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       1                 :            : // SPDX-License-Identifier: GPL-2.0
       2                 :            : #include <linux/kernel.h>
       3                 :            : #include <linux/init.h>
       4                 :            : #include <linux/memblock.h>
       5                 :            : 
       6                 :            : #include <asm/setup.h>
       7                 :            : #include <asm/bios_ebda.h>
       8                 :            : 
       9                 :            : /*
      10                 :            :  * This function reserves all conventional PC system BIOS related
      11                 :            :  * firmware memory areas (some of which are data, some of which
      12                 :            :  * are code), that must not be used by the kernel as available
      13                 :            :  * RAM.
      14                 :            :  *
      15                 :            :  * The BIOS places the EBDA/XBDA at the top of conventional
      16                 :            :  * memory, and usually decreases the reported amount of
      17                 :            :  * conventional memory (int 0x12) too.
      18                 :            :  *
      19                 :            :  * This means that as a first approximation on most systems we can
      20                 :            :  * guess the reserved BIOS area by looking at the low BIOS RAM size
      21                 :            :  * value and assume that everything above that value (up to 1MB) is
      22                 :            :  * reserved.
      23                 :            :  *
      24                 :            :  * But life in firmware country is not that simple:
      25                 :            :  *
      26                 :            :  * - This code also contains a quirk for Dell systems that neglect
      27                 :            :  *   to reserve the EBDA area in the 'RAM size' value ...
      28                 :            :  *
      29                 :            :  * - The same quirk also avoids a problem with the AMD768MPX
      30                 :            :  *   chipset: reserve a page before VGA to prevent PCI prefetch
      31                 :            :  *   into it (errata #56). (Usually the page is reserved anyways,
      32                 :            :  *   unless you have no PS/2 mouse plugged in.)
      33                 :            :  *
      34                 :            :  * - Plus paravirt systems don't have a reliable value in the
      35                 :            :  *   'BIOS RAM size' pointer we can rely on, so we must quirk
      36                 :            :  *   them too.
      37                 :            :  *
      38                 :            :  * Due to those various problems this function is deliberately
      39                 :            :  * very conservative and tries to err on the side of reserving
      40                 :            :  * too much, to not risk reserving too little.
      41                 :            :  *
      42                 :            :  * Losing a small amount of memory in the bottom megabyte is
      43                 :            :  * rarely a problem, as long as we have enough memory to install
      44                 :            :  * the SMP bootup trampoline which *must* be in this area.
      45                 :            :  *
      46                 :            :  * Using memory that is in use by the BIOS or by some DMA device
      47                 :            :  * the BIOS didn't shut down *is* a big problem to the kernel,
      48                 :            :  * obviously.
      49                 :            :  */
      50                 :            : 
      51                 :            : #define BIOS_RAM_SIZE_KB_PTR    0x413
      52                 :            : 
      53                 :            : #define BIOS_START_MIN          0x20000U        /* 128K, less than this is insane */
      54                 :            : #define BIOS_START_MAX          0x9f000U        /* 640K, absolute maximum */
      55                 :            : 
      56                 :         28 : void __init reserve_bios_regions(void)
      57                 :            : {
      58                 :         28 :         unsigned int bios_start, ebda_start;
      59                 :            : 
      60                 :            :         /*
      61                 :            :          * NOTE: In a paravirtual environment the BIOS reserved
      62                 :            :          * area is absent. We'll just have to assume that the
      63                 :            :          * paravirt case can handle memory setup correctly,
      64                 :            :          * without our help.
      65                 :            :          */
      66         [ +  - ]:         28 :         if (!x86_platform.legacy.reserve_bios_regions)
      67                 :            :                 return;
      68                 :            : 
      69                 :            :         /*
      70                 :            :          * BIOS RAM size is encoded in kilobytes, convert it
      71                 :            :          * to bytes to get a first guess at where the BIOS
      72                 :            :          * firmware area starts:
      73                 :            :          */
      74                 :         28 :         bios_start = *(unsigned short *)__va(BIOS_RAM_SIZE_KB_PTR);
      75                 :         28 :         bios_start <<= 10;
      76                 :            : 
      77                 :            :         /*
      78                 :            :          * If bios_start is less than 128K, assume it is bogus
      79                 :            :          * and bump it up to 640K.  Similarly, if bios_start is above 640K,
      80                 :            :          * don't trust it.
      81                 :            :          */
      82         [ +  - ]:         28 :         if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX)
      83                 :         28 :                 bios_start = BIOS_START_MAX;
      84                 :            : 
      85                 :            :         /* Get the start address of the EBDA page: */
      86         [ -  + ]:         28 :         ebda_start = get_bios_ebda();
      87                 :            : 
      88                 :            :         /*
      89                 :            :          * If the EBDA start address is sane and is below the BIOS region,
      90                 :            :          * then also reserve everything from the EBDA start address up to
      91                 :            :          * the BIOS region.
      92                 :            :          */
      93         [ -  + ]:         28 :         if (ebda_start >= BIOS_START_MIN && ebda_start < bios_start)
      94                 :          0 :                 bios_start = ebda_start;
      95                 :            : 
      96                 :            :         /* Reserve all memory between bios_start and the 1MB mark: */
      97                 :         28 :         memblock_reserve(bios_start, 0x100000 - bios_start);
      98                 :            : }