Subversion Repositories HelenOS

/*

 * Copyright (c) 2008 Jakub Jermar

 * All rights reserved.

 *

 * Redistribution and use in source and binary forms, with or without

 * modification, are permitted provided that the following conditions

 * are met:

 *

 * - Redistributions of source code must retain the above copyright

 *   notice, this list of conditions and the following disclaimer.

 * - Redistributions in binary form must reproduce the above copyright

 *   notice, this list of conditions and the following disclaimer in the

 *   documentation and/or other materials provided with the distribution.

 * - The name of the author may not be used to endorse or promote products

 *   derived from this software without specific prior written permission.

 *

 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR

 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES

 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.

 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,

 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT

 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,

 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY

 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT

 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF

 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 */

/** @addtogroup fs

 * @{

 */ 

/**

 * @file	fat_ops.c

 * @brief	Implementation of VFS operations for the FAT file system server.

 */

#include "fat.h"

#include "../../vfs/vfs.h"

#include <libfs.h>

#include <ipc/ipc.h>

#include <ipc/services.h>

#include <ipc/devmap.h>

#include <async.h>

#include <errno.h>

#include <string.h>

#include <byteorder.h>

#include <libadt/hash_table.h>

#include <libadt/list.h>

#include <assert.h>

#include <futex.h>

#include <sys/mman.h>

#include <align.h>

#define BS_BLOCK		0

#define BS_SIZE			512

/** Futex protecting the list of cached free FAT nodes. */

static futex_t ffn_futex = FUTEX_INITIALIZER;

/** List of cached free FAT nodes. */

static LIST_INITIALIZE(ffn_head);

#define FAT_NAME_LEN		8

#define FAT_EXT_LEN		3

#define FAT_PAD			' ' 

#define FAT_DENTRY_UNUSED	0x00

#define FAT_DENTRY_E5_ESC	0x05

#define FAT_DENTRY_DOT		0x2e

#define FAT_DENTRY_ERASED	0xe5

#define min(a, b)		((a) < (b) ? (a) : (b))

static void dentry_name_canonify(fat_dentry_t *d, char *buf)

{

	int i;

	for (i = 0; i < FAT_NAME_LEN; i++) {

		if (d->name[i] == FAT_PAD)

			break;

		if (d->name[i] == FAT_DENTRY_E5_ESC)

			*buf++ = 0xe5;

		else

			*buf++ = d->name[i];

	}

	if (d->ext[0] != FAT_PAD)

		*buf++ = '.';

	for (i = 0; i < FAT_EXT_LEN; i++) {

		if (d->ext[i] == FAT_PAD) {

			*buf = '\0';

			return;

		}

		if (d->ext[i] == FAT_DENTRY_E5_ESC)

			*buf++ = 0xe5;

		else

			*buf++ = d->ext[i];

	}

	*buf = '\0';

}

static int dev_phone = -1;		/* FIXME */

static void *dev_buffer = NULL;		/* FIXME */

/* TODO move somewhere else */

typedef struct {

	void *data;

	size_t size;

	bool dirty;

} block_t;

static block_t *block_get(dev_handle_t dev_handle, off_t offset, size_t bs)

{

	/* FIXME */

	block_t *b;

	off_t bufpos = 0;

	size_t buflen = 0;

	off_t pos = offset * bs;

	assert(dev_phone != -1);

	assert(dev_buffer);

	b = malloc(sizeof(block_t));

	if (!b)

		return NULL;

	b->data = malloc(bs);

	if (!b->data) {

		free(b);

		return NULL;

	}

	b->size = bs;

	if (!libfs_blockread(dev_phone, dev_buffer, &bufpos, &buflen, &pos,

	    b->data, bs, bs)) {

		free(b->data);

		free(b);

		return NULL;

	}

	return b;

}

static void block_put(block_t *block)

{

	/* FIXME */

	free(block->data);

	free(block);

}

#define FAT_BS(b)		((fat_bs_t *)((b)->data))

