coreboot-libre-fam15h-rdimm/Documentation/soc/amd/family17h.md

# AMD Family 17h in coreboot

## Abstract

Beginning with Family 17h products (a.k.a. “Zen” cores), AMD
changed their paradigm for initializing the system and this requires
major modifications to the execution flow of coreboot.  This file
discusses the new boot flow, and challenges, and the tradeoffs of the
initial port into coreboot.

## Introduction

Family 17h products are x86-based designs.  This documentation assumes
familiarity with x86, its reset state and its early initialization
requirements.

To the extent necessary, the role of the Platform Security Processor
(a.k.a. PSP) in system initialization is addressed here.  AMD has
historically required an NDA for access to the PSP
specification<sup>1</sup>.  coreboot relies on util/amdfwtool to build
the structures and add various other firmware to the final image.  The
Family 17h PSP design guide adds a new BIOS Directory Table, similar to
the PSP Directory Table.

Support in coreboot for modern AMD products is based on AMD’s
reference code:  AMD Generic Encapsulated Software Architecture
(AGESA<sup>TM</sup>).  AGESA contains the technology for enabling DRAM,
configuring proprietary core logic, assistance with generating ACPI
tables, and other features.

AGESA for products earlier than Family 17h is known as v5 or
Arch2008<sup>2</sup>.  Also note that coreboot currently contains both
open source AGESA and closed source implementations (binaryPI) compiled
from AGESA.

The first AMD Family 17h device ported to coreboot is codenamed
“Picasso”<sup>3</sup>, and will be added to soc/amd/picasso.

## Additional Definitions

*   PSP, Platform Security Processor:  Onboard ARM processor that runs
alongside the main x86 processor; may be viewed as analogous to the
Intel<sup>R</sup> Management Engine
*   FCH, Fusion Control Hub, the logical southbridge within the SOC
*   ABL - AGESA Bootloader - Processor initialization code that runs on
the PSP
*   PSP Directory Table - A structured list of pointers to PSP firmware
and other controller binaries
*   BIOS Directory Table - A structured list of pointers to BIOS
related firmware images
*   Embedded Firmware Structure - Signature and pointers used by the
PSP to locate the PSP Directory Table and BIOS Directory Table; these
items are generated during coreboot build and are located in the SPI ROM
*   Verstage - The code to verify the firmware contained in the
writable section of the SPI ROM
*   APCB - AMD PSP Customization Block - A binary containing PSP and
system configuration preferences (analogous to v5 BUILDOPT_ options),
and generated by APCBTool to be added to coreboot/utils later
*   APOB - AGESA PSP Output Buffer - A buffer in main memory for
storing AGESA BootLoader output.  There are no plans for this to be
parsed by coreboot

## Problem Statements

AMD has ported early AGESA features to the PSP, which now discovers,
enables and trains DRAM.  Unlike any other x86 device in coreboot, a
Picasso system has DRAM online prior to the first instruction fetch.

Cache-as-RAM (CAR) is no longer a supportable feature in AMD hardware.
Early code expecting CAR behavior <span
style="text-decoration:underline;">must</span> account for writes
escaping the L2 cache and going to DRAM.

Without any practical need for CAR, or DRAM initialization, coreboot
should arguably skip bootblock and romstage, and possibly use ramstage
as the BIOS image.  This approach presents a number of challenges:

*   At the entry of ramstage, x86 processors are in flat protected
mode.  Picasso’s initial state is nearly identical to any other x86
at reset, except its CS shadow register’s base and limit put its
execution within DRAM, not at 0xfffffff0.  Picasso requires initial
programming and entry into protected mode prior to ramstage.
*   coreboot expects cbmem initialization during romstage.

AGESA supporting Picasso is now at v9.  Unlike Arch2008, which defines
granular entry points for easy inclusion to a legacy BIOS, v9 is
rewritten for compilation into a UEFI.  The source follows UEFI
standards, i.e. assumes the presence of UEFI phases, implements
dependency expressions, much functionality is rewritten as libraries,
etc.  It would, in no way, fit into the v5 model used in coreboot.

*   For the foreseeable future, AGESA source will distributed only
under NDA.

## Basic Pre-x86 Boot Flow

The following steps occur prior to x86 processor operation.

