binman: Expand documentation for entries

At present only the more complex entries are documented. It is useful to
have documentation for all entries in one place.

As a first step, add and expand the documentation to cover all entries.

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

24 files changed, 305 insertions, 9 deletions

diff --git a/tools/binman/etype/_testing.py b/tools/binman/etype/_testing.py
index 3eeec72b360..02c165c0c3e 100644
--- a/tools/binman/etype/_testing.py
+++ b/tools/binman/etype/_testing.py
@@ -15,8 +15,30 @@ import tools
 class Entry__testing(Entry):
     """A fake entry used for testing
 
+    This entry should not be used in normal images. It is a special entry with
+    strange features used for testing.
+
+    Properties / Entry arguments
+        test-str-fdt: Test string, normally in the node
+        test-int-fdt: Test integer, normally in the node
+        test-str-arg: Test string, normally in the entry arguments
+        test-int-arg: Test integer, normally in the entry arguments
+
+    The entry has a single 'a' byte as its contents. Operation is controlled by
+    a number of properties in the node, as follows:
+
     Properties:
-        return_invalid_entry: Return an invalid entry from GetOffsets()
+        return-invalid-entry: Return an invalid entry from GetOffsets()
+        return-unknown-contents: Refuse to provide any contents (to cause a
+            failure)
+        bad-update-contents: Implement ProcessContents() incorrectly so as to
+            cause a failure
+        never-complete-process-fdt: Refund to process the FDT (to cause a
+            failure)
+        require-args: Require that all used args are present (generating an
+            error if not)
+        force-bad-datatype: Force a call to GetEntryArgsOrProps() with a bad
+            data type (generating an error)
     """
     def __init__(self, section, etype, node):
         Entry.__init__(self, section, etype, node)
@@ -26,6 +48,8 @@ class Entry__testing(Entry):
                                                      'return-unknown-contents')
         self.bad_update_contents = fdt_util.GetBool(self._node,
                                                     'bad-update-contents')
+
+        # Set to True when the entry is ready to process the FDT.
         self.process_fdt_ready = False
         self.never_complete_process_fdt = fdt_util.GetBool(self._node,
                                                 'never-complete-process-fdt')
diff --git a/tools/binman/etype/blob.py b/tools/binman/etype/blob.py
index 28e6651a935..692d8683c33 100644
--- a/tools/binman/etype/blob.py
+++ b/tools/binman/etype/blob.py
@@ -10,6 +10,18 @@ import fdt_util
 import tools
 
 class Entry_blob(Entry):
+    """Entry containing an arbitrary binary blob
+
+    Note: This should not be used by itself. It is normally used as a parent
+    class by other entry types.
+
+    Properties / Entry arguments:
+        - filename: Filename of file to read into entry
+
+    This entry reads data from a file and places it in the entry. The
+    default filename is often specified specified by the subclass. See for
+    example the 'u_boot' entry which provides the filename 'u-boot.bin'.
+    """
     def __init__(self, section, etype, node):
         Entry.__init__(self, section, etype, node)
         self._filename = fdt_util.GetString(self._node, "filename", self.etype)
diff --git a/tools/binman/etype/intel_cmc.py b/tools/binman/etype/intel_cmc.py
index b8621e0cc5e..fa6f7793c64 100644
--- a/tools/binman/etype/intel_cmc.py
+++ b/tools/binman/etype/intel_cmc.py
@@ -9,5 +9,15 @@ from entry import Entry
 from blob import Entry_blob
 
 class Entry_intel_cmc(Entry_blob):
+    """Entry containing an Intel Chipset Micro Code (CMC) file
+
+    Properties / Entry arguments:
+        - filename: Filename of file to read into entry
+
+    This file contains microcode for some devices in a special format. An
+    example filename is 'Microcode/C0_22211.BIN'.
+
+    See README.x86 for information about x86 binary blobs.
+    """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
diff --git a/tools/binman/etype/intel_descriptor.py b/tools/binman/etype/intel_descriptor.py
index 0a5151d466c..6acbbd8b7a5 100644
--- a/tools/binman/etype/intel_descriptor.py
+++ b/tools/binman/etype/intel_descriptor.py
@@ -13,6 +13,7 @@ from blob import Entry_blob
 FD_SIGNATURE   = struct.pack('<L', 0x0ff0a55a)
 MAX_REGIONS    = 5
 
