Porting Linux to a new architecture involves creating architecture-specific directories for kernel code, memory management code, boot code, and machine-specific platforms. It requires implementing kernel and memory management interfaces and modifying makefiles and kernel configuration files. Initial porting work focuses on basic CPU and architecture initialization functions.

1. Architecture Porting
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2. What to Expect?
Porting Linux for a new Architecture
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3. Architecture Branch
20+ architectures supported in Linux
Each typically contains
Makefile, Kconfig
kernel (Arch specific Kernel code)
mm (Arch specific MM code)
boot (for specific bootable target)
mach-* (LSPs)
configs (default configs)
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4. Porting to a New CPU
Create a similar directory structure
Do all the architecture specific implementations
for
Kernel interfaces
Memory Management interfaces
Create/Modify Makefile & Kconfig, as required
LSPs & configs would be added as & when the
porting for the specific board is done
Significantly challenging, if totally afresh
Typically, start from a closer reference
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5. Pre-requisites & Assumptions
System RAM is already initialized
System Memory Map is already initialized
Optionally, Serial port is configured
For early kernel boot messages
Have a closest baseline Kernel
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6. Starting Point
Modify the Architecture specific code
For example
For PPC, it is under arch/ppc/platforms
For MPC5200, the file is lite5200.c
A typical set would include few structures & functions
Typical Functions
*show_cpuinfo – CPU info texts
*map_irq – Hardware specific interrupt logic routing
*setup_cpu – CPU specific init
*setup_arch – Architecture specific init
platform_init – Board-specific init
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7. Recall the Startup Flow
machine_init
Called from arch/<arch>/kernel/head*.S
Parameter Setup / Passing before calling machine_init
These are typically setup by the bootloader
Called before MMU init. So, calls from it are memory restricted
Code is in arch/<arch>/kernel/setup.c
Calls platform_init with the parameters as is
platform_init
Sets up Board Information Structure (BIS)
If initrd, Start & End addresses of ramdisk image are saved
Store the Kernel Command Line parameters
This code should be taking care of “Early Variable Access”
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8. Early Variable Access
Kernel statically linked to well known, user
configured base address
KERNELBASE (Typically, 0xC0000000)
Kernel is relocated to RAM (usually to 0)
After MMU is enabled, this works all fine
But before that, it should be relocatable, and
access to symbols should be fixed up
And, hence calls for a particular way of for the
early variable access
By subtracting the offset KERNELBASE
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9. Setting up BIS
Usual ways
struct bd_info (From U-boot)
Address in r3 on PPC
struct bi_record (Attempt to unify)
Found by looking for a special tag
Contains stuff like
Command Line Arguments
Start & End address of initrd image
Machine Type
Memory Info
Optionally, many more hardware information
May or may not be used by the LSP
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10. Final Steps
Other Generic Architecture specific Functions
needed, could be
machine_restart
machine_power_off
machine_halt
Modify the preocessor / architecture specific C file
& header file as per
Hardware specifications
Schematics
Any other relevant hardware platform data
Update the Kconfig & Makefile, as appropriate
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11. What all have we learnt?
Porting Linux for a new Architecture
In particular, for a new CPU
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12. Any Queries?
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