devlink: add a file documenting devlink regions

Also document the regions created by the mlx4 driver. This is currently
the only in-tree driver that creates devlink region snapshots.
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Cc: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/devlink/devlink-region.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+==============
+Devlink Region
+==============
+
+``devlink`` regions enable access to driver defined address regions using
+devlink.
+
+Each device can create and register its own supported address regions. The
+region can then be accessed via the devlink region interface.
+
+Region snapshots are collected by the driver, and can be accessed via read
+or dump commands. This allows future analysis on the created snapshots.
+
+The major benefit to creating a region is to provide access to internal
+address regions that are otherwise inaccessible to the user. They can be
+used to provide an additional way to debug complex error states.
+
+example usage
+-------------
+
+.. code:: shell
+
+    $ devlink region help
+    $ devlink region show [ DEV/REGION ]
+    $ devlink region del DEV/REGION snapshot SNAPSHOT_ID
+    $ devlink region dump DEV/REGION [ snapshot SNAPSHOT_ID ]
+    $ devlink region read DEV/REGION [ snapshot SNAPSHOT_ID ]
+            address ADDRESS length length
+
+    # Show all of the exposed regions with region sizes:
+    $ devlink region show
+    pci/0000:00:05.0/cr-space: size 1048576 snapshot [1 2]
+    pci/0000:00:05.0/fw-health: size 64 snapshot [1 2]
+
+    # Delete a snapshot using:
+    $ devlink region del pci/0000:00:05.0/cr-space snapshot 1
+
+    # Dump a snapshot:
+    $ devlink region dump pci/0000:00:05.0/fw-health snapshot 1
+    0000000000000000 0014 95dc 0014 9514 0035 1670 0034 db30
+    0000000000000010 0000 0000 ffff ff04 0029 8c00 0028 8cc8
+    0000000000000020 0016 0bb8 0016 1720 0000 0000 c00f 3ffc
+    0000000000000030 bada cce5 bada cce5 bada cce5 bada cce5
+
+    # Read a specific part of a snapshot:
+    $ devlink region read pci/0000:00:05.0/fw-health snapshot 1 address 0
+            length 16
+    0000000000000000 0014 95dc 0014 9514 0035 1670 0034 db30
+
+As regions are likely very device or driver specific, no generic regions are
+defined. See the driver-specific documentation files for information on the
+specific regions a driver supports.

Documentation/networking/devlink/index.rst

@@ -16,6 +16,7 @@ general.
    devlink-health
    devlink-info
    devlink-params
+   devlink-region
    devlink-trap
    devlink-trap-netdevsim
 

Documentation/networking/devlink/mlx4.rst

@@ -41,3 +41,16 @@ parameters.
      - Enable using the 4k UAR.
 
 The ``mlx4`` driver supports reloading via ``DEVLINK_CMD_RELOAD``
+
+Regions
+=======
+
+The ``mlx4`` driver supports dumping the firmware PCI crspace and health
+buffer during a critical firmware issue.
+
+In case a firmware command times out, firmware getting stuck, or a non zero
+value on the catastrophic buffer, a snapshot will be taken by the driver.
+
+The ``cr-space`` region will contain the firmware PCI crspace contents. The
+``fw-health`` region will contain the device firmware's health buffer.
+Snapshots for both of these regions are taken on the same event triggers.