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/arch/index.rst

@@ -11,7 +11,7 @@ Architecture-specific doc
    m68k
    mips
    nios2
-   sandbox
+   sandbox/index
    sh
    x86
    xtensa

doc/arch/sandbox/block_impl.rst

@@ -0,0 +1,39 @@
+.. SPDX-License-Identifier: GPL-2.0+ */
+.. Copyright (c) 2014 The Chromium OS Authors.
+.. sectionauthor:: Simon Glass <sjg@chromium.org>
+
+Sandbox block devices (implementation)
+======================================
+
+(See :ref:`sandbox_blk` for operation)
+
+Sandbox block devices are implemented using the `UCLASS_HOST` uclass. Only one
+driver is provided (`host_sb_drv`) so all devices in the uclass use the same
+driver.
+
+The uclass has a simple API allowing files to be attached and detached.
+Attaching a file results in it appearing as a block device in sandbox. This
+allows filesystems and whole disk images to be accessed from U-Boot. This is
+particularly useful for tests.
+
+Devices are created using `host_create_device()`. This sets up a new
+`UCLASS_HOST`.
+
+The device can then be attached to a file with `host_attach_file()`. This
+creates the child block device (and bootdev device).
+
+The host device's block device must be probed before use, as normal.
+
+To destroy a device, call host_destroy_device(). This removes the device (and
+its children of course), then closes any attached file, then unbinds the device.
+
+There is no arbitrary limit to the number of host devices that can be created.
+
+
+Uclass API
+----------
+
+This is incomplete as it isn't clear how to make Sphinx do the right thing for
+struct host_ops. See `include/sandbox_host.h` for full details.
+
+.. kernel-doc:: include/sandbox_host.h

doc/arch/sandbox/index.rst

@@ -0,0 +1,12 @@
+.. SPDX-License-Identifier: GPL-2.0+ */
+.. Copyright 2022 Google LLC
+.. sectionauthor:: Simon Glass <sjg@chromium.org>
+
+Sandbox
+=======
+
+.. toctree::
+   :maxdepth: 2
+
+   sandbox
+   block_impl

doc/arch/sandbox/sandbox.rst

@@ -43,7 +43,7 @@ Note that standalone/API support is not available at present.
 Prerequisites
 -------------
 
-Install the dependencies noted in :doc:`../build/gcc`.
+Install the dependencies noted in :doc:`../../build/gcc`.
 
 
 Basic Operation
@@ -374,6 +374,7 @@ also use low-level SPI commands::
 This is issuing a READ_ID command and getting back 20 (ST Micro) part
 0x2015 (the M25P16).
 
+.. _sandbox_blk:
 
 Block Device Emulation
 ----------------------
@@ -401,6 +402,8 @@ or utilize the device described in test/py/make_test_disk.py::
    import make_test_disk
    make_test_disk.makeDisk()
 
+For more technical details, see :doc:`block_impl`.
+
 Writing Sandbox Drivers
 -----------------------
 
@@ -600,8 +603,8 @@ Testing
 U-Boot sandbox can be used to run various tests, mostly in the test/
 directory.
 
-See :doc:`../develop/tests_sandbox` for more information and
+See :doc:`../../develop/tests_sandbox` for more information and
-:doc:`../develop/testing` for information about testing generally.
+:doc:`../../develop/testing` for information about testing generally.
 
 
 Memory Map

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.

doc/usage/index.rst

@@ -52,6 +52,7 @@ Shell commands
    cmd/for
    cmd/fwu_mdata
    cmd/gpio
+   cmd/host
    cmd/load
    cmd/loadm
    cmd/loady