Merge tag 'docs/v5.3-1' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab/linux-media

Pull rst conversion of docs from Mauro Carvalho Chehab:
 "As agreed with Jon, I'm sending this big series directly to you, c/c
  him, as this series required a special care, in order to avoid
  conflicts with other trees"

* tag 'docs/v5.3-1' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab/linux-media: (77 commits)
  docs: kbuild: fix build with pdf and fix some minor issues
  docs: block: fix pdf output
  docs: arm: fix a breakage with pdf output
  docs: don't use nested tables
  docs: gpio: add sysfs interface to the admin-guide
  docs: locking: add it to the main index
  docs: add some directories to the main documentation index
  docs: add SPDX tags to new index files
  docs: add a memory-devices subdir to driver-api
  docs: phy: place documentation under driver-api
  docs: serial: move it to the driver-api
  docs: driver-api: add remaining converted dirs to it
  docs: driver-api: add xilinx driver API documentation
  docs: driver-api: add a series of orphaned documents
  docs: admin-guide: add a series of orphaned documents
  docs: cgroup-v1: add it to the admin-guide book
  docs: aoe: add it to the driver-api book
  docs: add some documentation dirs to the driver-api book
  docs: driver-model: move it to the driver-api book
  docs: lp855x-driver.rst: add it to the driver-api book
  ...

1 files changed, 116 insertions, 0 deletions

diff --git a/Documentation/admin-guide/device-mapper/era.rst b/Documentation/admin-guide/device-mapper/era.rst
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+======
+dm-era
+======
+
+Introduction
+============
+
+dm-era is a target that behaves similar to the linear target.  In
+addition it keeps track of which blocks were written within a user
+defined period of time called an 'era'.  Each era target instance
+maintains the current era as a monotonically increasing 32-bit
+counter.
+
+Use cases include tracking changed blocks for backup software, and
+partially invalidating the contents of a cache to restore cache
+coherency after rolling back a vendor snapshot.
+
+Constructor
+===========
+
+era <metadata dev> <origin dev> <block size>
+
+ ================ ======================================================
+ metadata dev     fast device holding the persistent metadata
+ origin dev	  device holding data blocks that may change
+ block size       block size of origin data device, granularity that is
+		  tracked by the target
+ ================ ======================================================
+
+Messages
+========
+
+None of the dm messages take any arguments.
+
+checkpoint
+----------
+
+Possibly move to a new era.  You shouldn't assume the era has
+incremented.  After sending this message, you should check the
+current era via the status line.
+
+take_metadata_snap
+------------------
+
+Create a clone of the metadata, to allow a userland process to read it.
+
+drop_metadata_snap
+------------------
+
+Drop the metadata snapshot.
+
+Status
+======
+
+<metadata block size> <#used metadata blocks>/<#total metadata blocks>
+<current era> <held metadata root | '-'>
+
+========================= ==============================================
+metadata block size	  Fixed block size for each metadata block in
+			  sectors
+#used metadata blocks	  Number of metadata blocks used
+#total metadata blocks	  Total number of metadata blocks
+current era		  The current era
+held metadata root	  The location, in blocks, of the metadata root
+			  that has been 'held' for userspace read
+			  access. '-' indicates there is no held root
+========================= ==============================================
+
+Detailed use case
+=================
+
+The scenario of invalidating a cache when rolling back a vendor
+snapshot was the primary use case when developing this target:
+
+Taking a vendor snapshot
+------------------------
+
+- Send a checkpoint message to the era target
+- Make a note of the current era in its status line
+- Take vendor snapshot (the era and snapshot should be forever
+  associated now).
+
+Rolling back to an vendor snapshot
+----------------------------------
+
+- Cache enters passthrough mode (see: dm-cache's docs in cache.txt)
+- Rollback vendor storage
+- Take metadata snapshot
+- Ascertain which blocks have been written since the snapshot was taken
+  by checking each block's era
+- Invalidate those blocks in the caching software
+- Cache returns to writeback/writethrough mode
+
+Memory usage
+============
+
+The target uses a bitset to record writes in the current era.  It also
+has a spare bitset ready for switching over to a new era.  Other than
+that it uses a few 4k blocks for updating metadata::
+
+   (4 * nr_blocks) bytes + buffers
+
+Resilience
+==========
+
+Metadata is updated on disk before a write to a previously unwritten
+block is performed.  As such dm-era should not be effected by a hard
+crash such as power failure.
+
+Userland tools
+==============
+
+Userland tools are found in the increasingly poorly named
+thin-provisioning-tools project:
+
+    https://github.com/jthornber/thin-provisioning-tools