Documentation/io_ordering.txt

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Ordering I/O writes to memory-mapped addresses
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On some platforms, so-called memory-mapped I/O is weakly ordered.  On such
platforms, driver writers are responsible for ensuring that I/O writes to
memory-mapped addresses on their device arrive in the order intended.  This is
typically done by reading a 'safe' device or bridge register, causing the I/O
chipset to flush pending writes to the device before any reads are posted.  A
driver would usually use this technique immediately prior to the exit of a
critical section of code protected by spinlocks.  This would ensure that
subsequent writes to I/O space arrived only after all prior writes (much like a
memory barrier op, mb(), only with respect to I/O).

A more concrete example from a hypothetical device driver::

		...
	CPU A:  spin_lock_irqsave(&dev_lock, flags)
	CPU A:  val = readl(my_status);
	CPU A:  ...
	CPU A:  writel(newval, ring_ptr);
	CPU A:  spin_unlock_irqrestore(&dev_lock, flags)
		...
	CPU B:  spin_lock_irqsave(&dev_lock, flags)
	CPU B:  val = readl(my_status);
	CPU B:  ...
	CPU B:  writel(newval2, ring_ptr);
	CPU B:  spin_unlock_irqrestore(&dev_lock, flags)
		...

In the case above, the device may receive newval2 before it receives newval,
which could cause problems.  Fixing it is easy enough though::

		...
	CPU A:  spin_lock_irqsave(&dev_lock, flags)
	CPU A:  val = readl(my_status);
	CPU A:  ...
	CPU A:  writel(newval, ring_ptr);
	CPU A:  (void)readl(safe_register); /* maybe a config register? */
	CPU A:  spin_unlock_irqrestore(&dev_lock, flags)
		...
	CPU B:  spin_lock_irqsave(&dev_lock, flags)
	CPU B:  val = readl(my_status);
	CPU B:  ...
	CPU B:  writel(newval2, ring_ptr);
	CPU B:  (void)readl(safe_register); /* maybe a config register? */
	CPU B:  spin_unlock_irqrestore(&dev_lock, flags)

Here, the reads from safe_register will cause the I/O chipset to flush any
pending writes before actually posting the read to the chipset, preventing
possible data corruption.