Merge tag 'arm64-mmiowb' of git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux

Pull mmiowb removal from Will Deacon:
 "Remove Mysterious Macro Intended to Obscure Weird Behaviours (mmiowb())

  Remove mmiowb() from the kernel memory barrier API and instead, for
  architectures that need it, hide the barrier inside spin_unlock() when
  MMIO has been performed inside the critical section.

  The only relatively recent changes have been addressing review
  comments on the documentation, which is in a much better shape thanks
  to the efforts of Ben and Ingo.

  I was initially planning to split this into two pull requests so that
  you could run the coccinelle script yourself, however it's been plain
  sailing in linux-next so I've just included the whole lot here to keep
  things simple"

* tag 'arm64-mmiowb' of git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux: (23 commits)
  docs/memory-barriers.txt: Update I/O section to be clearer about CPU vs thread
  docs/memory-barriers.txt: Fix style, spacing and grammar in I/O section
  arch: Remove dummy mmiowb() definitions from arch code
  net/ethernet/silan/sc92031: Remove stale comment about mmiowb()
  i40iw: Redefine i40iw_mmiowb() to do nothing
  scsi/qla1280: Remove stale comment about mmiowb()
  drivers: Remove explicit invocations of mmiowb()
  drivers: Remove useless trailing comments from mmiowb() invocations
  Documentation: Kill all references to mmiowb()
  riscv/mmiowb: Hook up mmwiob() implementation to asm-generic code
  powerpc/mmiowb: Hook up mmwiob() implementation to asm-generic code
  ia64/mmiowb: Add unconditional mmiowb() to arch_spin_unlock()
  mips/mmiowb: Add unconditional mmiowb() to arch_spin_unlock()
  sh/mmiowb: Add unconditional mmiowb() to arch_spin_unlock()
  m68k/io: Remove useless definition of mmiowb()
  nds32/io: Remove useless definition of mmiowb()
  x86/io: Remove useless definition of mmiowb()
  arm64/io: Remove useless definition of mmiowb()
  ARM/io: Remove useless definition of mmiowb()
  mmiowb: Hook up mmiowb helpers to spinlocks and generic I/O accessors
  ...

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

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