Linux-2.6.12-rc2v2.6.12-rc2

Initial git repository build. I'm not bothering with the full history,
even though we have it. We can create a separate "historical" git
archive of that later if we want to, and in the meantime it's about
3.2GB when imported into git - space that would just make the early
git days unnecessarily complicated, when we don't have a lot of good
infrastructure for it.

Let it rip!

1 files changed, 47 insertions, 0 deletions

diff --git a/Documentation/io_ordering.txt b/Documentation/io_ordering.txt
new file mode 100644
index 000000000000..9faae6f26d32
--- /dev/null
+++ b/Documentation/io_ordering.txt
@@ -0,0 +1,47 @@
+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.