#define FAT_CLST_RES0	0x0000

#define FAT_CLST_RES1	0x0001

#define FAT_CLST_FIRST	0x0002

#define FAT_CLST_BAD	0xfff7

#define FAT_CLST_LAST1	0xfff8

#define FAT_CLST_LAST8  0xffff

/* internally used to mark root directory's parent */

#define FAT_CLST_ROOTPAR	FAT_CLST_RES0

/* internally used to mark root directory */

#define FAT_CLST_ROOT		FAT_CLST_RES1

#define fat_block_get(np, off) \

    _fat_block_get((np)->idx->dev_handle, (np)->firstc, (off))

static block_t *

_fat_block_get(dev_handle_t dev_handle, fat_cluster_t firstc, off_t offset)

{

	block_t *bb;

	block_t *b;

	unsigned bps;

	unsigned spc;

	unsigned rscnt;		/* block address of the first FAT */

	unsigned fatcnt;

	unsigned rde;

	unsigned rds;		/* root directory size */

	unsigned sf;

	unsigned ssa;		/* size of the system area */

	unsigned clusters;

	fat_cluster_t clst = firstc;

	unsigned i;

	bb = block_get(dev_handle, BS_BLOCK, BS_SIZE);

	bps = uint16_t_le2host(FAT_BS(bb)->bps);

	spc = FAT_BS(bb)->spc;

	rscnt = uint16_t_le2host(FAT_BS(bb)->rscnt);

	fatcnt = FAT_BS(bb)->fatcnt;

	rde = uint16_t_le2host(FAT_BS(bb)->root_ent_max);

	sf = uint16_t_le2host(FAT_BS(bb)->sec_per_fat);

	block_put(bb);

	rds = (sizeof(fat_dentry_t) * rde) / bps;

	rds += ((sizeof(fat_dentry_t) * rde) % bps != 0);

	ssa = rscnt + fatcnt * sf + rds;

	if (firstc == FAT_CLST_ROOT) {

		/* root directory special case */

		assert(offset < rds);

		b = block_get(dev_handle, rscnt + fatcnt * sf + offset, bps);

		return b;

	}

	clusters = offset / spc; 

	for (i = 0; i < clusters; i++) {

		unsigned fsec;	/* sector offset relative to FAT1 */

		unsigned fidx;	/* FAT1 entry index */

		assert(clst >= FAT_CLST_FIRST && clst < FAT_CLST_BAD);

		fsec = (clst * sizeof(fat_cluster_t)) / bps;

		fidx = clst % (bps / sizeof(fat_cluster_t));

		/* read FAT1 */

		b = block_get(dev_handle, rscnt + fsec, bps);

		clst = uint16_t_le2host(((fat_cluster_t *)b->data)[fidx]);

		assert(clst != FAT_CLST_BAD);

		assert(clst < FAT_CLST_LAST1);

		block_put(b);

	}

	b = block_get(dev_handle, ssa + (clst - FAT_CLST_FIRST) * spc +

	    offset % spc, bps);

	return b;

}

/** Return number of blocks allocated to a file.

 *

 * @param dev_handle	Device handle of the device with the file.

 * @param firstc	First cluster of the file.

 *

 * @return		Number of blocks allocated to the file.

 */

static uint16_t 

_fat_blcks_get(dev_handle_t dev_handle, fat_cluster_t firstc)

{

	block_t *bb;

	block_t *b;

	unsigned bps;

	unsigned spc;

	unsigned rscnt;		/* block address of the first FAT */

	unsigned clusters = 0;

	fat_cluster_t clst = firstc;

	bb = block_get(dev_handle, BS_BLOCK, BS_SIZE);

	bps = uint16_t_le2host(FAT_BS(bb)->bps);

	spc = FAT_BS(bb)->spc;

	rscnt = uint16_t_le2host(FAT_BS(bb)->rscnt);

	block_put(bb);

	if (firstc == FAT_CLST_RES0) {

		/* No space allocated to the file. */

		return 0;

	}

	while (clst < FAT_CLST_LAST1) {

		unsigned fsec;	/* sector offset relative to FAT1 */

		unsigned fidx;	/* FAT1 entry index */

		assert(clst >= FAT_CLST_FIRST);

		fsec = (clst * sizeof(fat_cluster_t)) / bps;

		fidx = clst % (bps / sizeof(fat_cluster_t));

		/* read FAT1 */

		b = block_get(dev_handle, rscnt + fsec, bps);

		clst = uint16_t_le2host(((fat_cluster_t *)b->data)[fidx]);

		assert(clst != FAT_CLST_BAD);

		block_put(b);

		clusters++;

