Documentation: Explain FMAP and FMD

The Flashmap (FMAP) was not clearly documented. The new flashmap.md
explains where to find more details about that and how / why it was used
in coreboot. Also explained what is FMD and how to use it (based on
original README.fmaptool).

BUG=None
TEST=None (only documentation)

Change-Id: Ia389e56c632096d7c905ed221fd4f140dec382e6
Signed-off-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/31766
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

2 files changed, 160 insertions, 0 deletions

diff --git a/Documentation/lib/flashmap.md b/Documentation/lib/flashmap.md
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+# Flashmap and Flashmap Descriptor in coreboot
+
+## Flashmap
+
+[Flashmap](https://code.google.com/p/flashmap) (FMAP) is a binary format to
+describe partitions in a flash chip. It was added to coreboot to support the
+requirements of Chromium OS firmware but then was also used in other scenarios
+where precise placement of data in flash was necessary, or for data that is
+written to at runtime, as CBFS is considered too fragile for such situations.
+The Flashmap implementation inside coreboot is the de facto standard today.
+
+Flashmap partitions the image into clearly delimited sections and some of those
+sections may be CBFSes that can hold arbitrary-length files (at least one, the
+default CBFS, called `COREBOOT`). General guidance is that everything with
+strict layout requirements (e.g. must be aligned to erase blocks or
+something else) should have its own Flashmap section, and everything else should
+normally go into CBFS.
+
+The Flashmap itself starts with a header `struct fmap` and followed by a list of
+section descriptions in `strcut fmap_area`.
+
+### Header
+The header `struct fmap` has following fields:
+* `signature`: 8 characters as `"__FMAP__"`.
+* `ver_major`: one byte for major version (currently only 1).
+* `ver_minor`: one byte for minor version (current value is 1).
+* `base`: 64 bit integer for the address of the firmware binary.
+* `size`: 32 bit integer for the size of firmware binary in bytes.
+* `name`: 32 characters for the name of the firmware binary.
+* `nareas`: 16 bit integer for the number of area definitions (i.e., how many
+  sections are in this firmware image) following the header.
+
+### Area Definition
+The section is defined by `struct fmap_area` with following fields:
+* `offset`: 32 bit integer for where the area starts (relative to `base` in
+  header).
+* `size`: 32 bit integer for the size of area in bytes.
+* `name`: 32 characters for a descriptive name of this area. Should be unique to
+  all sections inside same Flashmap.
+* `flags`: 16 bit integer for attributes of this area (see below).
+
+### Area Flags
+Currently the defined values for `flags` in `struct fmap_area` are:
+* `FMAP_AREA_PRESERVE`: suggesting the section should be preserved when
+  updating firmware, usually for product data like serial number, MAC address,
+  or calibration and cache data.
+* `FMAP_AREA_STATIC`: Not really used today.
+* `FMAP_AREA_COMPRESSED`: Not really used today.
+* `FMAP_AREA_RO`: Not really used today.
+
+### FMAP section
+The whole Flashmap (`struct fmap` and list of `struct fmap_area`) should be
+stored in a standalone section named as `FMAP` (which should be also described
+by the Flashmap itself in `struct fmap_area`). There's no restriction for where
+it should be located (or how large), but usually we need to do a linear or
+binary search on whole firmware binary image to find Flashmap so a properly
+aligned address would be better.
+
+### COREBOOT section
+coreboot firmware images (`coreboot.rom`) should have at least one Flashmap
+section that is reserved for CBFS. Usually it is named as `COREBOOT`.
+
+## Flashmap Descriptor
+
+Since coreboot is starting to use a "partition" of Flashmap to describe the
+flash chip layout (both at runtime and when flashing a new image onto a
+chip), the project needs a reasonably expressive plain text format for
+representing such sections in the source tree.
+
+Flashmap Descriptor (FMD) is a [language and
+compiler](https://chromium-review.googlesource.com/#/c/255031) inside coreboot
+utility folder that can be used to generate final firmware images (i.e.
+`coreboot.rom`) formatted by Flashmap.
+
+The FMD implementation is in coreboot `utils/cbfstool` folder. Here's an
+informal language description:
+
+```
+# <line comment>
+<image name>[@<memory-mapped address>] <image size> {
