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-
-#### cli()/sti() removal guide, started by Ingo Molnar <mingo@redhat.com>
-
-
-as of 2.5.28, five popular macros have been removed on SMP, and
-are being phased out on UP:
-
- cli(), sti(), save_flags(flags), save_flags_cli(flags), restore_flags(flags)
-
-until now it was possible to protect driver code against interrupt
-handlers via a cli(), but from now on other, more lightweight methods
-have to be used for synchronization, such as spinlocks or semaphores.
-
-for example, driver code that used to do something like:
-
-        struct driver_data;
-
-        irq_handler (...)
-        {
-                ....
-                driver_data.finish = 1;
-                driver_data.new_work = 0;
-                ....
-        }
-
-        ...
-
-        ioctl_func (...)
-        {
-                ...
-                cli();
-                ...
-                driver_data.finish = 0;
-                driver_data.new_work = 2;
-                ...
-                sti();
-                ...
-        }
-
-was SMP-correct because the cli() function ensured that no
-interrupt handler (amongst them the above irq_handler()) function
-would execute while the cli()-ed section is executing.
-
-but from now on a more direct method of locking has to be used:
-
-        DEFINE_SPINLOCK(driver_lock);
-        struct driver_data;
-
-        irq_handler (...)
-        {
-                unsigned long flags;
-                ....
-                spin_lock_irqsave(&driver_lock, flags);
-                ....
-                driver_data.finish = 1;
-                driver_data.new_work = 0;
-                ....
-                spin_unlock_irqrestore(&driver_lock, flags);
-                ....
-        }
-
-        ...
-
-        ioctl_func (...)
-        {
-                ...
-                spin_lock_irq(&driver_lock);
-                ...
-                driver_data.finish = 0;
-                driver_data.new_work = 2;
-                ...
-                spin_unlock_irq(&driver_lock);
-                ...
-        }
-
-the above code has a number of advantages:
-
-- the locking relation is easier to understand - actual lock usage
-  pinpoints the critical sections. cli() usage is too opaque.
-  Easier to understand means it's easier to debug.
-
-- it's faster, because spinlocks are faster to acquire than the
-  potentially heavily-used IRQ lock. Furthermore, your driver does
-  not have to wait eg. for a big heavy SCSI interrupt to finish,
-  because the driver_lock spinlock is only used by your driver.
-  cli() on the other hand was used by many drivers, and extended
-  the critical section to the whole IRQ handler function - creating
-  serious lock contention.
-
- 
-to make the transition easier, we've still kept the cli(), sti(),
-save_flags(), save_flags_cli() and restore_flags() macros defined
-on UP systems - but their usage will be phased out until 2.6 is
-released.
-
-drivers that want to disable local interrupts (interrupts on the
-current CPU), can use the following five macros:
-
-  local_irq_disable(), local_irq_enable(), local_save_flags(flags),
-  local_irq_save(flags), local_irq_restore(flags)
-
-but beware, their meaning and semantics are much simpler, far from
-that of the old cli(), sti(), save_flags(flags) and restore_flags(flags)
-SMP meaning:
-
-    local_irq_disable()       => turn local IRQs off
-
-    local_irq_enable()        => turn local IRQs on
-
-    local_save_flags(flags)   => save the current IRQ state into flags. The
-                                 state can be on or off. (on some
-                                 architectures there's even more bits in it.)
-
-    local_irq_save(flags)     => save the current IRQ state into flags and
-                                 disable interrupts.
-
-    local_irq_restore(flags)  => restore the IRQ state from flags.
-
-(local_irq_save can save both irqs on and irqs off state, and
-local_irq_restore can restore into both irqs on and irqs off state.)
-
-another related change is that synchronize_irq() now takes a parameter:
-synchronize_irq(irq). This change too has the purpose of making SMP
-synchronization more lightweight - this way you can wait for your own
-interrupt handler to finish, no need to wait for other IRQ sources.
-
-
-why were these changes done? The main reason was the architectural burden
-of maintaining the cli()/sti() interface - it became a real problem. The
-new interrupt system is much more streamlined, easier to understand, debug,
-and it's also a bit faster - the same happened to it that will happen to
-cli()/sti() using drivers once they convert to spinlocks :-)
-