netboot/basemem.c

#include "etherboot.h"
#define DEBUG_BASEMEM
/* Routines to allocate base memory in a BIOS-compatible way, by
 * updating the Free Base Memory Size counter at 40:13h.
 *
 * Michael Brown <mbrown@fensystems.co.uk> (mcb30)
 * $Id: basemem.c,v 1.5 2004/06/17 12:48:08 fengshuo Exp $
 */

#define fbms ( ( uint16_t * ) phys_to_virt ( 0x413 ) )
#define BASE_MEMORY_MAX ( 640 )
#define FREE_BLOCK_MAGIC ( ('!'<<0) + ('F'<<8) + ('R'<<16) + ('E'<<24) )

typedef struct free_base_memory_block {
	uint32_t	magic;
	uint16_t	size_kb;
} free_base_memory_block_t;

/* Return amount of free base memory in bytes
 */

uint32_t get_free_base_memory ( void ) {
	return *fbms << 10;
}

/* Adjust the real mode stack pointer.  We keep the real mode stack at
 * the top of free base memory, rather than allocating space for it.
 */

static inline void adjust_real_mode_stack ( void ) {
/*  	real_mode_stack = ( *fbms << 10 ); */
}

/* Allocate N bytes of base memory.  Amount allocated will be rounded
 * up to the nearest kB, since that's the granularity of the BIOS FBMS
 * counter.  Returns NULL if memory cannot be allocated.
 */

void * allot_base_memory ( size_t size ) {
	uint16_t size_kb = ( size + 1023 ) >> 10;
	void *ptr = NULL;

#ifdef DEBUG_BASEMEM
	printf ( "Trying to allocate %d kB of base memory, %d kB free\n",
		 size_kb, *fbms );
#endif

	/* Free up any unused memory before we start */
	free_unused_base_memory();

	/* Check available base memory */
	if ( size_kb > *fbms ) { return NULL; }

	/* Reduce available base memory */
	*fbms -= size_kb;

	/* Calculate address of memory allocated */
	ptr = phys_to_virt ( *fbms << 10 );

#ifdef DEBUG_BASEMEM
	/* Zero out memory.  We do this so that allocation of
	 * already-used space will show up in the form of a crash as
	 * soon as possible.
	 */
	memset ( ptr, 0, size_kb << 10 );
#endif

	/* Adjust real mode stack pointer */
	adjust_real_mode_stack ();

	return ptr;
}

/* Free base memory allocated by allot_base_memory.  The BIOS provides
 * nothing better than a LIFO mechanism for freeing memory (i.e. it
 * just has the single "total free memory" counter), but we improve
 * upon this slightly; as long as you free all the allotted blocks, it
 * doesn't matter what order you free them in.  (This will only work
 * for blocks that are freed via forget_base_memory()).
 *
 * Yes, it's annoying that you have to remember the size of the blocks
 * you've allotted.  However, since our granularity of allocation is
 * 1K, the alternative is to risk wasting the occasional kB of base
 * memory, which is a Bad Thing.  Really, you should be using as
 * little base memory as possible, so consider the awkwardness of the
 * API to be a feature! :-)
 */

void forget_base_memory ( void *ptr, size_t size ) {
	uint16_t remainder = virt_to_phys(ptr) & 1023;
	uint16_t size_kb = ( size + remainder + 1023 ) >> 10;
	free_base_memory_block_t *free_block =
		( free_base_memory_block_t * ) ( ptr - remainder );
	
	if ( ( ptr == NULL ) || ( size == 0 ) ) { return; }

#ifdef DEBUG_BASEMEM
	printf ( "Trying to free %d bytes base memory at 0x%x\n",
		 size, virt_to_phys ( ptr ) );
	if ( remainder > 0 ) {
		printf ( "WARNING: destructively expanding free block "
			 "downwards to 0x%x\n",
			 virt_to_phys ( ptr - remainder ) );
	}
#endif

	/* Mark every kilobyte within this block as free.  This is
	 * overkill for normal purposes, but helps when something has
	 * allocated base memory with a granularity finer than the
	 * BIOS granularity of 1kB.  PXE ROMs tend to do this when
	 * they allocate their own memory.  This method allows us to
	 * free their blocks (admittedly in a rather dangerous,
	 * tread-on-anything-either-side sort of way, but there's no
	 * other way to do it).
	 *
	 * Since we're marking every kB as free, there's actually no
	 * need for recording the size of the blocks.  However, we
	 * keep this in so that debug messages are friendlier.  It
	 * probably adds around 8 bytes to the overall code size.
	 */
	while ( size_kb > 0 ) {
		/* Mark this block as unused */
		free_block->magic = FREE_BLOCK_MAGIC;
		free_block->size_kb = size_kb;
		/* Move up by 1 kB */
		free_block = (void *)free_block + ( 1 << 10 );
		size_kb--;
	}

	/* Free up unused base memory */
	free_unused_base_memory();
}

/* Do the actual freeing of memory.  This is split out from
 * forget_base_memory() so that it may be called separately.  It
 * should be called whenever base memory is deallocated by an external
 * entity (if we can detect that it has done so) so that we get the
 * chance to free up our own blocks.
 */
void free_unused_base_memory ( void ) {
	free_base_memory_block_t *free_block = NULL;

	/* Try to release memory back to the BIOS.  Free all
	 * consecutive blocks marked as free.
	 */
	while ( 1 ) {
		/* Calculate address of next potential free block */
		free_block = ( free_base_memory_block_t * )
			phys_to_virt ( *fbms << 10 );
		
		/* Stop processing if we're all the way up to 640K or
		 * if this is not a free block
		 */
		if ( ( *fbms == BASE_MEMORY_MAX ) ||
		     ( free_block->magic != FREE_BLOCK_MAGIC ) ) {
			break;
		}

		/* Return memory to BIOS */
		*fbms += free_block->size_kb;

#ifdef DEBUG_BASEMEM
		printf ( "Freed %d kB base memory, %d kB now free\n",
			 free_block->size_kb, *fbms );
		
		/* Zero out freed block.  We do this in case
		 * the block contained any structures that
		 * might be located by scanning through
		 * memory.
		 */
		memset ( free_block, 0, free_block->size_kb << 10 );
#endif			
	}

	/* Adjust real mode stack pointer */
	adjust_real_mode_stack ();
}