From mboxrd@z Thu Jan  1 00:00:00 1970
From: prakashk75@gmail.com (Prakash K.B.)
Date: Wed, 30 Mar 2011 20:49:39 +0530
Subject: ARM : Kernel : Setting up of MMU in head.S
Message-ID: <AANLkTikbzjKepm7+ZhmA-3Vq33vADPaXrVdJGRc284OS@mail.gmail.com>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

Hello.

Please do not hesitate to let me know if this must be posted elsewhere.

I have been trying to understand the code that sets up the MMU. I do have a
fair understanding of the way MMU is meant to be setup, but something in the
kernel code is tripping me.

The code that kickstarts setting up of MMU is __create_page_tables in
/arch/arm/kernel/head.S.

This code is position independent.

It basically
- Reserves 16KB of memory in RAM just before the start of the uncompressed
kernel.
- Clears the 16KB meant to serve as L1 lookup table containing 4096 entries
- Creates a section entry in L1 table for the kernel code to be mapped into
physical memory

It is this creation of section entry code that is puzzling me.

The index used to program this entry is based on physical address of the
kernel base.

The way it ought to work is this.  When the CPU issues a virtual address,
the top bits are used as an index into this L1 table and then through a
couple of table walk throughs, the physical address is arrived at. So the
index used to program the L1 table ought to have been

Now look at this code.

__create_page_tables:
    pgtbl    r4                @ page table address

    /*
     * Clear the 16K level 1 swapper page table
     */
    mov    r0, r4               @r0 = 0x80004000
    mov    r3, #0
    add    r6, r0, #0x4000      @r6 = 0x80008000
1:    str    r3, [r0], #4
    str    r3, [r0], #4
    str    r3, [r0], #4
    str    r3, [r0], #4
    teq    r0, r6
    bne    1b


  /* r10 contains proc_info pointer */
    ldr    r7, [r10, #PROCINFO_MM_MMUFLAGS] @ mm_mmuflags

    /*
     * Create identity mapping to cater for __enable_mmu.
     * This identity mapping will be removed by paging_init().
     */
    adr    r0, __enable_mmu_loc
    ldmia    r0, {r3, r5, r6}

    sub    r0, r0, r3            @ virt->phys offset
    add    r5, r5, r0            @ phys __enable_mmu
    add    r6, r6, r0            @ phys __enable_mmu_end

    mov    r5, r5, lsr #20
    mov    r6, r6, lsr #20

1:  orr    r3, r7, r5, lsl #20        @ flags + kernel base
    str    r3, [r4, r5, lsl #2]        @ identity mapping

    teq    r5, r6
    addne    r5, r5, #1            @ next section
    bne    1b


The 2 lines above
1:  orr    r3, r7, r5, lsl #20        @ flags + kernel base ==> Correct
    str    r3, [r4, r5, lsl #2]        @ identity mapping  ==> ??

create a section entry using index based on physical address.

Am I missing something here?

Regards,
Prakash
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