IFD Layout

A coreboot image for an Intel SoC contains two separate definitions of the layout of the flash. The Intel Flash Descriptor (IFD) which defines offsets and sizes of various regions of flash and the coreboot FMAP.

The FMAP should define all of the of the regions defined by the IFD to ensure that those regions are accounted for by coreboot and will not be accidentally modified.

IFD mapping

The names of the IFD regions in the FMAP should follow the convention of starting with the prefix SI_ which stands for silicon initialization as a way to categorize anything required by the SoC but not provided by coreboot.

IFD Region index	IFD Region name	FMAP Name	Notes
0	Flash Descriptor	SI_DESC	Always the top 4 KiB of flash
1	BIOS	SI_BIOS	This is the region that contains coreboot
2	Intel ME	SI_ME
3	Gigabit Ethernet	SI_GBE
4	Platform Data	SI_PDR
8	EC Firmware	SI_EC	Most ChromeOS devices do not use this region; EC firmware is stored in BIOS region of flash

Validation

The ifdtool can be used to manipulate a firmware image with a IFD. This tool will not take into account the FMAP while modifying the image which can lead to unexpected and hard to debug issues with the firmware image. For example if the ME region is defined at 6 MiB in the IFD but the FMAP only allocates 4 MiB for the ME, then when the ME is added by the ifdtool 6 MiB will be written which could overwrite 2 MiB of the BIOS.

In order to validate that the FMAP and the IFD are compatible the ifdtool provides –validate (-t) option. ifdtool -t will read both the IFD and the FMAP in the image and for every non empty region in the IFD if that region is defined in the FMAP but the offset or size is different then the tool will return an error.

Example:

foo@bar:~$ ifdtool -t bad_image.bin
Region mismatch between bios and SI_BIOS
 Descriptor region bios:
  offset: 0x00400000
  length: 0x01c00000
 FMAP area SI_BIOS:
  offset: 0x00800000
  length: 0x01800000
Region mismatch between me and SI_ME
 Descriptor region me:
  offset: 0x00103000
  length: 0x002f9000
 FMAP area SI_ME:
  offset: 0x00103000
  length: 0x006f9000
Region mismatch between pd and SI_PDR
 Descriptor region pd:
  offset: 0x003fc000
  length: 0x00004000
 FMAP area SI_PDR:
  offset: 0x007fc000
  length: 0x00004000