docs/memory-barriers.txt: Fix style, spacing and grammar in I/O section

Commit 4614bbde ("docs/memory-barriers.txt: Rewrite "KERNEL I/O
BARRIER EFFECTS" section") rewrote the I/O ordering section of
memory-barriers.txt.

Subsequently, Ingo noticed a number of issues with the style, spacing
and grammar of the rewritten section. Fix them based on his suggestions.

Link: https://lkml.kernel.org/r/20190410105833.GA116161@gmail.com
Fixes: 4614bbde ("docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER EFFECTS" section")
Reported-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

Documentation/memory-barriers.txt

@@ -2517,45 +2517,52 @@ guarantees:
 
  (*) readX(), writeX():
 
-     The readX() and writeX() MMIO accessors take a pointer to the peripheral
-     being accessed as an __iomem * parameter. For pointers mapped with the
-     default I/O attributes (e.g. those returned by ioremap()), then the
-     ordering guarantees are as follows:
+	The readX() and writeX() MMIO accessors take a pointer to the
+	peripheral being accessed as an __iomem * parameter. For pointers
+	mapped with the default I/O attributes (e.g. those returned by
+	ioremap()), the ordering guarantees are as follows:
 
 	1. All readX() and writeX() accesses to the same peripheral are ordered
-        with respect to each other. For example, 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 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. For example, 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.
+	   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. For example, 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.
+	   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. For example, 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().
+	   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():
+
+		writel(42, DEVICE_REGISTER_0); // Arrives at the device...
+		readl(DEVICE_REGISTER_0);
+		udelay(1);
+		writel(42, DEVICE_REGISTER_1); // ...at least 1us before this.
 
-     __iomem pointers obtained with non-default attributes (e.g. those returned
-     by ioremap_wc()) are unlikely to provide many of these guarantees.
+	The ordering properties of __iomem pointers obtained with non-default
+	attributes (e.g. those returned by ioremap_wc()) are specific to the
+	underlying architecture and therefore the guarantees listed above cannot
+	generally be relied upon for accesses to these types of mappings.
 
  (*) readX_relaxed(), writeX_relaxed():
 
 	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 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.
 
  (*) readsX(), writesX():
 
@@ -2572,9 +2579,10 @@ guarantees:
 	accessed is passed as an argument.
 
 	Since many CPU architectures ultimately access these peripherals via an
-     internal virtual memory mapping, the portable ordering guarantees provided
-     by inX() and outX() are the same as those provided by readX() and writeX()
-     respectively when accessing a mapping with the default I/O attributes.
+	internal virtual memory mapping, the portable ordering guarantees
+	provided by inX() and outX() are the same as those provided by readX()
+	and writeX() respectively when accessing a mapping with the default I/O
+	attributes.
 
 	Device drivers may expect outX() to emit a non-posted write transaction
 	that waits for a completion response from the I/O peripheral before
@@ -2584,10 +2592,10 @@ guarantees:
  (*) insX(), outsX():
 
 	As above, the insX() and outsX() accessors provide the same ordering
-     guarantees as readsX() and writesX() respectively when accessing a mapping
-     with the default I/O attributes.
+	guarantees as readsX() and writesX() respectively when accessing a
+	mapping with the default I/O attributes.
 
- (*) ioreadX(), iowriteX()
+ (*) ioreadX(), iowriteX():
 
 	These will perform appropriately for the type of access they're actually
 	doing, be it inX()/outX() or readX()/writeX().