dirty fix to merging

1 files changed, 47 insertions, 0 deletions

diff --git a/Documentation/io_ordering.txt b/Documentation/io_ordering.txt
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+++ b/Documentation/io_ordering.txt
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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.