[PATCH v3 1/5] cramfs: direct memory access support

[Nicolas Pitre]

Small embedded systems typically execute the kernel code in place (XIP)
directly from flash to save on precious RAM usage. This adds the ability
to consume filesystem data directly from flash to the cramfs filesystem
as well. Cramfs is particularly well suited to this feature as it is
very simple and its RAM usage is already very low, and with this feature
it is possible to use it with no block device support and even lower RAM
usage.

This patch was inspired by a similar patch from Shane Nay dated 17 years
ago that used to be very popular in embedded circles but never made it
into mainline. This is a cleaned-up implementation that uses far fewer
memory address at run time when both methods are configured in. In the
context of small IoT deployments, this functionality has become relevant
and useful again.

To distinguish between both access types, the cramfs_physmem filesystem
type must be specified when using a memory accessible cramfs image, and
the physaddr argument must provide the actual filesystem image's physical
memory location.

Signed-off-by: Nicolas Pitre <nico@xxxxxxxxxx>
Tested-by: Chris Brandt <chris.brandt@xxxxxxxxxxx>
---
 fs/cramfs/Kconfig |  29 +++++-
 fs/cramfs/inode.c | 264 +++++++++++++++++++++++++++++++++++++++++++-----------
 2 files changed, 241 insertions(+), 52 deletions(-)

diff --git a/fs/cramfs/Kconfig b/fs/cramfs/Kconfig
index 11b29d491b..5b4e0b7e13 100644
--- a/fs/cramfs/Kconfig
+++ b/fs/cramfs/Kconfig
@@ -1,6 +1,5 @@
 config CRAMFS
 	tristate "Compressed ROM file system support (cramfs) (OBSOLETE)"
-	depends on BLOCK
 	select ZLIB_INFLATE
 	help
 	  Saying Y here includes support for CramFs (Compressed ROM File
@@ -20,3 +19,31 @@ config CRAMFS
 	  in terms of performance and features.
 
 	  If unsure, say N.
+
+config CRAMFS_BLOCKDEV
+	bool "Support CramFs image over a regular block device" if EXPERT
+	depends on CRAMFS && BLOCK
+	default y
+	help
+	  This option allows the CramFs driver to load data from a regular
+	  block device such a disk partition or a ramdisk.
+
+config CRAMFS_PHYSMEM
+	bool "Support CramFs image directly mapped in physical memory"
+	depends on CRAMFS
+	default y if !CRAMFS_BLOCKDEV
+	help
+	  This option allows the CramFs driver to load data directly from
+	  a linear adressed memory range (usually non volatile memory
+	  like flash) instead of going through the block device layer.
+	  This saves some memory since no intermediate buffering is
+	  necessary.
+
+	  The filesystem type for this feature is "cramfs_physmem".
+	  The location of the CramFs image in memory is board
+	  dependent. Therefore, if you say Y, you must know the proper
+	  physical address where to store the CramFs image and specify
+	  it using the physaddr=0x******** mount option (for example:
+	  "mount -t cramfs_physmem -o physaddr=0x100000 none /mnt").
+
+	  If unsure, say N.
diff --git a/fs/cramfs/inode.c b/fs/cramfs/inode.c
index 7919967488..19f464a214 100644
--- a/fs/cramfs/inode.c
+++ b/fs/cramfs/inode.c
@@ -24,6 +24,7 @@
 #include <linux/mutex.h>
 #include <uapi/linux/cramfs_fs.h>
 #include <linux/uaccess.h>
+#include <linux/io.h>
 
 #include "internal.h"
 
@@ -36,6 +37,8 @@ struct cramfs_sb_info {
 	unsigned long blocks;
 	unsigned long files;
 	unsigned long flags;
+	void *linear_virt_addr;
+	phys_addr_t linear_phys_addr;
 };
 
 static inline struct cramfs_sb_info *CRAMFS_SB(struct super_block *sb)
@@ -140,6 +143,9 @@ static struct inode *get_cramfs_inode(struct super_block *sb,
  * BLKS_PER_BUF*PAGE_SIZE, so that the caller doesn't need to
  * worry about end-of-buffer issues even when decompressing a full
  * page cache.
+ *
+ * Note: This is all optimized away at compile time when
+ *       CONFIG_CRAMFS_BLOCKDEV=n.
  */
 #define READ_BUFFERS (2)
 /* NEXT_BUFFER(): Loop over [0..(READ_BUFFERS-1)]. */
@@ -160,10 +166,10 @@ static struct super_block *buffer_dev[READ_BUFFERS];
 static int next_buffer;
 