*   System power on
*   PSP executes immutable on-chip boot ROM
*   PSP locates the Embedded Firmware Table and PSP Directory Table in
the SPI ROM
*   PSP verifies and executes the PSP off-chip bootloader
*   ChromeOS systems:
    *   Off-chip bootloader attempts to locate verstage via the RO BIOS
Directory Table
    *   If verstage is not found, booting continues with ABLs below
    *   Verstage initializes, setting up GPIOs, UART if needed,
communication path to the EC, and the SPI controller for direct access
to the flash device.
    *   Verstage verifies the RW sections (as is typically performed by
the main processor)
    *   Verstage locates the Embedded Firmware Directory within the
verified FMAP section and passes a pointer to the PSP bootloader.  If
the verification fails, it passes a pointer to the RO header to the
bootloader.
*   PSP parses the PSP Directory Table to find the ABLs and executes
them
*   An ABL parses the APCB for system configuration preferences
*   An ABL initializes system main memory, locates the compressed BIOS
image in the SPI ROM, and decompresses it into DRAM
*   An ABL writes the APOB to DRAM  for consumption by the x86-based
AGESA
*   PSP releases the x86 processor from reset.  The x86 core fetches
and executes instructions from the reset vector

## Picasso Reset Vector and First Instructions

As mentioned above, prior to releasing the x86 main core from reset,
the PSP decompresses a BIOS image into DRAM.  The PSP uses a specific
BIOS Directory Table entry type to determine the source address (in
flash), the destination address (in DRAM), and the destination size.
The decompressed image is at the top of the destination region.  The
PSP then

Calculates the x86 reset vector as

    reset_vector	= dest_addr + dest_size - 0x10

Sets x86 CS descriptor shadow register to

    base		= dest_addr + dest_size - 0x10000
    limit		= 0xffff

Like all x86 devices, the main core is allowed to begin executing
instructions with

    CS:IP		= 0xf000:0xfff0

For example, assume the BIOS Directory Table indicates

    destination	= 0x9b00000
    size		= 0x300000

… then the BIOS image is placed at the topmost position the region
0x9b00000-0x9dfffff and

    reset_vector	= 0x9dffff0
    CS_shdw_base	= 0x9df0000
    CS:IP		= 0xf000:0xfff0

Although the x86 behaves as though it began executing at 0xfffffff0
i.e. 0xf000:0xfff0, the initial GDT load must use the physical address
of the table and not the typical CS-centric address.  And, the first
jump to protected mode must jump to the physical address in DRAM.  Any
code that is position-dependent must be linked to run at the final
destination.

## Initial coreboot Implementation

Supporting Picasso doesn’t fit well with many of the coreboot
assumptions.  Initial porting shall attempt to fit within existing
coreboot paradigms and make minimal changes to common code.

### CAR and bootblock

The coreboot bootblock contains features Picasso doesn’t require or
can’t use, and is assumed to execute in an unusable location.
Picasso’s requirement for bootblock in coreboot will be eliminated.

### Hybrid romstage

Picasso’s x86 reset state doesn’t meet the coreboot expectations
for jumping directly to ramstage.  The primary feature of romstage is
also not needed, however there are other important features that are
typically in romstage that Picasso does need.

The romstage architecture is designed around the presence of CAR.
Several features implement ROMSTAGE_CBMEM_INIT_HOOK, expecting to move
data from CAR to cbmem.  The hybrid romstage consumes DRAM for the
purpose of implementing the expected CAR storage.  This region as well
as the DRAM where romstage is decompressed must be reserved and
unavailable to the OS.

The initial Picasso port implements a hybrid romstage that contains the
first instruction fetched at the reset vector.  It minimally configures
flat protected mode, initializes cbmem, then loads the next stage.
Future work will consider breaking the dependencies mentioned above
and/or potentially loading ramstage directly from the PSP.

## AGESA v9 on Picasso

Due to the current inability to publish AGESA source, a pre-built
binary solution remains a requirement.  The rewrite from v5 to v9 for
direct inclusion into UEFI source makes modifying it for conforming to
the existing v5 interface impractical.

Given the UEFI nature of modern AGESA, and the existing open source
work from Intel, Picasso shall support AGESA via an FSP-like prebuilt
image.  The Intel Firmware Support Package<sup>4</sup> combines
reference code with EDK II source to create a modular image with
discoverable entry points.  coreboot source already contains knowledge
of FSP, how to parse it, integrate it, and how to communicate with it.

## Footnotes

1. “AMD Platform Security Processor BIOS Architecture Design Guide
for AMD Family 17h Processors” (PID #55758) and “AMD Platform
Security Processor BIOS Architecture Design Guide” (PID #54267) for
earlier products
2. [https://www.amd.com/system/files/TechDocs/44065_Arch2008.pdf](https://www.amd.com/system/files/TechDocs/44065_Arch2008.pdf)
3. [https://en.wikichip.org/wiki/amd/cores/picasso](https://en.wikichip.org/wiki/amd/cores/picasso)
4. [https://www.intel.com/content/www/us/en/intelligent-systems/intel-firmware-support-package/intel-fsp-overview.html](https://www.intel.com/content/www/us/en/intelligent-systems/intel-firmware-support-package/intel-fsp-overview.html)