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+# cbfstool: Handling memory mapped boot media
+
+`cbfstool` is a utility used for managing coreboot file system (CBFS)
+components in a ROM image. x86 platforms are special since they have
+the SPI flash boot media memory mapped into host address space at
+runtime. This requires `cbfstool` to deal with two separate address
+spaces for any CBFS components that are eXecute-In-Place (XIP) - one
+is the SPI flash address space and other is the host address space
+where the SPI flash gets mapped.
+
+By default, all x86 platforms map a maximum of 16MiB of SPI flash at
+the top of 4G in host address space. If the flash is greater than
+16MiB, then only the top 16MiB of the flash is mapped in the host
+address space. If the flash is smaller than 16MiB, then the entire SPI
+flash is mapped at the top of 4G and the rest of the space remains
+unused.
+
+In more recent platforms like Tiger Lake (TGL), it is possible to map
+more than 16MiB of SPI flash. Since the host address space has legacy
+fixed device addresses mapped below `4G - 16M`, the SPI flash is split
+into separate windows when being mapped to the host address space.
+Default decode window of maximum 16MiB size still lives just below the
+4G boundary. The additional decode window is free to live in any
+available MMIO space that the SoC chooses.
+
+Following diagram shows different combinations of SPI flash being
+mapped into host address space when using multiple windows:
+
+![MMAP window combinations with different flash sizes][mmap_windows]
+
+*(a) SPI flash of size 16MiB (b) SPI flash smaller than 16MiB (c) SPI flash
+of size (16MiB+ext window size) (d) SPI flash smaller than (16MiB+ext
+window size)*
+
+The location of standard decode window is fixed in host address space
+`(4G - 16M) to 4G`. However, the platform is free to choose where the
+extended window lives in the host address space. Since `cbfstool`
+needs to know the exact location of the extended window, it allows the
+platform to pass in two parameters `ext-win-base` and `ext-win-size`
+that provide the base and the size of the extended window in host
+address space.
+
+`cbfstool` creates two memory map windows using the knowledge about the
+standard decode window and the information passed in by the platform
+about the extended decode window. These windows are useful in
+converting addresses from one space to another (flash space and host
+space) when dealing with XIP components.
+
+## Assumptions
+
+1. Top 16MiB is still decoded in the fixed decode window just below 4G
+   boundary.
+1. Rest of the SPI flash below the top 16MiB is mapped at the top of
+   the extended window. Even though the platform might support a
+   larger extended window, the SPI flash part used by the mainboard
+   might not be large enough to be mapped in the entire window. In
+   such cases, the mapping is assumed to be in the top part of the
+   extended window with the bottom part remaining unused.
+
+## Example
+
+If the platform supports extended window and the SPI flash size is
+greater, then `cbfstool` creates a mapping for the extended window as
+well.
+
+```
+ext_win_base = 0xF8000000
+ext_win_size = 32 * MiB
+ext_win_limit = ext_win_base + ext_win_size - 1 = 0xF9FFFFFF
+```
+
+If SPI flash is 32MiB, then top 16MiB is mapped from `0xFF000000 -
+0xFFFFFFFF` whereas the bottom 16MiB is mapped from `0xF9000000 -
+0xF9FFFFFF`. The extended window `0xF8000000 - 0xF8FFFFFF` remains
+unused.
+
+[mmap_windows]: mmap_windows.svg