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