+# Region numbers supported by the Intel firmware format
 (REGION_DESCRIPTOR, REGION_BIOS, REGION_ME, REGION_GBE,
         REGION_PDATA) = range(5)
 
@@ -27,10 +28,21 @@ class Region:
 class Entry_intel_descriptor(Entry_blob):
     """Intel flash descriptor block (4KB)
 
-    This is placed at the start of flash and provides information about
+    Properties / Entry arguments:
+        filename: Filename of file containing the descriptor. This is typically
+            a 4KB binary file, sometimes called 'descriptor.bin'
+
+    This entry is placed at the start of flash and provides information about
     the SPI flash regions. In particular it provides the base address and
-    size of the ME region, allowing us to place the ME binary in the right
-    place.
+    size of the ME (Management Engine) region, allowing us to place the ME
+    binary in the right place.
+
+    With this entry in your image, the position of the 'intel-me' entry will be
+    fixed in the image, which avoids you needed to specify an offset for that
+    region. This is useful, because it is not possible to change the position
+    of the ME region without updating the descriptor.
+
+    See README.x86 for information about x86 binary blobs.
     """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
diff --git a/tools/binman/etype/intel_fsp.py b/tools/binman/etype/intel_fsp.py
index cb80a617c32..00a78e70832 100644
--- a/tools/binman/etype/intel_fsp.py
+++ b/tools/binman/etype/intel_fsp.py
@@ -9,5 +9,19 @@ from entry import Entry
 from blob import Entry_blob
 
 class Entry_intel_fsp(Entry_blob):
+    """Entry containing an Intel Firmware Support Package (FSP) file
+
+    Properties / Entry arguments:
+        - filename: Filename of file to read into entry
+
+    This file contains binary blobs which are used on some devices to make the
+    platform work. U-Boot executes this code since it is not possible to set up
+    the hardware using U-Boot open-source code. Documentation is typically not
+    available in sufficient detail to allow this.
+
+    An example filename is 'FSP/QUEENSBAY_FSP_GOLD_001_20-DECEMBER-2013.fd'
+
+    See README.x86 for information about x86 binary blobs.
+    """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
diff --git a/tools/binman/etype/intel_me.py b/tools/binman/etype/intel_me.py
index 0682ead9439..247c5b33866 100644
--- a/tools/binman/etype/intel_me.py
+++ b/tools/binman/etype/intel_me.py
@@ -9,5 +9,20 @@ from entry import Entry
 from blob import Entry_blob
 
 class Entry_intel_me(Entry_blob):
+    """Entry containing an Intel Management Engine (ME) file
+
+    Properties / Entry arguments:
+        - filename: Filename of file to read into entry
+
+    This file contains code used by the SoC that is required to make it work.
+    The Management Engine is like a background task that runs things that are
+    not clearly documented, but may include keyboard, deplay and network
+    access. For platform that use ME it is not possible to disable it. U-Boot
+    does not directly execute code in the ME binary.
+
+    A typical filename is 'me.bin'.
+
+    See README.x86 for information about x86 binary blobs.
+    """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
diff --git a/tools/binman/etype/intel_mrc.py b/tools/binman/etype/intel_mrc.py
index 305ac98888b..4dbc99a04f2 100644
--- a/tools/binman/etype/intel_mrc.py
+++ b/tools/binman/etype/intel_mrc.py
@@ -9,6 +9,17 @@ from entry import Entry
 from blob import Entry_blob
 
 class Entry_intel_mrc(Entry_blob):
+    """Entry containing an Intel Memory Reference Code (MRC) file
+
+    Properties / Entry arguments:
+        - filename: Filename of file to read into entry
+
+    This file contains code for setting up the SDRAM on some Intel systems. This
+    is executed by U-Boot when needed early during startup. A typical filename
+    is 'mrc.bin'.
+
+    See README.x86 for information about x86 binary blobs.
+    """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
 
