Merge tag 'docs-5.7' of git://git.lwn.net/linux

Pull documentation updates from Jonathan Corbet:
 "This has been a busy cycle for documentation work.

  Highlights include:

   - Lots of RST conversion work by Mauro, Daniel ALmeida, and others.
     Maybe someday we'll get to the end of this stuff...maybe...

   - Some organizational work to bring some order to the core-api
     manual.

   - Various new docs and additions to the existing documentation.

   - Typo fixes, warning fixes, ..."

* tag 'docs-5.7' of git://git.lwn.net/linux: (123 commits)
  Documentation: x86: exception-tables: document CONFIG_BUILDTIME_TABLE_SORT
  MAINTAINERS: adjust to filesystem doc ReST conversion
  docs: deprecated.rst: Add BUG()-family
  doc: zh_CN: add translation for virtiofs
  doc: zh_CN: index files in filesystems subdirectory
  docs: locking: Drop :c:func: throughout
  docs: locking: Add 'need' to hardirq section
  docs: conf.py: avoid thousands of duplicate label warning on Sphinx
  docs: prevent warnings due to autosectionlabel
  docs: fix reference to core-api/namespaces.rst
  docs: fix pointers to io-mapping.rst and io_ordering.rst files
  Documentation: Better document the softlockup_panic sysctl
  docs: hw-vuln: tsx_async_abort.rst: get rid of an unused ref
  docs: perf: imx-ddr.rst: get rid of a warning
  docs: filesystems: fuse.rst: supress a Sphinx warning
  docs: translations: it: avoid duplicate refs at programming-language.rst
  docs: driver.rst: supress two ReSt warnings
  docs: trace: events.rst: convert some new stuff to ReST format
  Documentation: Add io_ordering.rst to driver-api manual
  Documentation: Add io-mapping.rst to driver-api manual
  ...

1 files changed, 51 insertions, 0 deletions

diff --git a/Documentation/driver-api/io_ordering.rst b/Documentation/driver-api/io_ordering.rst
new file mode 100644
index 000000000000..2ab303ce9a0d
--- /dev/null
+++ b/Documentation/driver-api/io_ordering.rst
@@ -0,0 +1,51 @@
+==============================================
+Ordering I/O writes to memory-mapped addresses
+==============================================
+
+On some platforms, so-called memory-mapped I/O is weakly ordered.  On such
+platforms, driver writers are responsible for ensuring that I/O writes to
+memory-mapped addresses on their device arrive in the order intended.  This is
+typically done by reading a 'safe' device or bridge register, causing the I/O
+chipset to flush pending writes to the device before any reads are posted.  A
+driver would usually use this technique immediately prior to the exit of a
+critical section of code protected by spinlocks.  This would ensure that
+subsequent writes to I/O space arrived only after all prior writes (much like a
+memory barrier op, mb(), only with respect to I/O).
+
+A more concrete example from a hypothetical device driver::
+
+		...
+	CPU A:  spin_lock_irqsave(&dev_lock, flags)
+	CPU A:  val = readl(my_status);
+	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:  val = readl(my_status);
+	CPU B:  ...
+	CPU B:  writel(newval2, ring_ptr);
+	CPU B:  spin_unlock_irqrestore(&dev_lock, flags)
+		...
+
+In the case above, the device may receive newval2 before it receives newval,
+which could cause problems.  Fixing it is easy enough though::
+
+		...
+	CPU A:  spin_lock_irqsave(&dev_lock, flags)
+	CPU A:  val = readl(my_status);
+	CPU A:  ...
+	CPU A:  writel(newval, ring_ptr);
+	CPU A:  (void)readl(safe_register); /* maybe a config register? */
+	CPU A:  spin_unlock_irqrestore(&dev_lock, flags)
+		...
+	CPU B:  spin_lock_irqsave(&dev_lock, flags)
+	CPU B:  val = readl(my_status);
+	CPU B:  ...
+	CPU B:  writel(newval2, ring_ptr);
+	CPU B:  (void)readl(safe_register); /* maybe a config register? */
+	CPU B:  spin_unlock_irqrestore(&dev_lock, flags)
+
+Here, the reads from safe_register will cause the I/O chipset to flush any
+pending writes before actually posting the read to the chipset, preventing
+possible data corruption.