Difference between revisions of "Unit BootRPi2"

Latest revision as of 01:18, 22 April 2022

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Description

Ultibo Initialization code for Raspberry Pi 2 unit

Raspberry Pi 2

SoC: Broadcom BCM2836

CPU: Cortex A7 (ARMv7) (4 @ 900MHz) Cache: L1 32KB (Per Core)/L2 512KB (Shared by all Cores)

FPU: VFPV3

GPU: Broadcom VideoCore IV (VC4)

RAM: 1GB

USB: Synopsys DesignWare Hi-Speed USB 2.0 On-The-Go Controller (DWCOTG)

LAN: SMSC LAN9514 (SMSC95XX)

SD/MMC: Arasan (BCM2709)

WiFi: (None)

Bluetooth: (None)

Other: GPIO/SPI/I2C/I2S/PL011 (UART)/PWM/SMI/Watchdog (PM)/Random(RNG)/Timer

Boot RPi2

The boot loader on the Raspberry Pi 2 will load this code at address 0x00008000 onwards and set the following registers before jumping to this code.

R0 - Zero

R1 - Machine Type (Raspberry Pi 2 or BCM2709 = 0x0C42)

R2 - Address of the ARM Tags structure (Normally 0x0100)

On entry to this code the processor will be in the following state:

World - Non Secure (Firmware causes a switch to Non Secure before jumping to Startup code)

Mode - Supervisor (ARM_MODE_SVC) Note: Firmware later than 2/10/2015 will boot in Hypervisor mode (ARM_MODE_HYP)

MMU - Disabled

FPU - Disabled

L1 Data Cache - Enabled (Firmware enabled prior to Non Secure switch)

L1 Instruction Cache - Enabled (Firmware enabled prior to Non Secure switch)

Branch Predication - Disabled

Unaligned Data Access - Enabled (Always enabled on ARMv7)

SMP Coherence - Enabled (Firmware enabled prior to Non Secure switch)

If the processor is in Hypervisor mode (Firmware behaviour after 2/10/2015) then Ultibo switches it to Supervisor mode during initial boot.

If the configuration option ARMSecureBoot is set to 1 then Ultibo switches the processor back to Secure world during initial boot (see note below).

Ultibo then switches the processor to System mode for all operations and remains in either the Secure or Non Secure World as per the option above.

The initialization process enables the MMU, FPU, L1 Cache and other performance optimizations.

Returning to Secure World:

The Raspberry Pi 2 firmware always switches to Non Secure world in order to:

a) Reset CNTVOFF (Virtual Offset register) to zero

b) Switch to Hypervisor mode (Firmware later than 2/10/2015)

It is not normally possible (by design) to return from the Non Secure world to the Secure world except by invoking the Secure Monitor which then handles any requests in the Secure world.

However due to the fact that this switch is performed by the boot loader code (loaded between 0x00000000 and 0x000000FF by the firmware) and that the code appears to set the secure, non secure and monitor vector base addresses to 0x00000000 and because all memory is writable from non secure at boot (the MMU is disabled) then the option exists to return to the Secure world as follows:

- Install a new Secure Monitor Call vector at the appropriate point in the vector table

- Clean the data cache by Modified Virtual Address at address 0x00000000

- Perform a data synchronisation barrier

- Invalidate the entire instruction cache

- Perform a data synchronisation barrier

- Perform an instruction synchronisation barrier

- Perform a Secure Monitor Call (SMC)

- From within the newly installed Secure Monitor Call read the Secure Configuration Register (SCR) mask off the Non Secure (NS) bit, rewrite the SCR and then return to Supervisor mode.

Secondary cores also switch back to the Secure world using the same technique however they do not need to install the new SMC vector and instead use the one installed by the primary core.

Firmware changes from October 2020 disable the ability to perform the SMC instruction from non secure mode which means this technique will no longer work with the default firmware. To allow continued support for returning to the Secure world a custom ARM boot stub is now included with Ultibo that allows the SMC instruction to be performed.

This custom stub contains a marker that allows the boot process to detect it and disable the attempt to switch back to the Secure world if the default boot stub is used instead.

As the default boot stub continues to evolve in response to demands from the Linux community this technique will allow Ultibo to continue to provide the features we want without having to make frequent changes to combat items that are beyond our control.

Thanks to the Circle project for introducing the custom boot stub technique to work around these changes.

Constants

None defined

Type definitions

None defined

Public variables

None defined

Function declarations

Boot functions

procedure Startup; assembler; nostackframe; public name '_START';

procedure Vectors; assembler; nostackframe;

procedure SecureVectors; assembler; nostackframe;

procedure SecureMonitor; assembler; nostackframe;

procedure StartupSwitch; assembler; nostackframe;

procedure StartupSecure; assembler; nostackframe;

procedure StartupHandler; assembler; nostackframe;

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