 /*
- * Returns a pointer to a buffer containing at least LEN bytes of
- * filesystem starting at byte offset OFFSET into the filesystem.
+ * Populate our block cache and return a pointer from it.
  */
-static void *cramfs_read(struct super_block *sb, unsigned int offset, unsigned int len)
+static void *cramfs_blkdev_read(struct super_block *sb, unsigned int offset,
+				unsigned int len)
 {
 	struct address_space *mapping = sb->s_bdev->bd_inode->i_mapping;
 	struct page *pages[BLKS_PER_BUF];
@@ -239,7 +245,39 @@ static void *cramfs_read(struct super_block *sb, unsigned int offset, unsigned i
 	return read_buffers[buffer] + offset;
 }
 
-static void cramfs_kill_sb(struct super_block *sb)
+/*
+ * Return a pointer to the linearly addressed cramfs image in memory.
+ */
+static void *cramfs_direct_read(struct super_block *sb, unsigned int offset,
+				unsigned int len)
+{
+	struct cramfs_sb_info *sbi = CRAMFS_SB(sb);
+
+	if (!len)
+		return NULL;
+	if (len > sbi->size || offset > sbi->size - len)
+	       return page_address(ZERO_PAGE(0));
+	return sbi->linear_virt_addr + offset;
+}
+
+/*
+ * Returns a pointer to a buffer containing at least LEN bytes of
+ * filesystem starting at byte offset OFFSET into the filesystem.
+ */
+static void *cramfs_read(struct super_block *sb, unsigned int offset,
+			 unsigned int len)
+{
+	struct cramfs_sb_info *sbi = CRAMFS_SB(sb);
+
+	if (IS_ENABLED(CONFIG_CRAMFS_PHYSMEM) && sbi->linear_virt_addr)
+		return cramfs_direct_read(sb, offset, len);
+	else if (IS_ENABLED(CONFIG_CRAMFS_BLOCKDEV))
+		return cramfs_blkdev_read(sb, offset, len);
+	else
+		return NULL;
+}
+
+static void cramfs_blkdev_kill_sb(struct super_block *sb)
 {
 	struct cramfs_sb_info *sbi = CRAMFS_SB(sb);
 
@@ -247,6 +285,16 @@ static void cramfs_kill_sb(struct super_block *sb)
 	kfree(sbi);
 }
 
+static void cramfs_physmem_kill_sb(struct super_block *sb)
+{
+	struct cramfs_sb_info *sbi = CRAMFS_SB(sb);
+
+	if (sbi->linear_virt_addr)
+		memunmap(sbi->linear_virt_addr);
+	kill_anon_super(sb);
+	kfree(sbi);
+}
+
 static int cramfs_remount(struct super_block *sb, int *flags, char *data)
 {
 	sync_filesystem(sb);
@@ -254,34 +302,24 @@ static int cramfs_remount(struct super_block *sb, int *flags, char *data)
 	return 0;
 }
 
-static int cramfs_fill_super(struct super_block *sb, void *data, int silent)
+static int cramfs_read_super(struct super_block *sb,
+			     struct cramfs_super *super, int silent)
 {
-	int i;
-	struct cramfs_super super;
+	struct cramfs_sb_info *sbi = CRAMFS_SB(sb);
 	unsigned long root_offset;
-	struct cramfs_sb_info *sbi;
-	struct inode *root;
-
-	sb->s_flags |= MS_RDONLY;
-
-	sbi = kzalloc(sizeof(struct cramfs_sb_info), GFP_KERNEL);
-	if (!sbi)
-		return -ENOMEM;
-	sb->s_fs_info = sbi;
 
-	/* Invalidate the read buffers on mount: think disk change.. */
-	mutex_lock(&read_mutex);
-	for (i = 0; i < READ_BUFFERS; i++)
-		buffer_blocknr[i] = -1;
+	/* We don't know the real size yet */
+	sbi->size = PAGE_SIZE;
 
 	/* Read the first block and get the superblock from it */
-	memcpy(&super, cramfs_read(sb, 0, sizeof(super)), sizeof(super));
+	mutex_lock(&read_mutex);
+	memcpy(super, cramfs_read(sb, 0, sizeof(*super)), sizeof(*super));
 	mutex_unlock(&read_mutex);
 
