#include <asm/bios_ebda.h>
/*
+ * This function reserves all conventional PC system BIOS related
+ * firmware memory areas (some of which are data, some of which
+ * are code), that must not be used by the kernel as available
+ * RAM.
+ *
* The BIOS places the EBDA/XBDA at the top of conventional
* memory, and usually decreases the reported amount of
- * conventional memory (int 0x12) too. This also contains a
- * workaround for Dell systems that neglect to reserve EBDA.
- * The same workaround also avoids a problem with the AMD768MPX
- * chipset: reserve a page before VGA to prevent PCI prefetch
- * into it (errata #56). Usually the page is reserved anyways,
- * unless you have no PS/2 mouse plugged in.
+ * conventional memory (int 0x12) too.
+ *
+ * This means that as a first approximation on most systems we can
+ * guess the reserved BIOS area by looking at the low BIOS RAM size
+ * value and assume that everything above that value (up to 1MB) is
+ * reserved.
+ *
+ * But life in firmware country is not that simple:
+ *
+ * - This code also contains a quirk for Dell systems that neglect
+ * to reserve the EBDA area in the 'RAM size' value ...
+ *
+ * - The same quirk also avoids a problem with the AMD768MPX
+ * chipset: reserve a page before VGA to prevent PCI prefetch
+ * into it (errata #56). (Usually the page is reserved anyways,
+ * unless you have no PS/2 mouse plugged in.)
+ *
+ * - Plus paravirt systems don't have a reliable value in the
+ * 'BIOS RAM size' pointer we can rely on, so we must quirk
+ * them too.
+ *
+ * Due to those various problems this function is deliberately
+ * very conservative and tries to err on the side of reserving
+ * too much, to not risk reserving too little.
+ *
+ * Losing a small amount of memory in the bottom megabyte is
+ * rarely a problem, as long as we have enough memory to install
+ * the SMP bootup trampoline which *must* be in this area.
*
- * This functions is deliberately very conservative. Losing
- * memory in the bottom megabyte is rarely a problem, as long
- * as we have enough memory to install the trampoline. Using
- * memory that is in use by the BIOS or by some DMA device
- * the BIOS didn't shut down *is* a big problem.
+ * Using memory that is in use by the BIOS or by some DMA device
+ * the BIOS didn't shut down *is* a big problem to the kernel,
+ * obviously.
*/
-#define BIOS_LOWMEM_KILOBYTES 0x413
-#define LOWMEM_CAP 0x9f000U /* Absolute maximum */
-#define INSANE_CUTOFF 0x20000U /* Less than this = insane */
+#define BIOS_RAM_SIZE_KB_PTR 0x413
-void __init reserve_ebda_region(void)
+#define BIOS_START_MIN 0x20000U /* 128K, less than this is insane */
+#define BIOS_START_MAX 0x9f000U /* 640K, absolute maximum */
+
+void __init reserve_bios_regions(void)
{
- unsigned int lowmem, ebda_addr;
+ unsigned int bios_start, ebda_start;
/*
- * To determine the position of the EBDA and the
- * end of conventional memory, we need to look at
- * the BIOS data area. In a paravirtual environment
- * that area is absent. We'll just have to assume
- * that the paravirt case can handle memory setup
- * correctly, without our help.
+ * NOTE: In a paravirtual environment the BIOS reserved
+ * area is absent. We'll just have to assume that the
+ * paravirt case can handle memory setup correctly,
+ * without our help.
*/
- if (!x86_platform.legacy.ebda_search)
+ if (!x86_platform.legacy.reserve_bios_regions)
return;
- /* end of low (conventional) memory */
- lowmem = *(unsigned short *)__va(BIOS_LOWMEM_KILOBYTES);
- lowmem <<= 10;
-
- /* start of EBDA area */
- ebda_addr = get_bios_ebda();
+ /* Get the start address of the EBDA page: */
+ ebda_start = get_bios_ebda();
/*
- * Note: some old Dells seem to need 4k EBDA without
- * reporting so, so just consider the memory above 0x9f000
- * to be off limits (bugzilla 2990).
+ * Quirk: some old Dells seem to have a 4k EBDA without
+ * reporting so in their BIOS RAM size value, so just
+ * consider the memory above 640K to be off limits
+ * (bugzilla 2990).
+ *
+ * We detect this case by filtering for nonsensical EBDA
+ * addresses below 128K, where we can assume that they
+ * are bogus and bump it up to a fixed 640K value:
*/
+ if (ebda_start < BIOS_START_MIN)
+ ebda_start = BIOS_START_MAX;
- /* If the EBDA address is below 128K, assume it is bogus */
- if (ebda_addr < INSANE_CUTOFF)
- ebda_addr = LOWMEM_CAP;
+ /*
+ * BIOS RAM size is encoded in kilobytes, convert it
+ * to bytes to get a first guess at where the BIOS
+ * firmware area starts:
+ */
+ bios_start = *(unsigned short *)__va(BIOS_RAM_SIZE_KB_PTR);
+ bios_start <<= 10;
- /* If lowmem is less than 128K, assume it is bogus */
- if (lowmem < INSANE_CUTOFF)
- lowmem = LOWMEM_CAP;
+ /*
+ * If bios_start is less than 128K, assume it is bogus
+ * and bump it up to 640K:
+ */
+ if (bios_start < BIOS_START_MIN)
+ bios_start = BIOS_START_MAX;
- /* Use the lower of the lowmem and EBDA markers as the cutoff */
- lowmem = min(lowmem, ebda_addr);
- lowmem = min(lowmem, LOWMEM_CAP); /* Absolute cap */
+ /*
+ * Use the lower of the bios_start and ebda_start
+ * as the starting point, but don't allow it to
+ * go beyond 640K:
+ */
+ bios_start = min(bios_start, ebda_start);
+ bios_start = min(bios_start, BIOS_START_MAX);
- /* reserve all memory between lowmem and the 1MB mark */
- memblock_reserve(lowmem, 0x100000 - lowmem);
+ /* Reserve all memory between bios_start and the 1MB mark: */
+ memblock_reserve(bios_start, 0x100000 - bios_start);
}
projects / openwrt / staging / blogic.git / commitdiff