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Diffstat (limited to 'Documentation/io_ordering.txt')
| -rw-r--r-- | Documentation/io_ordering.txt | 51 |
1 files changed, 51 insertions, 0 deletions
diff --git a/Documentation/io_ordering.txt b/Documentation/io_ordering.txt new file mode 100644 index 000000000..2ab303ce9 --- /dev/null +++ b/Documentation/io_ordering.txt @@ -0,0 +1,51 @@ +============================================== +Ordering I/O writes to memory-mapped addresses +============================================== + +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. |