diff --git a/tools/binman/etype/intel_vbt.py b/tools/binman/etype/intel_vbt.py
index d4e8c3f797d..d93dd196341 100644
--- a/tools/binman/etype/intel_vbt.py
+++ b/tools/binman/etype/intel_vbt.py
@@ -8,5 +8,15 @@ from entry import Entry
 from blob import Entry_blob
 
 class Entry_intel_vbt(Entry_blob):
+    """Entry containing an Intel Video BIOS Table (VBT) file
+
+    Properties / Entry arguments:
+        - filename: Filename of file to read into entry
+
+    This file contains code that sets up the integrated graphics subsystem on
+    some Intel SoCs. U-Boot executes this when the display is started up.
+
+    See README.x86 for information about Intel binary blobs.
+    """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
diff --git a/tools/binman/etype/intel_vga.py b/tools/binman/etype/intel_vga.py
index 140dd40dda6..40982c86656 100644
--- a/tools/binman/etype/intel_vga.py
+++ b/tools/binman/etype/intel_vga.py
@@ -9,5 +9,17 @@ from entry import Entry
 from blob import Entry_blob
 
 class Entry_intel_vga(Entry_blob):
+    """Entry containing an Intel Video Graphics Adaptor (VGA) file
+
+    Properties / Entry arguments:
+        - filename: Filename of file to read into entry
+
+    This file contains code that sets up the integrated graphics subsystem on
+    some Intel SoCs. U-Boot executes this when the display is started up.
+
+    This is similar to the VBT file but in a different format.
+
+    See README.x86 for information about Intel binary blobs.
+    """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
diff --git a/tools/binman/etype/section.py b/tools/binman/etype/section.py
index b90b80e4ce7..2e68f276f0d 100644
--- a/tools/binman/etype/section.py
+++ b/tools/binman/etype/section.py
@@ -13,6 +13,23 @@ import tools
 import bsection
 
 class Entry_section(Entry):
+    """Entry that contains other entries
+
+    Properties / Entry arguments: (see binman README for more information)
+        - size: Size of section in bytes
+        - align-size: Align size to a particular power of two
+        - pad-before: Add padding before the entry
+        - pad-after: Add padding after the entry
+        - pad-byte: Pad byte to use when padding
+        - sort-by-offset: Reorder the entries by offset
+        - end-at-4gb: Used to build an x86 ROM which ends at 4GB (2^32)
+        - name-prefix: Adds a prefix to the name of every entry in the section
+            when writing out the map
+
+    A section is an entry which can contain other entries, thus allowing
+    hierarchical images to be created. See 'Sections and hierarchical images'
+    in the binman README for more information.
+    """
     def __init__(self, image, etype, node):
         Entry.__init__(self, image, etype, node)
         self._section = bsection.Section(node.name, node)
diff --git a/tools/binman/etype/text.py b/tools/binman/etype/text.py
index 1c69afa72ef..7a1cddf4af8 100644
--- a/tools/binman/etype/text.py
+++ b/tools/binman/etype/text.py
@@ -13,7 +13,14 @@ class Entry_text(Entry):
     """An entry which contains text
 
     The text can be provided either in the node itself or by a command-line
-    argument.
+    argument. There is a level of indirection to allow multiple text strings
+    and sharing of text.
+
+    Properties / Entry arguments:
+        text-label: The value of this string indicates the property / entry-arg
+            that contains the string to place in the entry
+        <xxx> (actual name is the value of text-label): contains the string to
+            place in the entry.
 
     Example node:
 
diff --git a/tools/binman/etype/u_boot.py b/tools/binman/etype/u_boot.py
index b6058bf6b52..23dd12ce435 100644
--- a/tools/binman/etype/u_boot.py
+++ b/tools/binman/etype/u_boot.py
@@ -9,6 +9,22 @@ from entry import Entry
 from blob import Entry_blob
 
 class Entry_u_boot(Entry_blob):
+    """U-Boot flat binary
+
+    Properties / Entry arguments:
+        - filename: Filename of u-boot.bin (default 'u-boot.bin')
+
+    This is the U-Boot binary, containing relocation information to allow it
+    to relocate itself at runtime. The binary typically includes a device tree
+    blob at the end of it. Use u_boot_nodtb if you want to package the device
+    tree separately.
+
+    U-Boot can access binman symbols at runtime. See:
+
+        'Access to binman entry offsets at run time (fdt)'
+
+    in the binman README for more information.
+    """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
 
