docs/memory-barriers.txt: Update I/O section to be clearer about CPU vs thread

The revised I/O ordering section of memory-barriers.txt introduced in
4614bbdee3 ("docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER
EFFECTS" section") loosely refers to "the CPU", whereas the ordering
guarantees generally apply within a thread of execution that can migrate
between cores, with the scheduler providing the relevant barrier
semantics.

Reword the section to refer to "CPU thread" and call out ordering of
MMIO writes separately from ordering of writes to memory. Ben also
spotted that the string accessors are native-endian, so fix that up too.

Link: https://lkml.kernel.org/r/080d1ec73e3e29d6ffeeeb50b39b613da28afb37.camel@kernel.crashing.org
Fixes: 4614bbdee3 ("docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER EFFECTS" section")
Reported-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

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.
+	   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() 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.
+	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 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.
+	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 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():
+	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.
 
 
 ========================================