Contents of /alx-src/tags/kernel26-2.6.12-alx-r9/Documentation/io_ordering.txt
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Wed Mar 4 11:03:09 2009 UTC (15 years, 6 months ago) by niro
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Wed Mar 4 11:03:09 2009 UTC (15 years, 6 months ago) by niro
File MIME type: text/plain
File size: 1923 byte(s)
Tag kernel26-2.6.12-alx-r9
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. |