diff --git a/tools/binman/etype/u_boot_dtb.py b/tools/binman/etype/u_boot_dtb.py
index dd3c5b27901..fb3dd1cf642 100644
--- a/tools/binman/etype/u_boot_dtb.py
+++ b/tools/binman/etype/u_boot_dtb.py
@@ -9,6 +9,15 @@ from entry import Entry
 from blob import Entry_blob
 
 class Entry_u_boot_dtb(Entry_blob):
+    """U-Boot device tree
+
+    Properties / Entry arguments:
+        - filename: Filename of u-boot.dtb (default 'u-boot.dtb')
+
+    This is the U-Boot device tree, containing configuration information for
+    U-Boot. U-Boot needs this to know what devices are present and which drivers
+    to activate.
+    """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
 
diff --git a/tools/binman/etype/u_boot_dtb_with_ucode.py b/tools/binman/etype/u_boot_dtb_with_ucode.py
index 55d3a38a64f..752d0ac0ba2 100644
--- a/tools/binman/etype/u_boot_dtb_with_ucode.py
+++ b/tools/binman/etype/u_boot_dtb_with_ucode.py
@@ -14,8 +14,16 @@ import tools
 class Entry_u_boot_dtb_with_ucode(Entry_blob):
     """A U-Boot device tree file, with the microcode removed
 
-    See Entry_u_boot_ucode for full details of the 3 entries involved in this
-    process.
+    Properties / Entry arguments:
+        - filename: Filename of u-boot.dtb (default 'u-boot.dtb')
+
+    See Entry_u_boot_ucode for full details of the three entries involved in
+    this process. This entry provides the U-Boot device-tree file, which
+    contains the microcode. If the microcode is not being collated into one
+    place then the offset and size of the microcode is recorded by this entry,
+    for use by u_boot_with_ucode_ptr. If it is being collated, then this
+    entry deletes the microcode from the device tree (to save space) and makes
+    it available to u_boot_ucode.
     """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
diff --git a/tools/binman/etype/u_boot_img.py b/tools/binman/etype/u_boot_img.py
index 6e0b736af14..1ec0757c7f8 100644
--- a/tools/binman/etype/u_boot_img.py
+++ b/tools/binman/etype/u_boot_img.py
@@ -9,6 +9,17 @@ from entry import Entry
 from blob import Entry_blob
 
 class Entry_u_boot_img(Entry_blob):
+    """U-Boot legacy image
+
+    Properties / Entry arguments:
+        - filename: Filename of u-boot.img (default 'u-boot.img')
+
+    This is the U-Boot binary as a packaged image, in legacy format. It has a
+    header which allows it to be loaded at the correct address for execution.
+
+    You should use FIT (Flat Image Tree) instead of the legacy image for new
+    applications.
+    """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
 
diff --git a/tools/binman/etype/u_boot_nodtb.py b/tools/binman/etype/u_boot_nodtb.py
index ca9e53a0947..a4b95a4390a 100644
--- a/tools/binman/etype/u_boot_nodtb.py
+++ b/tools/binman/etype/u_boot_nodtb.py
@@ -9,6 +9,17 @@ from entry import Entry
 from blob import Entry_blob
 
 class Entry_u_boot_nodtb(Entry_blob):
+    """U-Boot flat binary without device tree appended
+
+    Properties / Entry arguments:
+        - filename: Filename of u-boot.bin (default 'u-boot-nodtb.bin')
+
+    This is the U-Boot binary, containing relocation information to allow it
+    to relocate itself at runtime. It does not include a device tree blob at
+    the end of it so normally cannot work without it. You can add a u_boot_dtb
+    entry after this one, or use a u_boot entry instead (which contains both
+    U-Boot and the device tree).
+    """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
 
diff --git a/tools/binman/etype/u_boot_spl.py b/tools/binman/etype/u_boot_spl.py
index 9edd2dad030..0e99a2d2a70 100644
--- a/tools/binman/etype/u_boot_spl.py
+++ b/tools/binman/etype/u_boot_spl.py
@@ -11,6 +11,27 @@ from entry import Entry
 from blob import Entry_blob
 
