arch/x86/kernel/head.c

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