	}

	return clusters * spc;

}

static void fat_node_initialize(fat_node_t *node)

{

	futex_initialize(&node->lock, 1);

	node->idx = NULL;

	node->type = 0;

	link_initialize(&node->ffn_link);

	node->size = 0;

	node->lnkcnt = 0;

	node->refcnt = 0;

	node->dirty = false;

}

static uint16_t fat_bps_get(dev_handle_t dev_handle)

{

	block_t *bb;

	uint16_t bps;

	bb = block_get(dev_handle, BS_BLOCK, BS_SIZE);

	assert(bb != NULL);

	bps = uint16_t_le2host(FAT_BS(bb)->bps);

	block_put(bb);

	return bps;

}

typedef enum {

	FAT_DENTRY_SKIP,

	FAT_DENTRY_LAST,

	FAT_DENTRY_VALID

} fat_dentry_clsf_t;

static fat_dentry_clsf_t fat_classify_dentry(fat_dentry_t *d)

{

	if (d->attr & FAT_ATTR_VOLLABEL) {

		/* volume label entry */

		return FAT_DENTRY_SKIP;

	}

	if (d->name[0] == FAT_DENTRY_ERASED) {

		/* not-currently-used entry */

		return FAT_DENTRY_SKIP;

	}

	if (d->name[0] == FAT_DENTRY_UNUSED) {

		/* never used entry */

		return FAT_DENTRY_LAST;

	}

	if (d->name[0] == FAT_DENTRY_DOT) {

		/*

		 * Most likely '.' or '..'.

		 * It cannot occur in a regular file name.

		 */

		return FAT_DENTRY_SKIP;

	}

	return FAT_DENTRY_VALID;

}

static void fat_node_sync(fat_node_t *node)

{

	/* TODO */

}

/** Internal version of fat_node_get().

 *

 * @param idxp		Locked index structure.

 */

static void *fat_node_get_core(fat_idx_t *idxp)

{

	block_t *b;

	fat_dentry_t *d;

	fat_node_t *nodep = NULL;

	unsigned bps;

	unsigned dps;

	if (idxp->nodep) {

		/*

		 * We are lucky.

		 * The node is already instantiated in memory.

		 */

		futex_down(&idxp->nodep->lock);

		if (!idxp->nodep->refcnt++)

			list_remove(&idxp->nodep->ffn_link);

		futex_up(&idxp->nodep->lock);

		return idxp->nodep;

	}

	/*

	 * We must instantiate the node from the file system.

	 */

	assert(idxp->pfc);

	futex_down(&ffn_futex);

	if (!list_empty(&ffn_head)) {

		/* Try to use a cached free node structure. */

		fat_idx_t *idxp_tmp;

		nodep = list_get_instance(ffn_head.next, fat_node_t, ffn_link);

		if (futex_trydown(&nodep->lock) == ESYNCH_WOULD_BLOCK)

			goto skip_cache;

		idxp_tmp = nodep->idx;

		if (futex_trydown(&idxp_tmp->lock) == ESYNCH_WOULD_BLOCK) {

			futex_up(&nodep->lock);

			goto skip_cache;

		}

		list_remove(&nodep->ffn_link);

		futex_up(&ffn_futex);

		if (nodep->dirty)

			fat_node_sync(nodep);

		idxp_tmp->nodep = NULL;

		futex_up(&nodep->lock);

		futex_up(&idxp_tmp->lock);

	} else {

skip_cache:

		/* Try to allocate a new node structure. */

		futex_up(&ffn_futex);

		nodep = (fat_node_t *)malloc(sizeof(fat_node_t));

		if (!nodep)

			return NULL;

	}

	fat_node_initialize(nodep);

	bps = fat_bps_get(idxp->dev_handle);

	dps = bps / sizeof(fat_dentry_t);

	/* Read the block that contains the dentry of interest. */

	b = _fat_block_get(idxp->dev_handle, idxp->pfc,

	    (idxp->pdi * sizeof(fat_dentry_t)) / bps);

	assert(b);

	d = ((fat_dentry_t *)b->data) + (idxp->pdi % dps);

	if (d->attr & FAT_ATTR_SUBDIR) {

		/* 

		 * The only directory which does not have this bit set is the

		 * root directory itself. The root directory node is handled

		 * and initialized elsewhere.