 class Entry_u_boot_spl(Entry_blob):
+    """U-Boot SPL binary
+
+    Properties / Entry arguments:
+        - filename: Filename of u-boot-spl.bin (default 'spl/u-boot-spl.bin')
+
+    This is the U-Boot SPL (Secondary Program Loader) binary. This is a small
+    binary which loads before U-Boot proper, typically into on-chip SRAM. It is
+    responsible for locating, loading and jumping to U-Boot. Note that SPL is
+    not relocatable so must be loaded to the correct address in SRAM, or written
+    to run from the correct address is direct flash execution is possible (e.g.
+    on x86 devices).
+
+    SPL can access binman symbols at runtime. See:
+
+        'Access to binman entry offsets at run time (symbols)'
+
+    in the binman README for more information.
+
+    The ELF file 'spl/u-boot-spl' must also be available for this to work, since
+    binman uses that to look up symbols to write into the SPL binary.
+    """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
         self.elf_fname = 'spl/u-boot-spl'
diff --git a/tools/binman/etype/u_boot_spl_bss_pad.py b/tools/binman/etype/u_boot_spl_bss_pad.py
index 65f631d3c5e..00b7ac50040 100644
--- a/tools/binman/etype/u_boot_spl_bss_pad.py
+++ b/tools/binman/etype/u_boot_spl_bss_pad.py
@@ -14,6 +14,22 @@ from blob import Entry_blob
 import tools
 
 class Entry_u_boot_spl_bss_pad(Entry_blob):
+    """U-Boot SPL binary padded with a BSS region
+
+    Properties / Entry arguments:
+        None
+
+    This is similar to u_boot_spl except that padding is added after the SPL
+    binary to cover the BSS (Block Started by Symbol) region. This region holds
+    the various used by SPL. It is set to 0 by SPL when it starts up. If you
+    want to append data to the SPL image (such as a device tree file), you must
+    pad out the BSS region to avoid the data overlapping with U-Boot variables.
+    This entry is useful in that case. It automatically pads out the entry size
+    to cover both the code, data and BSS.
+
+    The ELF file 'spl/u-boot-spl' must also be available for this to work, since
+    binman uses that to look up the BSS address.
+    """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
 
diff --git a/tools/binman/etype/u_boot_spl_dtb.py b/tools/binman/etype/u_boot_spl_dtb.py
index eefa1ff53a4..6a30edc83c8 100644
--- a/tools/binman/etype/u_boot_spl_dtb.py
+++ b/tools/binman/etype/u_boot_spl_dtb.py
@@ -9,6 +9,15 @@ from entry import Entry
 from blob import Entry_blob
 
 class Entry_u_boot_spl_dtb(Entry_blob):
+    """U-Boot SPL device tree
+
+    Properties / Entry arguments:
+        - filename: Filename of u-boot.dtb (default 'spl/u-boot-spl.dtb')
+
+    This is the SPL device tree, containing configuration information for
+    SPL. SPL needs this to know what devices are present and which drivers
+    to activate.
+    """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
 
diff --git a/tools/binman/etype/u_boot_spl_nodtb.py b/tools/binman/etype/u_boot_spl_nodtb.py
index 99e56ebe3bf..41c17366b1d 100644
--- a/tools/binman/etype/u_boot_spl_nodtb.py
+++ b/tools/binman/etype/u_boot_spl_nodtb.py
@@ -9,6 +9,18 @@ from entry import Entry
 from blob import Entry_blob
 
 class Entry_u_boot_spl_nodtb(Entry_blob):
+    """SPL binary without device tree appended
+
+    Properties / Entry arguments:
+        - filename: Filename of spl/u-boot-spl-nodtb.bin (default
+            'spl/u-boot-spl-nodtb.bin')
+
+    This is the U-Boot SPL binary, It does not include a device tree blob at
+    the end of it so may not be able to work without it, assuming SPL needs
+    a device tree to operation on your platform. You can add a u_boot_spl_dtb
+    entry after this one, or use a u_boot_spl entry instead (which contains
+    both SPL and the device tree).
+    """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
 
diff --git a/tools/binman/etype/u_boot_ucode.py b/tools/binman/etype/u_boot_ucode.py
index 922a607cfe6..0f1761afc8f 100644
--- a/tools/binman/etype/u_boot_ucode.py
+++ b/tools/binman/etype/u_boot_ucode.py
@@ -12,6 +12,12 @@ import tools
 class Entry_u_boot_ucode(Entry_blob):
     """U-Boot microcode block
 
