ia64/linux-2.6.18-xen.hg

view Documentation/io_ordering.txt @ 854:950b9eb27661

usbback: fix urb interval value for interrupt urbs.

Signed-off-by: Noboru Iwamatsu <n_iwamatsu@jp.fujitsu.com>
author Keir Fraser <keir.fraser@citrix.com>
date Mon Apr 06 13:51:20 2009 +0100 (2009-04-06)
parents 831230e53067
children
line source
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).
11 A more concrete example from a hypothetical device driver:
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 ...
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:
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)
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.