		 */

		nodep->type = FAT_DIRECTORY;

		/*

		 * Unfortunately, the 'size' field of the FAT dentry is not

		 * defined for the directory entry type. We must determine the

		 * size of the directory by walking the FAT.

		 */

		nodep->size = bps * _fat_blcks_get(idxp->dev_handle,

		    uint16_t_le2host(d->firstc));

	} else {

		nodep->type = FAT_FILE;

		nodep->size = uint32_t_le2host(d->size);

	}

	nodep->firstc = uint16_t_le2host(d->firstc);

	nodep->lnkcnt = 1;

	nodep->refcnt = 1;

	block_put(b);

	/* Link the idx structure with the node structure. */

	nodep->idx = idxp;

	idxp->nodep = nodep;

	return nodep;

}

/** Instantiate a FAT in-core node. */

static void *fat_node_get(dev_handle_t dev_handle, fs_index_t index)

{

	void *node;

	fat_idx_t *idxp;

	idxp = fat_idx_get_by_index(dev_handle, index);

	if (!idxp)

		return NULL;

	/* idxp->lock held */

	node = fat_node_get_core(idxp);

	futex_up(&idxp->lock);

	return node;

}

static void fat_node_put(void *node)

{

	fat_node_t *nodep = (fat_node_t *)node;

	futex_down(&nodep->lock);

	if (!--nodep->refcnt) {

		futex_down(&ffn_futex);

		list_append(&nodep->ffn_link, &ffn_head);

		futex_up(&ffn_futex);

	}

	futex_up(&nodep->lock);

}

static void *fat_create(int flags)

{

	return NULL;	/* not supported at the moment */

}

static int fat_destroy(void *node)

{

	return ENOTSUP;	/* not supported at the moment */

}

static bool fat_link(void *prnt, void *chld, const char *name)

{

	return false;	/* not supported at the moment */

}

static int fat_unlink(void *prnt, void *chld)

{

	return ENOTSUP;	/* not supported at the moment */

}

static void *fat_match(void *prnt, const char *component)

{

	fat_node_t *parentp = (fat_node_t *)prnt;

	char name[FAT_NAME_LEN + 1 + FAT_EXT_LEN + 1];

	unsigned i, j;

	unsigned bps;		/* bytes per sector */

	unsigned dps;		/* dentries per sector */

	unsigned blocks;

	fat_dentry_t *d;

	block_t *b;

	futex_down(&parentp->idx->lock);

	bps = fat_bps_get(parentp->idx->dev_handle);

	dps = bps / sizeof(fat_dentry_t);

	blocks = parentp->size / bps + (parentp->size % bps != 0);

	for (i = 0; i < blocks; i++) {

		unsigned dentries;

		b = fat_block_get(parentp, i);

		dentries = (i == blocks - 1) ?

		    parentp->size % sizeof(fat_dentry_t) :

		    dps;

		for (j = 0; j < dentries; j++) { 

			d = ((fat_dentry_t *)b->data) + j;

			switch (fat_classify_dentry(d)) {

			case FAT_DENTRY_SKIP:

				continue;

			case FAT_DENTRY_LAST:

				block_put(b);

				futex_up(&parentp->idx->lock);

				return NULL;

			default:

			case FAT_DENTRY_VALID:

				dentry_name_canonify(d, name);

				break;

			}

			if (stricmp(name, component) == 0) {

				/* hit */

				void *node;

				/*

				 * Assume tree hierarchy for locking.  We

				 * already have the parent and now we are going

				 * to lock the child.  Never lock in the oposite

				 * order.

				 */

				fat_idx_t *idx = fat_idx_get_by_pos(

				    parentp->idx->dev_handle, parentp->firstc,

				    i * dps + j);

				futex_up(&parentp->idx->lock);

				if (!idx) {

					/*

					 * Can happen if memory is low or if we

					 * run out of 32-bit indices.