+    Properties / Entry arguments:
+        None
+
+    The contents of this entry are filled in automatically by other entries
+    which must also be in the image.
+
     U-Boot on x86 needs a single block of microcode. This is collected from
     the various microcode update nodes in the device tree. It is also unable
     to read the microcode from the device tree on platforms that use FSP
diff --git a/tools/binman/etype/u_boot_with_ucode_ptr.py b/tools/binman/etype/u_boot_with_ucode_ptr.py
index 97e58a0bfcf..51e7ba48f59 100644
--- a/tools/binman/etype/u_boot_with_ucode_ptr.py
+++ b/tools/binman/etype/u_boot_with_ucode_ptr.py
@@ -17,8 +17,13 @@ import tools
 class Entry_u_boot_with_ucode_ptr(Entry_blob):
     """U-Boot with embedded microcode pointer
 
-    See Entry_u_boot_ucode for full details of the 3 entries involved in this
-    process.
+    Properties / Entry arguments:
+        - filename: Filename of u-boot-nodtb.dtb (default 'u-boot-nodtb.dtb')
+
+    See Entry_u_boot_ucode for full details of the three entries involved in
+    this process. This entry updates U-Boot with the offset and size of the
+    microcode, to allow early x86 boot code to find it without doing anything
+    complicated. Otherwise it is the same as the u_boot entry.
     """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
diff --git a/tools/binman/etype/x86_start16.py b/tools/binman/etype/x86_start16.py
index 23d27f07040..7d32ecd321b 100644
--- a/tools/binman/etype/x86_start16.py
+++ b/tools/binman/etype/x86_start16.py
@@ -9,6 +9,20 @@ from entry import Entry
 from blob import Entry_blob
 
 class Entry_x86_start16(Entry_blob):
+    """x86 16-bit start-up code for U-Boot
+
+    Properties / Entry arguments:
+        - filename: Filename of u-boot-x86-16bit.bin (default
+            'u-boot-x86-16bit.bin')
+
+    x86 CPUs start up in 16-bit mode, even if they are 32-bit CPUs. This code
+    must be placed at a particular address. This entry holds that code. It is
+    typically placed at offset CONFIG_SYS_X86_START16. The code is responsible
+    for changing to 32-bit mode and jumping to U-Boot's entry point, which
+    requires 32-bit mode (for 32-bit U-Boot).
+
+    For 64-bit U-Boot, the 'x86_start16_spl' entry type is used instead.
+    """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)
 
diff --git a/tools/binman/etype/x86_start16_spl.py b/tools/binman/etype/x86_start16_spl.py
index 176420ba88c..d85909e7ae8 100644
--- a/tools/binman/etype/x86_start16_spl.py
+++ b/tools/binman/etype/x86_start16_spl.py
@@ -9,6 +9,20 @@ from entry import Entry
 from blob import Entry_blob
 
 class Entry_x86_start16_spl(Entry_blob):
+    """x86 16-bit start-up code for SPL
+
+    Properties / Entry arguments:
+        - filename: Filename of spl/u-boot-x86-16bit-spl.bin (default
+            'spl/u-boot-x86-16bit-spl.bin')
+
+    x86 CPUs start up in 16-bit mode, even if they are 64-bit CPUs. This code
+    must be placed at a particular address. This entry holds that code. It is
+    typically placed at offset CONFIG_SYS_X86_START16. The code is responsible
+    for changing to 32-bit mode and starting SPL, which in turn changes to
+    64-bit mode and jumps to U-Boot (for 64-bit U-Boot).
+
+    For 32-bit U-Boot, the 'x86_start16' entry type is used instead.
+    """
     def __init__(self, section, etype, node):
         Entry_blob.__init__(self, section, etype, node)