https://github.com/torvalds/linux
- HEAD
- refs/heads/arm64-uaccess
- refs/heads/dependabot/pip/drivers/gpu/drm/ci/xfails/certifi-2024.7.4
- refs/heads/dependabot/pip/drivers/gpu/drm/ci/xfails/idna-3.7
- refs/heads/dependabot/pip/drivers/gpu/drm/ci/xfails/requests-2.32.2
- refs/heads/dependabot/pip/drivers/gpu/drm/ci/xfails/setuptools-70.0.0
- refs/heads/dependabot/pip/drivers/gpu/drm/ci/xfails/urllib3-2.2.2
- refs/heads/master
- 413541dd66d51f791a0b169d9b9014e4f56be13c
- v6.9-rc7
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- v2.6.20-rc6
- v2.6.20-rc5
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Linux 3.13-rc5
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.
Computing file changes ...