+    <section name>[(flags)][@<offset from start of image>] [<section size>] [{
+        <subsection name>[@<offset from start of parent section>] [<subsection size>] [{
+            # Sections can be nested as deeply as desired
+            <subsubsection name>[(flags)][@...] [...] [{...}]
+        }]
+        [<subsection name>[(flags)][@...] [...] [{...}]]
+        # There can be many subsections at each level of nesting: they will be inserted
+        # sequentially, and although gaps are allowed, any provided offsets are always
+        # relative to the closest parent node's and must be strictly increasing with neither
+        # overlapping nor degenerate-size sections.
+    }]
+}
+```
+
+Note that the above example contains a few symbols that are actually meta
+syntax, and therefore have neither meaning nor place in a real file. The `<.*>`s
+indicate placeholders for parameters:
+
+* The names are strings, which are provided as single-word (no white space)
+  groups of syntactically unimportant symbols (i.e. every thing except `@`, `{`,
+  and `}`): they are not surrounded by quotes or any other form of delimiter.
+* The other fields are non-negative integers, which may be given as decimal or
+  hexadecimal; in either case, a `K`, `M`, or `G` may be appended (without
+  intermediate white space) as a multiplier.
+* Comments consist of anything one manages to enter, provided it doesn't start a
+  new line.
+
+The `[.*]`s indicate that a portion of the file could be omitted altogether:
+
+* Just because something is noted as optional doesn't mean it is in every case:
+  the answer might actually depend on which other information is---or
+  isn't---provided.
+* The "flag" specifies the attribute or type for given section. The most
+  important supported flag is "CBFS", which indicates the section will contain
+  a CBFS structure.
+* In particular, it is only legal to place a (CBFS) flag on a leaf section; that
+  is, choosing to add child sections excludes the possibility of putting a CBFS
+  in their parent. Such flags are only used to decide where CBFS empty file
+  headers should be created, and do not result in the storage of any additional
+  metadata in the resulting FMAP section.
+
+Additionally, it's important to note these properties of the overall file and
+its values:
+
+* Other than within would-be strings and numbers, white space is ignored. It
+  goes without saying that such power comes with responsibility, which is why
+  this sentence is here.
+* Although the `section name` must be globally unique, one of them may (but is
+  not required to) match the image name.
+* It is a syntax error to supply a number (besides 0) that begins with the
+  character `0`, as there is no intention of adding octals to the mix.
+* The image's memory address should be present on (and only on) layouts for
+  memory-mapped chips.
+* Although it may be evident from above, all `section` offsets are relative only
+  to the immediate parent. There is no way to include an absolute offset (i.e.
+  from the beginning of flash), which means that it is "safe" to reorder the
+  sections within a particular level of nesting, as long as the change doesn't
+  cause their positions and sizes to necessitate overlap or zero sizes.
+* A `section` with omitted offset is assumed to start at as low a position as
+  possible (with no consideration of alignment) and one with omitted size is
+  assumed to fill the remaining space until the next sibling or before the end
+  of its parent.
+* It's fine to omit any `section`'s offset, size, or both, provided its position
+  and size are still unambiguous in the context of its *sibling* sections and
+  its parent's *size*. In particular, knowledge of one .*section 's children or
+  the `section`s' common parent's siblings will not be used for this purpose.
+* Although `section`s are not required to have children, the flash chip as a
+  whole must have at least one.
+* Though the braces after `section`s may be omitted for those that have no
+  children, if they are present, they must contain at least one child.
+
+To see the formal description of the language, please refer to the Lex and Yacc
+files: `fmd_scanner.l` and `fmd_scanner.y`.
diff --git a/Documentation/lib/index.md b/Documentation/lib/index.md
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+# Library-specific documentation
+
+This section contains documentation about coreboot internal technical
+information and libraries.
+
+# Structure and layout
+- [Flashmap and Flashmap Descriptor](flashmap.md)