doc: Document partition specifications

This documents the way U-Boot understands partitions specifications.
This also updates the fastboot documentation for the changes in the
previous commit.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/android/fastboot.rst

@@ -154,6 +154,10 @@ The device index starts from ``a`` and refers to the interface (e.g. USB
 controller, SD/MMC controller) or disk index. The partition index starts
 from ``1`` and describes the partition number on the particular device.
 
+Alternatively, partition types may be specified using :ref:`U-Boot's partition
+syntax <partitions>`. This allows specifying partitions like ``0.1``,
+``0#boot``, or ``:3``. The interface is always ``mmc``.
+
 Writing Partition Table
 -----------------------
 

doc/usage/index.rst

@@ -6,6 +6,7 @@ Use U-Boot
 
    fdt_overlays
    netconsole
+   partitions
 
 Shell commands
 --------------

doc/usage/partitions.rst

@@ -0,0 +1,80 @@
+.. SPDX-License-Identifier: GPL-2.0+
+.. _partitions:
+
+Partitions
+==========
+
+Synopsis
+--------
+
+::
+
+    <command> <interface> [devnum][.hwpartnum][:partnum|#partname]
+
+Description
+-----------
+
+Many U-Boot commands allow specifying partitions (or whole disks) using a
+generic syntax.
+
+interface
+        The interface used to access the partition's device, like ``mmc`` or
+        ``scsi``. For a full list of supported interfaces, consult the
+        ``if_typename_str`` array in ``drivers/block/blk-uclass.c``
+
+devnum
+        The device number. This defaults to 0.
+
+hwpartnum
+        The hardware partition number. All devices have at least one hardware
+        partition. On most devices, hardware partition 0 specifies the whole
+        device. On eMMC devices, hardware partition 0 is the user partition,
+        hardware partitions 1 and 2 are the boot partitions, hardware partition
+        3 is the RPMB partition, and further partitions are general-purpose
+        user-created partitions. The default hardware partition number is 0.
+
+partnum
+        The partition number, starting from 1. The partition number 0 specifies
+        that the whole device is to be used as one "partition."
+
+partname
+        The partition name. This is the partition label for GPT partitions. For
+        MBR partitions, the following syntax is used::
+
+                <devtype><devletter><partnum>
+
+        devtype
+                A device type like ``mmcsd`` or ``hd``. See the
+                ``part_set_generic_name`` function in ``disk/part.c`` for a
+                complete list.
+
+        devletter
+                The device number as an offset from ``a``. For example, device
+                number 2 would have a device letter of ``c``.
+
+        partnum
+                The partition number. This is the same as above.
+
+If neither ``partname`` nor ``partnum`` is specified and there is a partition
+table, then partition 1 is used. If there is no partition table, then the whole
+device is used as one "partition." If none of ``devnum``, ``hwpartnum``,
+``partnum``, or ``partname`` is specified, or only ``-`` is specified, then
+``devnum`` defaults to the value of the ``bootdevice`` environmental variable.
+
+Examples
+--------
+
+List the root directory contents on MMC device 2, hardware partition 1,
+and partition number 3::
+
+        ls mmc 2.1:3 /
+
+Load ``/kernel.itb`` to address ``0x80000000`` from SCSI device 0, hardware partition
+0, and the partition labeled ``boot``::
+
+        load scsi #boot 0x80000000 /kernel.itb
+
+Print the partition UUID of the SATA device ``$bootdevice``, hardware partition
+0, and partition number 0::
+
+        part uuid sata -