diff options
Diffstat (limited to 'Documentation/driver-api')
| -rw-r--r-- | Documentation/driver-api/device-io.rst | 45 | ||||
| -rw-r--r-- | Documentation/driver-api/pci/p2pdma.rst | 4 |
2 files changed, 0 insertions, 49 deletions
diff --git a/Documentation/driver-api/device-io.rst b/Documentation/driver-api/device-io.rst index b00b23903078..0e389378f71d 100644 --- a/Documentation/driver-api/device-io.rst +++ b/Documentation/driver-api/device-io.rst @@ -103,51 +103,6 @@ continuing execution:: ha->flags.ints_enabled = 0; } -In addition to write posting, on some large multiprocessing systems -(e.g. SGI Challenge, Origin and Altix machines) posted writes won't be -strongly ordered coming from different CPUs. Thus it's important to -properly protect parts of your driver that do memory-mapped writes with -locks and use the :c:func:`mmiowb()` to make sure they arrive in the -order intended. Issuing a regular readX() will also ensure write ordering, -but should only be used when the -driver has to be sure that the write has actually arrived at the device -(not that it's simply ordered with respect to other writes), since a -full readX() is a relatively expensive operation. - -Generally, one should use :c:func:`mmiowb()` prior to releasing a spinlock -that protects regions using :c:func:`writeb()` or similar functions that -aren't surrounded by readb() calls, which will ensure ordering -and flushing. The following pseudocode illustrates what might occur if -write ordering isn't guaranteed via :c:func:`mmiowb()` or one of the -readX() functions:: - - CPU A: spin_lock_irqsave(&dev_lock, flags) - 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: writel(newval2, ring_ptr); - CPU B: ... - CPU B: spin_unlock_irqrestore(&dev_lock, flags) - -In the case above, newval2 could be written to ring_ptr before newval. -Fixing it is easy though:: - - CPU A: spin_lock_irqsave(&dev_lock, flags) - CPU A: ... - CPU A: writel(newval, ring_ptr); - CPU A: mmiowb(); /* ensure no other writes beat us to the device */ - CPU A: spin_unlock_irqrestore(&dev_lock, flags) - ... - CPU B: spin_lock_irqsave(&dev_lock, flags) - CPU B: writel(newval2, ring_ptr); - CPU B: ... - CPU B: mmiowb(); - CPU B: spin_unlock_irqrestore(&dev_lock, flags) - -See tg3.c for a real world example of how to use :c:func:`mmiowb()` - PCI ordering rules also guarantee that PIO read responses arrive after any outstanding DMA writes from that bus, since for some devices the result of a readb() call may signal to the driver that a DMA transaction is diff --git a/Documentation/driver-api/pci/p2pdma.rst b/Documentation/driver-api/pci/p2pdma.rst index 6d85b5a2598d..44deb52beeb4 100644 --- a/Documentation/driver-api/pci/p2pdma.rst +++ b/Documentation/driver-api/pci/p2pdma.rst @@ -132,10 +132,6 @@ precludes passing these pages to userspace. P2P memory is also technically IO memory but should never have any side effects behind it. Thus, the order of loads and stores should not be important and ioreadX(), iowriteX() and friends should not be necessary. -However, as the memory is not cache coherent, if access ever needs to -be protected by a spinlock then :c:func:`mmiowb()` must be used before -unlocking the lock. (See ACQUIRES VS I/O ACCESSES in -Documentation/memory-barriers.txt) P2P DMA Support Library |