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/Documentation/io_ordering.txt

https://bitbucket.org/evzijst/gittest
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 1On some platforms, so-called memory-mapped I/O is weakly ordered.  On such
 2platforms, driver writers are responsible for ensuring that I/O writes to
 3memory-mapped addresses on their device arrive in the order intended.  This is
 4typically done by reading a 'safe' device or bridge register, causing the I/O
 5chipset to flush pending writes to the device before any reads are posted.  A
 6driver would usually use this technique immediately prior to the exit of a
 7critical section of code protected by spinlocks.  This would ensure that
 8subsequent writes to I/O space arrived only after all prior writes (much like a
 9memory barrier op, mb(), only with respect to I/O).
10
11A more concrete example from a hypothetical device driver:
12
13        ...
14CPU A:  spin_lock_irqsave(&dev_lock, flags)
15CPU A:  val = readl(my_status);
16CPU A:  ...
17CPU A:  writel(newval, ring_ptr);
18CPU A:  spin_unlock_irqrestore(&dev_lock, flags)
19        ...
20CPU B:  spin_lock_irqsave(&dev_lock, flags)
21CPU B:  val = readl(my_status);
22CPU B:  ...
23CPU B:  writel(newval2, ring_ptr);
24CPU B:  spin_unlock_irqrestore(&dev_lock, flags)
25        ...
26
27In the case above, the device may receive newval2 before it receives newval,
28which could cause problems.  Fixing it is easy enough though:
29
30        ...
31CPU A:  spin_lock_irqsave(&dev_lock, flags)
32CPU A:  val = readl(my_status);
33CPU A:  ...
34CPU A:  writel(newval, ring_ptr);
35CPU A:  (void)readl(safe_register); /* maybe a config register? */
36CPU A:  spin_unlock_irqrestore(&dev_lock, flags)
37        ...
38CPU B:  spin_lock_irqsave(&dev_lock, flags)
39CPU B:  val = readl(my_status);
40CPU B:  ...
41CPU B:  writel(newval2, ring_ptr);
42CPU B:  (void)readl(safe_register); /* maybe a config register? */
43CPU B:  spin_unlock_irqrestore(&dev_lock, flags)
44
45Here, the reads from safe_register will cause the I/O chipset to flush any
46pending writes before actually posting the read to the chipset, preventing
47possible data corruption.