Nov, 2004 Amit Bhor Codito Technologies www'codito'com

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Nov, 2004Amit Bhor Codito Technologies (
www.codito.com )
Porting Linux to 32 bit RISC processors
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The Start

• What do you need to start a Linux port
• A new 32 bit architecture ( of course ) and a
  development board
• The GNU tool chain for the new architecture
• A debugger, preferably over JTAG
• The sources to the linux kernel
• Architecture caveats
• Memory Management Unit ?
• TRAP or software interrupt mechanism ?
• Mis-aligned data access ?
• Defined ABI
• Similar architectures
• Cache aliasing

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Where to add support

• Architecture specific code
• TOPDIR/arch
• alpha arm ia64 mips parisc ppc64 s390x
  sparc x86_64
• cris i386 m68k mips64 ppc s390 sh
  sparc64
• TOPDIR/include/asm-
• asm-alpha asm-generic asm-mips asm-ppc64
  asm-sparc
• asm-i386 asm-mips64 asm-s390 asm-sparc64
• asm-arm asm-ia64 asm-parisc asm-s390x
  asm-x86_64
• asm-cris asm-m68k asm-ppc asm-sh
• Device drivers serial, ethernet etc.
• Add new directories for your new processor
• TOPDIR/arch/arc
• TOPDIR/arch/asm-arc

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Code Layout

• Multiple platforms / boards
• Multiple processors variants / versions
• Design the code layout and Architecture HAL
  accordingly
• Main level layout
• boot config.in defconfig kernel lib Makefile
  mm proc tools
• Second level layout
• Processor specific arc700 arc600 a5 a4
• Platform specific ( none for ARC )

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Start Coding

• Setup Makefiles
• Kernel linker script linux.lds.in
• Reuse as much as possible
• Start with generic implementations
• TOPDIR/proc/arc700/head.S
• Read processor identity
• Initialize BSS
• Setup frame pointer and stack pointer for the
  kernel
• mov fp, 0
• mov sp, _init_task_union 8192
• Jump to start of the kernel initialization
  start_kernel()
• sub_s sp, sp, 4
• jal _start_kernel

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Important header files

• TOPDIR/include/asm-arc/system.h
• __cli() , __sti(), __save_flags_cli(),
  __save_flags(), __restore_flags()
• define __save_flags_cli(x) \
• __asm__ __volatile__ ( \
• "lr r20, status32 \n\t" \
• "mov 0, r20 \n\t" \
• "and r20, r20, 1 \n\t" \
• "flag r20 \n\t" \
• "r" (x) \
• "n" ((STATUS_E1_MASK STATUS_E2_MASK)) \
• "r20", "memory")
• Memory barrier
• define mb() __asm__ __volatile__ (""
  "memory")
• The context switching macro switch_to()

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Important header files

• TOPDIR/include/asm-arc/atomic.h
• Macros on top of the routines in system.h
• Use generic atomic_ macros
• TOPDIR/include/asm-arc/entry.h
• Macros to save and restore context
• Caller saved and Callee saved registers
• Interrupt / exception / system call contexts

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Important header files

• TOPDIR/include/asm-arc/ptrace.h
• struct pt_regs
• / this struct defines the way the registers are
  stored on the
• stack during a system call. /
• struct pt_regs
• long stack_place_holder
• long bta / bta_l1, bta_l2, erbta /
• long lp_start
• long lp_end
• long lp_count
• long status32 / status32_l1, status32_l2,
  erstatus /
• long ret / ilink1, ilink2 or eret /
• long blink
• long fp
• long r26 / gp /
• long r12

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Important header files

• byteorder.h little / big endian
• checksum.h start with generic C implementations
• irq.h - irq design , irq register / unregister
  API
• mmu.h
• MMU context ( if supported )
• typedef struct
• unsigned long asid
• ifdef CONFIG_ARC_TLB_DBG
• struct task_struct tsk
• endif
• mm_context_t
• Page Table Entries and associated bit masks
• Hardware supported
• Software emulated

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Important header files

• unistd.h
• System call interface, TRAP mechanism
• uaccess.h
• Access to user space virtual memory
• processor.h
• Page size
• User space process size
• define TASK_SIZE (PAGE_OFFSET -
  (CONFIG_VMALLOC_SIZE 1024 1024)
  ARC_SOME_PADDING)
• struct thread_struct architecture specific
  additions to the linux task ( struct task_struct
  )
• init task and its initializations