					 */

					block_put(b);

					return NULL;

				}

				node = fat_node_get_core(idx);

				futex_up(&idx->lock);

				block_put(b);

				return node;

			}

		}

		block_put(b);

	}

	futex_up(&parentp->idx->lock);

	return NULL;

}

static fs_index_t fat_index_get(void *node)

{

	fat_node_t *fnodep = (fat_node_t *)node;

	if (!fnodep)

		return 0;

	return fnodep->idx->index;

}

static size_t fat_size_get(void *node)

{

	return ((fat_node_t *)node)->size;

}

static unsigned fat_lnkcnt_get(void *node)

{

	return ((fat_node_t *)node)->lnkcnt;

}

static bool fat_has_children(void *node)

{

	fat_node_t *nodep = (fat_node_t *)node;

	unsigned bps;

	unsigned dps;

	unsigned blocks;

	block_t *b;

	unsigned i, j;

	if (nodep->type != FAT_DIRECTORY)

		return false;

	futex_down(&nodep->idx->lock);

	bps = fat_bps_get(nodep->idx->dev_handle);

	dps = bps / sizeof(fat_dentry_t);

	blocks = nodep->size / bps + (nodep->size % bps != 0);

	for (i = 0; i < blocks; i++) {

		unsigned dentries;

		fat_dentry_t *d;

		b = fat_block_get(nodep, i);

		dentries = (i == blocks - 1) ?

		    nodep->size % sizeof(fat_dentry_t) :

		    dps;

		for (j = 0; j < dentries; j++) {

			d = ((fat_dentry_t *)b->data) + j;

			switch (fat_classify_dentry(d)) {

			case FAT_DENTRY_SKIP:

				continue;

			case FAT_DENTRY_LAST:

				block_put(b);

				futex_up(&nodep->idx->lock);

				return false;

			default:

			case FAT_DENTRY_VALID:

				block_put(b);

				futex_up(&nodep->idx->lock);

				return true;

			}

			block_put(b);

			futex_up(&nodep->idx->lock);

			return true;

		}

		block_put(b);

	}

	futex_up(&nodep->idx->lock);

	return false;

}

static void *fat_root_get(dev_handle_t dev_handle)

{

	return fat_node_get(dev_handle, 0);

}

static char fat_plb_get_char(unsigned pos)

{

	return fat_reg.plb_ro[pos % PLB_SIZE];

}

static bool fat_is_directory(void *node)

{

	return ((fat_node_t *)node)->type == FAT_DIRECTORY;

}

static bool fat_is_file(void *node)

{

	return ((fat_node_t *)node)->type == FAT_FILE;

}

/** libfs operations */

libfs_ops_t fat_libfs_ops = {

	.match = fat_match,

	.node_get = fat_node_get,

	.node_put = fat_node_put,

	.create = fat_create,

	.destroy = fat_destroy,

	.link = fat_link,

	.unlink = fat_unlink,

	.index_get = fat_index_get,

	.size_get = fat_size_get,

	.lnkcnt_get = fat_lnkcnt_get,

	.has_children = fat_has_children,

	.root_get = fat_root_get,

	.plb_get_char =	fat_plb_get_char,

	.is_directory = fat_is_directory,

	.is_file = fat_is_file

};

void fat_mounted(ipc_callid_t rid, ipc_call_t *request)

{

	dev_handle_t dev_handle = (dev_handle_t) IPC_GET_ARG1(*request);

	block_t *bb;

	uint16_t bps;

	uint16_t rde;

	int rc;

	/*

	 * For now, we don't bother to remember dev_handle, dev_phone or

	 * dev_buffer in some data structure. We use global variables because we

	 * know there will be at most one mount on this file system.

	 * Of course, this is a huge TODO item.

	 */

	dev_buffer = mmap(NULL, BS_SIZE, PROTO_READ | PROTO_WRITE,

	    MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);

	if (!dev_buffer) {

		ipc_answer_0(rid, ENOMEM);

		return;

	}

	dev_phone = ipc_connect_me_to(PHONE_NS, SERVICE_DEVMAP,

	    DEVMAP_CONNECT_TO_DEVICE, dev_handle);

	if (dev_phone < 0) {

		munmap(dev_buffer, BS_SIZE);

		ipc_answer_0(rid, dev_phone);

		return;

	}

	rc = ipc_share_out_start(dev_phone, dev_buffer,

	    AS_AREA_READ | AS_AREA_WRITE);

	if (rc != EOK) {

	    	munmap(dev_buffer, BS_SIZE);

		ipc_answer_0(rid, rc);

		return;

