docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER EFFECTS" section

The "KERNEL I/O BARRIER EFFECTS" section of memory-barriers.txt is vague,
x86-centric, out-of-date, incomplete and demonstrably incorrect in places.
This is largely because I/O ordering is a horrible can of worms, but also
because the document has stagnated as our understanding has evolved.

Attempt to address some of that, by rewriting the section based on
recent(-ish) discussions with Arnd, BenH and others. Maybe one day we'll
find a way to formalise this stuff, but for now let's at least try to
make the English easier to understand.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Daniel Lustig <dlustig@nvidia.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: "Maciej W. Rozycki" <macro@linux-mips.org>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Paul E. McKenney <paulmck@linux.ibm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

Documentation/memory-barriers.txt

@@ -2599,72 +2599,97 @@ likely, then interrupt-disabling locks should be used to guarantee ordering.
 KERNEL I/O BARRIER EFFECTS
 ==========================
 
-When accessing I/O memory, drivers should use the appropriate accessor
-functions:
+Interfacing with peripherals via I/O accesses is deeply architecture and device
+specific. Therefore, drivers which are inherently non-portable may rely on
+specific behaviours of their target systems in order to achieve synchronization
+in the most lightweight manner possible. For drivers intending to be portable
+between multiple architectures and bus implementations, the kernel offers a
+series of accessor functions that provide various degrees of ordering
+guarantees:
 
- (*) inX(), outX():
+ (*) readX(), writeX():
 
-     These are intended to talk to I/O space rather than memory space, but
-     that's primarily a CPU-specific concept.  The i386 and x86_64 processors
-     do indeed have special I/O space access cycles and instructions, but many
-     CPUs don't have such a concept.
+     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 PCI bus, amongst others, defines an I/O space concept which - on such
-     CPUs as i386 and x86_64 - readily maps to the CPU's concept of I/O
-     space.  However, it may also be mapped as a virtual I/O space in the CPU's
-     memory map, particularly on those CPUs that don't support alternate I/O
-     spaces.
+     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.
 
-     Accesses to this space may be fully synchronous (as on i386), but
-     intermediary bridges (such as the PCI host bridge) may not fully honour
-     that.
+     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.
 
-     They are guaranteed to be fully ordered with respect to each other.
+     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.
 
-     They are not guaranteed to be fully ordered with respect to other types of
-     memory and I/O operation.
+     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().
 
- (*) readX(), writeX():
+     __iomem pointers obtained with non-default attributes (e.g. those returned
+     by ioremap_wc()) are unlikely to provide many of these guarantees.
 
-     Whether these are guaranteed to be fully ordered and uncombined with
-     respect to each other on the issuing CPU depends on the characteristics
-     defined for the memory window through which they're accessing.  On later
-     i386 architecture machines, for example, this is controlled by way of the
-     MTRR registers.
+ (*) readX_relaxed(), writeX_relaxed():
 
-     Ordinarily, these will be guaranteed to be fully ordered and uncombined,
-     provided they're not accessing a prefetchable device.
+     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.
 
-     However, intermediary hardware (such as a PCI bridge) may indulge in
-     deferral if it so wishes; to flush a store, a load from the same location
-     is preferred[*], but a load from the same device or from configuration
-     space should suffice for PCI.
+ (*) readsX(), writesX():
 
-     [*] NOTE! attempting to load from the same location as was written to may
-	 cause a malfunction - consider the 16550 Rx/Tx serial registers for
-	 example.
+     The readsX() and writesX() MMIO accessors are designed for accessing
+     register-based, memory-mapped FIFOs residing on peripherals that are not
+     capable of performing DMA. Consequently, they provide only the ordering
+     guarantees of readX_relaxed() and writeX_relaxed(), as documented above.
 
-     Used with prefetchable I/O memory, an mmiowb() barrier may be required to
-     force stores to be ordered.
+ (*) inX(), outX():
 
-     Please refer to the PCI specification for more information on interactions
-     between PCI transactions.
+     The inX() and outX() accessors are intended to access legacy port-mapped
+     I/O peripherals, which may require special instructions on some
+     architectures (notably x86). The port number of the peripheral being
+     accessed is passed as an argument.
 
- (*) readX_relaxed(), writeX_relaxed()
+     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.
 
-     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 (e.g. DMA buffers) nor do they guarantee
-     ordering with respect to LOCK or UNLOCK operations.  If the latter is
-     required, an mmiowb() barrier can be used.  Note that relaxed accesses to
-     the same peripheral are guaranteed to be ordered with respect to each
-     other.
+     Device drivers may expect outX() to emit a non-posted write transaction
+     that waits for a completion response from the I/O peripheral before
+     returning. This is not guaranteed by all architectures and is therefore
+     not part of the portable ordering semantics.
+
+ (*) 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.
 
  (*) ioreadX(), iowriteX()
 
      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.
+
+Composing I/O ordering barriers with SMP ordering barriers and LOCK/UNLOCK
+operations is a dangerous sport which may require the use of mmiowb(). See the
+subsection "Acquires vs I/O accesses" for more information.
 
 ========================================
 ASSUMED MINIMUM EXECUTION ORDERING MODEL