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Linux kernel initialization

• TOPDIR/init/main.c
• start_kernel()
• asmlinkage void __init start_kernel(void)
• char command_line
• extern char saved_command_line
• /
• Interrupts are still disabled. Do necessary
  setups, then
• enable them
• /
• lock_kernel()
• printk(linux_banner)
• setup_arch(command_line)
• printk("Kernel command line s\n",
  saved_command_line)
• parse_options(command_line)
• trap_init()
• init_IRQ()
• sched_init()
• softirq_init()

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Linux kernel initialization

• console_init()
• sti()
• calibrate_delay()
• ifdef CONFIG_BLK_DEV_INITRD
• if (initrd_start !initrd_below_start_ok
• initrd_start lt min_low_pfn ltlt PAGE_SHIFT)
• printk(KERN_CRIT "initrd overwritten (0x08lx lt
  0x08lx) - "
• "disabling it.\n",initrd_start,min_low_pfn ltlt
  PAGE_SHIFT)
• initrd_start 0
• endif
• mem_init()
• kmem_cache_sizes_init()
• pgtable_cache_init()
• rest_init()

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Linux kernel initialization

• The first linux thread
• static void rest_init(void)
• kernel_thread(init, NULL, CLONE_FS CLONE_FILES
  CLONE_SIGNAL)
• unlock_kernel()
• current-gtneed_resched 1
• cpu_idle()

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Architecture support

• arc/kernel/setup.c
• setup_arch() Pseudo Code
• Enable I / D caches
• Parse commandline for architecture specific
  parameters
• Probe for available memory
• Register Boot memory (bootmem)
• bootmap_size init_bootmem_node(NODE_DATA(0),
  start_pfn, min_low_pfn, max_low_pfn)
• free_bootmem(PFN_PHYS(start_pfn),
  PFN_PHYS(size))
• reserve_bootmem(PFN_PHYS(start_pfn),
  bootmap_size)
• Initialize Paging paging_init()
• arc/kernel/init_task.c
• The init_task_union
• Associated file_struct , fs_struct and mm_struct

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Architecture support

• arc/kernel/irq.c
• Contains all of the interrupt handling and
  registering functionality
• NR_IRQ macro defined to max number of interrupt
  types
• init_IRQ() called from stat_kernel()
• Initialize interrupt unit
• Initialize / Copy vector table
• Request an interrupt handler
• int request_irq(unsigned int irq, void
  (handler)( int , void , struct pt_regs ),
  unsigned long flags, const char devname, void
  dev_id)
• Free an interrupt handler
• void free_irq(unsigned int irq, void dev_id)
• Common interrupt handler and interrupt
  multiplexer
• void process_interrupt(unsigned int irq, struct
  pt_regs fp)

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Architecture support

• arc/ kernel/time.c
• Has the timer functionality to manage the
  heartbeat or jiffies of the linux kernel
• Timer initialization time_init()
• void __init time_init(void)
• Setup / register irq handler
• Initialize the system timer to interrupt at 100hz
  ( 1 tick every 10ms)
• Timer interrupt handler
• Call do_timer() - architecture independent
  function to handle system tick ( See
  TOPDIR/kernel/timer.c )
• arc/kernel/semaphore.c
• Use generic versions ( ARM, i386 ) initially
• arc/kernel/sys_arc.c
• Remains unchanged across most architectures

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Architecture support

• arc/proc/arc700/entry.S
• System vector table
• System call interface / entry
• Switch from user to kernel stack
• Renable exceptions/interrupt
• Set up parameters (ABI)
• Call system call through the syscall table
• System call exit
• Interrupt exit
• Exception exit
• Syscall table
• System call wrappers passing the pt_regs
  pointer

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Architecture MMU support

• arc/mm/
• Memory initializations
• mem_init()
• paging_init()
• Cache handling
• Keep caches turned off during initial porting
• Translation lookaside buffers (TLB)
• Software managed ?
• ITLB/DTLB miss handlers
• Software emulation of unsupported paging bits.
• Page fault handling
• do_page_fault() - main page fault handler
• Major / minor page faults

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Questions

• Email - amit.bhor_at_codito.com
• ARC linux arc-support_at_codito.com