dm: Add documentation for host command and implementation

Document the 'host' command and also the internals of how it is
implemented.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/usage/cmd/host.rst

@@ -0,0 +1,116 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+host command
+============
+
+Synopis
+-------
+
+::
+
+    host bind [-r] <label> [<filename>]
+    host unbind <label|seq>
+    host info [<label|seq>]
+    host dev [<label|seq>]
+
+Description
+-----------
+
+The host command provides a way to attach disk images on the host to U-Boot
+sandbox. This can be useful for testing U-Boot's filesystem implementations.
+
+Common arguments:
+
+<label|seq>
+    This is used to specify a host device. It can either be a label (a string)
+    or the sequence number of the device. An invalid value causes the command
+    to fail.
+
+
+host bind
+~~~~~~~~~
+
+This creates a new host device and binds a file to it.
+
+Arguments:
+
+label
+    Label to use to identify this binding. This can be any string.
+
+filename:
+    Host filename to bind to
+
+Flags:
+
+-r
+    Mark the device as removable
+
+
+host unbind
+~~~~~~~~~~~
+
+This unbinds a host device that was previously bound. The sequence numbers of
+other devices remain unchanged.
+
+
+host info
+~~~~~~~~~
+
+Provides information about a particular host binding, or all of them.
+
+
+host dev
+~~~~~~~~
+
+Allowing selecting a particular device, or (with no arguments) seeing which one
+is selected.
+
+
+Example
+-------
+
+Initially there are no devices::
+
+    => host info
+    dev       blocks label           path
+
+Bind a device::
+
+    => host bind -r test2 2MB.ext2.img
+    => host bind fat 1MB.fat32.img
+    => host info
+    dev       blocks label           path
+      0         4096 test2           2MB.ext2.img
+      1         2048 fat             1MB.fat32.img
+
+Select a device by label or sequence number::
+
+    => host dev fat
+    Current host device: 1: fat
+    => host dev 0
+    Current host device: 0: test2
+
+Write a file::
+
+    => ext4write host 0 0 /dump 1e00
+    File System is consistent
+    7680 bytes written in 3 ms (2.4 MiB/s)
+    => ext4ls host 0
+    <DIR>       4096 .
+    <DIR>       4096 ..
+    <DIR>      16384 lost+found
+                4096 testing
+                7680 dump
+
+Unbind a device::
+
+    => host unbind test2
+    => host info
+    dev       blocks label           path
+      1         2048 fat             1MB.fat32.img
+
+
+Return value
+------------
+
+The return value $? indicates whether the command succeeded.