ARM平台下Uboot启动Linux内核前的必备条件

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Linux内核是由Uboot进行调用的，在执行内核前，需要满足什么条件呢？

我们来看下内核文档Documentation/ARM/booting。内核版本为3.7.6。

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' r& _2 f% s7 _+ e& @5. Calling the kernel image
---------------------------

Existing boot loaders:        MANDATORY
; a3 r+ L( D- }! _6 nNew boot loaders:        MANDATORY
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There are two options for calling the kernel zImage.  If the zImage
7 E+ z+ C2 V4 _, K: e$ c  D- bis stored in flash, and is linked correctly to be run from flash,
then it is legal for the boot loader to call the zImage in flash
directly.

: u. k0 R5 O6 r7 N2 A6 SThe zImage may also be placed in system RAM (at any location) and
called there.  Note that the kernel uses 16K of RAM below the image
1 n- x% e( @' ?. v4 m; H) \, l$ mto store page tables.  The recommended placement is 32KiB into RAM.

In either case, the following conditions must be met:

& |, `% h" T, w6 A- Quiesce all DMA capable devices so that memory does not get
' N7 r" |( _- {8 a/ n) H* m( h* _  corrupted by bogus network packets or disk data. This will save
" W' |/ X) z- w7 f  you many hours of debug.
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- CPU register settings
) K  c( V3 p3 T' }, c+ l  r0 = 0,
3 i! n; s. l( L5 R6 }2 g/ E  r1 = machine type number discovered in (3) above.
/ n& q3 e6 I4 b% ~% T  r2 = physical address of tagged list in system RAM, or
       physical address of device tree block (dtb) in system RAM
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- CPU mode
& m: b% b; P' m/ z% E, |7 j  All forms of interrupts must be disabled (IRQs and FIQs)

  For CPUs which do not include the ARM virtualization extensions, the
. \+ w, [; `; D; s  CPU must be in SVC mode.  (A special exception exists for Angel)
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  CPUs which include support for the virtualization extensions can be
& P1 K# o$ O- _, e' T6 e- ^  T% z# P  entered in HYP mode in order to enable the kernel to make full use of
1 H! o; J( f% U, K% k* R: C  these extensions.  This is the recommended boot method for such CPUs,
$ w! O" Y2 y" y3 s# N' b0 `2 z  unless the virtualisations are already in use by a pre-installed
! |: r& C. v+ p* A- f; Q& s; \  hypervisor.
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/ m0 R0 W' N# U9 y3 Y9 O. ?, O6 {  If the kernel is not entered in HYP mode for any reason, it must be
# ~) x8 c2 N0 X1 m1 R9 c+ k: S. L  entered in SVC mode.

- Caches, MMUs
  The MMU must be off.
& F+ }, w; U- [' b0 L3 j  Instruction cache may be on or off.
* Z; |1 E$ G. V  Data cache must be off.
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  If the kernel is entered in HYP mode, the above requirements apply to
  the HYP mode configuration in addition to the ordinary PL1 (privileged
* A  |8 O  r1 H! j1 a5 h+ R  kernel modes) configuration.  In addition, all traps into the
  hypervisor must be disabled, and PL1 access must be granted for all
  peripherals and CPU resources for which this is architecturally
+ p8 K" \- X+ ?: ~  possible.  Except for entering in HYP mode, the system configuration
  should be such that a kernel which does not include support for the
  virtualization extensions can boot correctly without extra help.
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- The boot loader is expected to call the kernel image by jumping
# x1 P! G  p8 c5 P0 f# f  directly to the first instruction of the kernel image.
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  On CPUs supporting the ARM instruction set, the entry must be
* P1 X. O8 k9 s8 P  made in ARM state, even for a Thumb-2 kernel.
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' A" C1 k8 z! A3 W; _; K5 q; B7 F4 R  On CPUs supporting only the Thumb instruction set such as
4 x" n, B# p3 K" v2 t  Cortex-M class CPUs, the entry must be made in Thumb state.
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# w. Y7 ^; }+ c7 X这里，省略了该文档中不感兴趣的部分。

根据文档，
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第一， 必须禁止所有使用DMA的设备。 感觉这个有点多余，Uboot使用的设备，包括网卡之类的，一般都不会使用DMA。

第二，必须设置r0，r1和r2寄存器为相应的值。这个是由下面的函数调用实现的。

: {% Z# y- @# ^0 u& B. P  G7 q该函数在Uboot/lib_arm/armlinux.c的do_bootm_linux函数中被调用。

    theKernel (0, bd->bi_arch_number, bd->bi_boot_params);

根据ATPCS的规则，函数的实参将分别传入r0到r3寄存器。这里有正好传入三个参数对应着0，机器码和参数列表的地址。
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第三，必须禁止中断，并且将CPU置入SVC（管理）模式。这些工作已经由start.S完成了。

第四，必须禁止数据cache和MMU，可以使能或禁止指令cache。这个是由下面的函数调用实现的。

该函数位在Uboot/lib_arm/armlinux.c的do_bootm_linux函数中被调用。

5 W8 v4 w  v$ m* r, z: J( n* ]! }下列函数位于：Uboot/cpu/arm920t/cpu.c中。

int cleanup_before_linux (void)
{
+ s# M; a5 @) W: m- [9 K        /*
% X, \  S6 `( Y+ h) B. g         * this function is called just before we call linux
         * it prepares the processor for linux
: c* \8 j; k& ?1 T) |         *
. v1 [2 O& q. n2 K. ]         * we turn off caches etc ...
  D0 N. r# E! k" B6 S: O# v" \         */

/ L+ B& }6 g! \3 ~1 X+ {. C/ L        unsigned long i;
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; o" u- O, E4 a* w, V, O4 z        disable_interrupts ();

        /* turn off I/D-cache */
8 K0 ^+ ?, Y! F" |        asm ("mrc p15, 0, %0, c1, c0, 0":"=r" (i));
        i &= ~(C1_DC | C1_IC);
        asm ("mcr p15, 0, %0, c1, c0, 0": :"r" (i));

        /* flush I/D-cache */
, f( B7 i* o) U( d' R        i = 0;
        asm ("mcr p15, 0, %0, c7, c7, 0": :"r" (i));
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) p" \8 [0 o3 C( T$ m& n        return (0);
}
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可以看出该函数禁止了数据和指令cache，同时冲刷了cache。
这里并没有禁止MMU，MMU禁止是在start.S中完成的。

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ARM平台下Uboot启动Linux内核前的必备条件