1da177e4c3
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
48 lines
1.9 KiB
Plaintext
48 lines
1.9 KiB
Plaintext
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.
|