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Documentation / io_ordering.txt




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Based on kernel version 3.9. Page generated on 2013-05-02 23:09 EST.

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