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