	}

	/* Read the number of root directory entries. */

	bb = block_get(dev_handle, BS_BLOCK, BS_SIZE);

	bps = uint16_t_le2host(FAT_BS(bb)->bps);

	rde = uint16_t_le2host(FAT_BS(bb)->root_ent_max);

	block_put(bb);

	if (bps != BS_SIZE) {

		munmap(dev_buffer, BS_SIZE);

		ipc_answer_0(rid, ENOTSUP);

		return;

	}

	rc = fat_idx_init_by_dev_handle(dev_handle);

	if (rc != EOK) {

	    	munmap(dev_buffer, BS_SIZE);

		ipc_answer_0(rid, rc);

		return;

	}

	/* Initialize the root node. */

	fat_node_t *rootp = (fat_node_t *)malloc(sizeof(fat_node_t));

	if (!rootp) {

	    	munmap(dev_buffer, BS_SIZE);

		fat_idx_fini_by_dev_handle(dev_handle);

		ipc_answer_0(rid, ENOMEM);

		return;

	}

	fat_node_initialize(rootp);

	fat_idx_t *ridxp = fat_idx_get_by_pos(dev_handle, FAT_CLST_ROOTPAR, 0);

	if (!ridxp) {

	    	munmap(dev_buffer, BS_SIZE);

		free(rootp);

		fat_idx_fini_by_dev_handle(dev_handle);

		ipc_answer_0(rid, ENOMEM);

		return;

	}

	assert(ridxp->index == 0);

	/* ridxp->lock held */

	rootp->type = FAT_DIRECTORY;

	rootp->firstc = FAT_CLST_ROOT;

	rootp->refcnt = 1;

	rootp->lnkcnt = 0;	/* FS root is not linked */

	rootp->size = rde * sizeof(fat_dentry_t);

	rootp->idx = ridxp;

	ridxp->nodep = rootp;

	futex_up(&ridxp->lock);

	ipc_answer_3(rid, EOK, ridxp->index, rootp->size, rootp->lnkcnt);

}

void fat_mount(ipc_callid_t rid, ipc_call_t *request)

{

	ipc_answer_0(rid, ENOTSUP);

}

void fat_lookup(ipc_callid_t rid, ipc_call_t *request)

{

	libfs_lookup(&fat_libfs_ops, fat_reg.fs_handle, rid, request);

}

void fat_read(ipc_callid_t rid, ipc_call_t *request)

{

	dev_handle_t dev_handle = (dev_handle_t)IPC_GET_ARG1(*request);

	fs_index_t index = (fs_index_t)IPC_GET_ARG2(*request);

	off_t pos = (off_t)IPC_GET_ARG3(*request);

	fat_node_t *nodep = (fat_node_t *)fat_node_get(dev_handle, index);

	uint16_t bps = fat_bps_get(dev_handle);

	size_t bytes;

	block_t *b;

	if (!nodep) {

		ipc_answer_0(rid, ENOENT);

		return;

	}

	ipc_callid_t callid;

	size_t len;

	if (!ipc_data_read_receive(&callid, &len)) {

		fat_node_put(nodep);

		ipc_answer_0(callid, EINVAL);

		ipc_answer_0(rid, EINVAL);

		return;

	}

	if (nodep->type == FAT_FILE) {

		/*

		 * Our strategy for regular file reads is to read one block at

		 * most and make use of the possibility to return less data than

		 * requested. This keeps the code very simple.

		 */

		bytes = min(len, bps - pos % bps);

		b = fat_block_get(nodep, pos / bps);

		(void) ipc_data_read_finalize(callid, b->data + pos % bps,

		    bytes);

		block_put(b);

	} else {

		unsigned bnum;

		off_t spos = pos;

		char name[FAT_NAME_LEN + 1 + FAT_EXT_LEN + 1];

		fat_dentry_t *d;

		assert(nodep->type == FAT_DIRECTORY);

		assert(nodep->size % bps == 0);

		assert(bps % sizeof(fat_dentry_t) == 0);

		/*

		 * Our strategy for readdir() is to use the position pointer as

		 * an index into the array of all dentries. On entry, it points

		 * to the first unread dentry. If we skip any dentries, we bump

		 * the position pointer accordingly.

