/smp.c
> @@ -96,16 +96,25 @@ static void arc_default_smp_cpu_kick(int cpu,
> unsigned long pc)
>   wake_flag = cpu;
>  }
> 
> +static void arc_default_smp_wait_to_boot(int cpu) {
> + while (wake_flag != cpu)
> + ;
> +
> + wake_flag = 0;

Why don't we convert "wake_flag" into bit-field so each core uses its special 
bit.
It is IMHO beneficial for 2 reasons:
 1. If we ever decide to have master core with ARCID != 0 implementation of 
that procedure won't change,
because "wake_flag" for core with ARCID=0 will be 1 but not 0, see for 
example http://patchwork.ozlabs.org/patch/703645/

 2. There's no need in resetting "wake_flag" to 0 at all as well because each 
core has its own bit and they not affect anybody else.
And in that case ...

> +}
> +
>  void arc_platform_smp_wait_to_boot(int cpu)  {
>   /* for halt-on-reset, we've waited already */
>   if (IS_ENABLED(CONFIG_ARC_SMP_HALT_ON_RESET))
>   return;

...we may just remove above part. Master core by that time has already set our 
bit in "wake_flag" so we
will effectively fall through the following "while".

> - while (wake_flag != cpu)
> - ;

-Alexey


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