Docs/cbfstool: Add details about memory mapped window handling

This change adds details memory mapped window handling in cbfstool
required for x86 platforms. It also captures the details about the
newly added support for multiple decode windows.

BUG=b:171534504

Change-Id: Icf970f951e56d717e6a4f8845fc73f10d5a21dd0
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48477
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>

1 files changed, 77 insertions, 0 deletions

diff --git a/Documentation/cbfstool/mmap_windows.md b/Documentation/cbfstool/mmap_windows.md
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+++ b/Documentation/cbfstool/mmap_windows.md
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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