diff options
| -rw-r--r-- | Documentation/memory-barriers.txt | 67 |
1 files changed, 40 insertions, 27 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 1660dde75e14..f70ebcdfe592 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -2523,27 +2523,37 @@ guarantees: ioremap()), the ordering guarantees are as follows: 1. All readX() and writeX() accesses to the same peripheral are ordered - with respect to each other. This ensures that MMIO register writes by - the CPU to a particular device will arrive in program order. - - 2. A writeX() by the CPU to the peripheral will first wait for the - completion of all prior CPU writes to memory. This ensures that - writes by the CPU to an outbound DMA buffer allocated by - dma_alloc_coherent() will be visible to a DMA engine when the CPU - writes to its MMIO control register to trigger the transfer. - - 3. A readX() by the CPU from the peripheral will complete before any - subsequent CPU reads from memory can begin. This ensures that reads - by the CPU from an incoming DMA buffer allocated by - dma_alloc_coherent() will not see stale data after reading from the - DMA engine's MMIO status register to establish that the DMA transfer - has completed. - - 4. A readX() by the CPU from the peripheral will complete before any - subsequent delay() loop can begin execution. This ensures that two - MMIO register writes by the CPU to a peripheral will arrive at least - 1us apart if the first write is immediately read back with readX() - and udelay(1) is called prior to the second writeX(): + with respect to each other. This ensures that MMIO register accesses + by the same CPU thread to a particular device will arrive in program + order. + + 2. A writeX() issued by a CPU thread holding a spinlock is ordered + before a writeX() to the same peripheral from another CPU thread + issued after a later acquisition of the same spinlock. This ensures + that MMIO register writes to a particular device issued while holding + a spinlock will arrive in an order consistent with acquisitions of + the lock. + + 3. A writeX() by a CPU thread to the peripheral will first wait for the + completion of all prior writes to memory either issued by, or + propagated to, the same thread. This ensures that writes by the CPU + to an outbound DMA buffer allocated by dma_alloc_coherent() will be + visible to a DMA engine when the CPU writes to its MMIO control + register to trigger the transfer. + + 4. A readX() by a CPU thread from the peripheral will complete before + any subsequent reads from memory by the same thread can begin. This + ensures that reads by the CPU from an incoming DMA buffer allocated + by dma_alloc_coherent() will not see stale data after reading from + the DMA engine's MMIO status register to establish that the DMA + transfer has completed. + + 5. A readX() by a CPU thread from the peripheral will complete before + any subsequent delay() loop can begin execution on the same thread. + This ensures that two MMIO register writes by the CPU to a peripheral + will arrive at least 1us apart if the first write is immediately read + back with readX() and udelay(1) is called prior to the second + writeX(): writel(42, DEVICE_REGISTER_0); // Arrives at the device... readl(DEVICE_REGISTER_0); @@ -2559,10 +2569,11 @@ guarantees: These are similar to readX() and writeX(), but provide weaker memory ordering guarantees. Specifically, they do not guarantee ordering with - respect to normal memory accesses or delay() loops (i.e. bullets 2-4 - above) but they are still guaranteed to be ordered with respect to other - accesses to the same peripheral when operating on __iomem pointers - mapped with the default I/O attributes. + respect to locking, normal memory accesses or delay() loops (i.e. + bullets 2-5 above) but they are still guaranteed to be ordered with + respect to other accesses from the same CPU thread to the same + peripheral when operating on __iomem pointers mapped with the default + I/O attributes. (*) readsX(), writesX(): @@ -2600,8 +2611,10 @@ guarantees: These will perform appropriately for the type of access they're actually doing, be it inX()/outX() or readX()/writeX(). -All of these accessors assume that the underlying peripheral is little-endian, -and will therefore perform byte-swapping operations on big-endian architectures. +With the exception of the string accessors (insX(), outsX(), readsX() and +writesX()), all of the above assume that the underlying peripheral is +little-endian and will therefore perform byte-swapping operations on big-endian +architectures. ======================================== |