arch/x86/kernel/head.c

   1 #include <linux/kernel.h>
   2 #include <linux/init.h>
   3 #include <linux/memblock.h>
   4
   5 #include <asm/setup.h>
   6 #include <asm/bios_ebda.h>
   7
   8 #define BIOS_LOWMEM_KILOBYTES 0x413
   9
  10 /*
  11  * The BIOS places the EBDA/XBDA at the top of conventional
  12  * memory, and usually decreases the reported amount of
  13  * conventional memory (int 0x12) too. This also contains a
  14  * workaround for Dell systems that neglect to reserve EBDA.
  15  * The same workaround also avoids a problem with the AMD768MPX
  16  * chipset: reserve a page before VGA to prevent PCI prefetch
  17  * into it (errata #56). Usually the page is reserved anyways,
  18  * unless you have no PS/2 mouse plugged in.
  19  */
  20 void __init reserve_ebda_region(void)
  21 {
  22         unsigned int lowmem, ebda_addr;
  23
  24         /* To determine the position of the EBDA and the */
  25         /* end of conventional memory, we need to look at */
  26         /* the BIOS data area. In a paravirtual environment */
  27         /* that area is absent. We'll just have to assume */
  28         /* that the paravirt case can handle memory setup */
  29         /* correctly, without our help. */
  30         if (paravirt_enabled())
  31                 return;
  32
  33         /* end of low (conventional) memory */
  34         lowmem = *(unsigned short *)__va(BIOS_LOWMEM_KILOBYTES);
  35         lowmem <<= 10;
  36
  37         /* start of EBDA area */
  38         ebda_addr = get_bios_ebda();
  39
  40         /* Fixup: bios puts an EBDA in the top 64K segment */
  41         /* of conventional memory, but does not adjust lowmem. */
  42         if ((lowmem - ebda_addr) <= 0x10000)
  43                 lowmem = ebda_addr;
  44
  45         /* Fixup: bios does not report an EBDA at all. */
  46         /* Some old Dells seem to need 4k anyhow (bugzilla 2990) */
  47         if ((ebda_addr == 0) && (lowmem >= 0x9f000))
  48                 lowmem = 0x9f000;
  49
  50         /* Paranoia: should never happen, but... */
  51         if ((lowmem == 0) || (lowmem >= 0x100000))
  52                 lowmem = 0x9f000;
  53
  54         /* reserve all memory between lowmem and the 1MB mark */
  55         memblock_reserve(lowmem, 0x100000 - lowmem);
  56 }