Driver-specific spinlock for cxd56_serial.c

[masayuki2009]

Summary

• This PR consists of the following 2 commits
• commit 1: include: nuttx: Introduce spinlock_t for #ifndef CONFIG_SPINLOCK
  • This commit introduces spinlock_t for #ifndef CONFIG_SPINLOCK
    which is useful for the non-SMP case because it does not consume
    memory
• commit 2: arch: cxd56xx: Introduce driver specific spinlock in cxd56_serial.c
  • This commit introduces driver-specific spinlock in cxd56_serial.c
    to improve performance
• NOTE: Implement the fine-grained spin lock #2213

Impact

• SMP only

Testing

• Tested with spresense:wifi and spresense:wifi_smp

[gustavonihei]

I'm trying to understand the outcome of this change when CONFIG_SPINLOCK is not defined.
From what I understood, &priv->lock should always evaluate to NULL when CONFIG_SPINLOCK is not defined.
I've tried to isolate this behavior in an example, but I couldn't achieve the same result. Instead, the pointer to the empty struct is never NULL.

https://godbolt.org/z/467afd

Did I do something wrong in my test?

[masayuki2009]

Hi @gustavonihei,

Actually, when CONFIG_SPINLOCK is not defined CONFIG_SMP is not defined either.
So, finally, spin_lock_irqsave(l) will be just up_irq_save().

include/nuttx/irq.h

...
#if defined(CONFIG_SMP)                                                                                                                                                
irqstate_t spin_lock_irqsave(spinlock_t *lock);                                                                                                                        
#else                                                                                                                                                                  
#  define spin_lock_irqsave(l) up_irq_save()                                                                                                                           
#endif  
...

[gustavonihei]

Thanks for the clarification, @masayuki2009!