关于DMA zone的疑问

junnyg

回复 4# chishanmingshen
在C0001000 - C0FFFFFF吧，物理内存的前16M（除起始的0x1000之外）作为DMA区域，一一映射后就到C0000000之后的16M位置了，但内核使用它的方式不是访问这块虚拟地址，而是基于ZONE_DMA的页分配进行管理的，而且除了一些ISA设备需要使用这块内存作为DMA内存，现在的设备一般都支持全地址的DMA访问


   

junnyg

内核代码段的加载位置是PAGE_OFFSET + CONFIG_PHYSICAL_START吧

1. config PHYSICAL_START
   
2.         hex "Physical address where the kernel is loaded" if (EMBEDDED || CRASH_DUMP)
   
3.         default "0x1000000"
   
4.         ---help---
   
5.           This gives the physical address where the kernel is loaded.
   
6. 
   
7.           If kernel is a not relocatable (CONFIG_RELOCATABLE=n) then
   
8.           bzImage will decompress itself to above physical address and
   
9.           run from there. Otherwise, bzImage will run from the address where
   
10.           it has been loaded by the boot loader and will ignore above physical
    
11.           address.
    
12. 
    
13.           In normal kdump cases one does not have to set/change this option
    
14.           as now bzImage can be compiled as a completely relocatable image
    
15.           (CONFIG_RELOCATABLE=y) and be used to load and run from a different
    
16.           address. This option is mainly useful for the folks who don't want
    
17.           to use a bzImage for capturing the crash dump and want to use a
    
18.           vmlinux instead. vmlinux is not relocatable hence a kernel needs
    
19.           to be specifically compiled to run from a specific memory area
    
20.           (normally a reserved region) and this option comes handy.
    
21. 
    
22.           So if you are using bzImage for capturing the crash dump,
    
23.           leave the value here unchanged to 0x1000000 and set
    
24.           CONFIG_RELOCATABLE=y.  Otherwise if you plan to use vmlinux
    
25.           for capturing the crash dump change this value to start of
    
26.           the reserved region.  In other words, it can be set based on
    
27.           the "X" value as specified in the "crashkernel=YM@XM"
    
28.           command line boot parameter passed to the panic-ed
    
29.           kernel. Please take a look at Documentation/kdump/kdump.txt
    
30.           for more details about crash dumps.
    
31. 
    
32.           Usage of bzImage for capturing the crash dump is recommended as
    
33.           one does not have to build two kernels. Same kernel can be used
    
34.           as production kernel and capture kernel. Above option should have
    
35.           gone away after relocatable bzImage support is introduced. But it
    
36.           is present because there are users out there who continue to use
    
37.           vmlinux for dump capture. This option should go away down the
    
38.           line.
    
39. 
    
40.           Don't change this unless you know what you are doing.

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详见http://bbs.chinaunix.net/thread-2029114-1-1.html