		 */

		bnum = (pos * sizeof(fat_dentry_t)) / bps;

		while (bnum < nodep->size / bps) {

			off_t o;

			b = fat_block_get(nodep, bnum);

			for (o = pos % (bps / sizeof(fat_dentry_t));

			    o < bps / sizeof(fat_dentry_t);

			    o++, pos++) {

				d = ((fat_dentry_t *)b->data) + o;

				switch (fat_classify_dentry(d)) {

				case FAT_DENTRY_SKIP:

					continue;

				case FAT_DENTRY_LAST:

					block_put(b);

					goto miss;

				default:

				case FAT_DENTRY_VALID:

					dentry_name_canonify(d, name);

					block_put(b);

					goto hit;

				}

			}

			block_put(b);

			bnum++;

		}

miss:

		fat_node_put(nodep);

		ipc_answer_0(callid, ENOENT);

		ipc_answer_1(rid, ENOENT, 0);

		return;

hit:

		(void) ipc_data_read_finalize(callid, name, strlen(name) + 1);

		bytes = (pos - spos) + 1;

	}

	fat_node_put(nodep);

	ipc_answer_1(rid, EOK, (ipcarg_t)bytes);

}

static void fat_fill_gap(fat_node_t *nodep, off_t start, off_t stop)

{

	/* TODO */

}

void fat_write(ipc_callid_t rid, ipc_call_t *request)

{

	dev_handle_t dev_handle = (dev_handle_t)IPC_GET_ARG1(*request);

	fs_index_t index = (fs_index_t)IPC_GET_ARG2(*request);

	off_t pos = (off_t)IPC_GET_ARG3(*request);

	fat_node_t *nodep = (fat_node_t *)fat_node_get(dev_handle, index);

	size_t bytes;

	block_t *b, *bb;

	uint16_t bps;

	unsigned spc;

	off_t clst_boundary;

	if (!nodep) {

		ipc_answer_0(rid, ENOENT);

		return;

	}

	/* XXX remove me when you are ready */

	{

		ipc_answer_0(rid, ENOTSUP);

		fat_node_put(nodep);

		return;

	}

	ipc_callid_t callid;

	size_t len;

	if (!ipc_data_write_receive(&callid, &len)) {

		fat_node_put(nodep);

		ipc_answer_0(callid, EINVAL);

		ipc_answer_0(rid, EINVAL);

		return;

	}

	/*

	 * In all scenarios, we will attempt to write out only one block worth

	 * of data at maximum. There might be some more efficient approaches,

	 * but this one greatly simplifies fat_write(). Note that we can afford

	 * to do this because the client must be ready to handle the return

	 * value signalizing a smaller number of bytes written. 

	 */ 

	bytes = min(len, bps - pos % bps);

	bb = block_get(dev_handle, BS_BLOCK, BS_SIZE);

	bps = uint16_t_le2host(FAT_BS(bb)->bps);

	spc = FAT_BS(bb)->spc;

	block_put(bb);

	clst_boundary = ROUND_UP(nodep->size, bps * spc);

	if (pos < clst_boundary) {

		/*

		 * This is the easier case - we are either overwriting already

		 * existing contents or writing behind the EOF, but still within

		 * the limits of the last cluster. The node size may grow to the

		 * next block size boundary.

		 */

		if (pos > nodep->size) 

			fat_fill_gap(nodep, nodep->size, pos);

		b = fat_block_get(nodep, pos / bps);

		(void) ipc_data_write_finalize(callid, b->data + pos % bps,

		    bytes);

		b->dirty = true;		/* need to sync block */

		if (pos + bytes > nodep->size) {

			nodep->size = pos + bytes;

			nodep->dirty = true;	/* need to sync node */

		}

		block_put(b);

		fat_node_put(nodep);

		ipc_answer_1(rid, EOK, bytes);	

		return;

	} else {

		/*

		 * This is the more difficult case. We must allocate new

		 * clusters for the node and zero them out.

		 */

		unsigned nclsts;

		nclsts = (ROUND_UP(pos + bytes, bps * spc) - clst_boundary) /

		    bps * spc; 

	}

}

/**

 * @}

 */