minix/servers/iso9660fs/cache.c
2012-03-25 21:58:14 +02:00

114 lines
3.2 KiB
C

/* The file system maintains a buffer cache to reduce the number of disk
* accesses needed. Whenever a read or write to the disk is done, a check is
* first made to see if the block is in the cache. This file manages the
* cache.
*
* The entry points into this file are:
* get_block: request to fetch a block for reading or writing from cache
* put_block: return a block previously requested with get_block
*
* Private functions:
* read_block: read physically the block
*/
#include "inc.h"
#include <minix/com.h>
#include <minix/u64.h>
#include <minix/bdev.h>
#include "buf.h"
static int read_block(struct buf *);
struct buf *bp_to_pickup = buf; /* This is a pointer to the next node in the
* buffer cache to pick up*/
/*===========================================================================*
* get_block *
*===========================================================================*/
struct buf *get_block(block)
register block_t block; /* which block is wanted? */
{
register struct buf *bp, *free_bp;
free_bp = NULL;
/* Find if the block is already loaded */
for (bp = &buf[0]; bp < &buf[NR_BUFS]; bp++)
if (bp->b_blocknr == block) {
/* Block found. Increment count and return it */
bp->b_count++;
return bp;
} else
if (bp == bp_to_pickup) {
if (bp->b_count == 0)
free_bp = bp;
else /* Increment the node to pickup */
if (bp_to_pickup < &buf[NR_BUFS] - 1)
bp_to_pickup++;
else
bp_to_pickup = buf;
}
if (free_bp == NULL &&
bp_to_pickup == buf &&
bp_to_pickup->b_count == 0)
free_bp = bp_to_pickup;
if (free_bp != NULL) {
/* Set fields of data structure */
free_bp->b_blocknr = block;
if (read_block(free_bp) != OK) return NULL;
free_bp->b_count = 1;
if (bp_to_pickup < &buf[NR_BUFS] - 1)
bp_to_pickup++;
else
bp_to_pickup = buf;
return free_bp;
} else {
/* No free blocks. Return NULL */
return NULL;
}
}
/*===========================================================================*
* put_block *
*===========================================================================*/
void put_block(bp)
register struct buf *bp; /* pointer to the buffer to be released */
{
if (bp == NULL) return; /* it is easier to check here than in caller */
bp->b_count--; /* there is one use fewer now */
}
/*===========================================================================*
* read_block *
*===========================================================================*/
static int read_block(bp)
register struct buf *bp; /* buffer pointer */
{
int r;
u64_t pos;
vir_bytes block_size;
block_size = v_pri.logical_block_size_l; /* The block size is indicated by
* the superblock */
pos = mul64u(bp->b_blocknr, block_size); /* get absolute position */
r = bdev_read(fs_dev, pos, bp->b_data, block_size, BDEV_NOFLAGS);
if (r != (ssize_t) block_size) {
if (r == OK) r = END_OF_FILE;
else printf("ISOFS(%d) I/O error on device %d/%d, block %u\n",
SELF_E, (fs_dev>>MAJOR)&BYTE, (fs_dev>>MINOR)&BYTE,
bp->b_blocknr);
rdwt_err = r;
return EINVAL;
}
return OK;
}