 	/* Do sanity checks on the superblock */
-	if (super.magic != CRAMFS_MAGIC) {
+	if (super->magic != CRAMFS_MAGIC) {
 		/* check for wrong endianness */
-		if (super.magic == CRAMFS_MAGIC_WEND) {
+		if (super->magic == CRAMFS_MAGIC_WEND) {
 			if (!silent)
 				pr_err("wrong endianness\n");
 			return -EINVAL;
@@ -289,10 +327,10 @@ static int cramfs_fill_super(struct super_block *sb, void *data, int silent)
 
 		/* check at 512 byte offset */
 		mutex_lock(&read_mutex);
-		memcpy(&super, cramfs_read(sb, 512, sizeof(super)), sizeof(super));
+		memcpy(super, cramfs_read(sb, 512, sizeof(*super)), sizeof(*super));
 		mutex_unlock(&read_mutex);
-		if (super.magic != CRAMFS_MAGIC) {
-			if (super.magic == CRAMFS_MAGIC_WEND && !silent)
+		if (super->magic != CRAMFS_MAGIC) {
+			if (super->magic == CRAMFS_MAGIC_WEND && !silent)
 				pr_err("wrong endianness\n");
 			else if (!silent)
 				pr_err("wrong magic\n");
@@ -301,34 +339,34 @@ static int cramfs_fill_super(struct super_block *sb, void *data, int silent)
 	}
 
 	/* get feature flags first */
-	if (super.flags & ~CRAMFS_SUPPORTED_FLAGS) {
+	if (super->flags & ~CRAMFS_SUPPORTED_FLAGS) {
 		pr_err("unsupported filesystem features\n");
 		return -EINVAL;
 	}
 
 	/* Check that the root inode is in a sane state */
-	if (!S_ISDIR(super.root.mode)) {
+	if (!S_ISDIR(super->root.mode)) {
 		pr_err("root is not a directory\n");
 		return -EINVAL;
 	}
 	/* correct strange, hard-coded permissions of mkcramfs */
-	super.root.mode |= (S_IRUSR | S_IXUSR | S_IRGRP | S_IXGRP | S_IROTH | S_IXOTH);
+	super->root.mode |= (S_IRUSR | S_IXUSR | S_IRGRP | S_IXGRP | S_IROTH | S_IXOTH);
 
-	root_offset = super.root.offset << 2;
-	if (super.flags & CRAMFS_FLAG_FSID_VERSION_2) {
-		sbi->size = super.size;
-		sbi->blocks = super.fsid.blocks;
-		sbi->files = super.fsid.files;
+	root_offset = super->root.offset << 2;
+	if (super->flags & CRAMFS_FLAG_FSID_VERSION_2) {
+		sbi->size = super->size;
+		sbi->blocks = super->fsid.blocks;
+		sbi->files = super->fsid.files;
 	} else {
 		sbi->size = 1<<28;
 		sbi->blocks = 0;
 		sbi->files = 0;
 	}
-	sbi->magic = super.magic;
-	sbi->flags = super.flags;
+	sbi->magic = super->magic;
+	sbi->flags = super->flags;
 	if (root_offset == 0)
 		pr_info("empty filesystem");
-	else if (!(super.flags & CRAMFS_FLAG_SHIFTED_ROOT_OFFSET) &&
+	else if (!(super->flags & CRAMFS_FLAG_SHIFTED_ROOT_OFFSET) &&
 		 ((root_offset != sizeof(struct cramfs_super)) &&
 		  (root_offset != 512 + sizeof(struct cramfs_super))))
 	{
@@ -336,9 +374,18 @@ static int cramfs_fill_super(struct super_block *sb, void *data, int silent)
 		return -EINVAL;
 	}
 
+	return 0;
+}
+
+static int cramfs_finalize_super(struct super_block *sb,
+				 struct cramfs_inode *cramfs_root)
+{
+	struct inode *root;
+
 	/* Set it all up.. */
+	sb->s_flags |= MS_RDONLY;
 	sb->s_op = &cramfs_ops;
-	root = get_cramfs_inode(sb, &super.root, 0);
+	root = get_cramfs_inode(sb, cramfs_root, 0);
 	if (IS_ERR(root))
 		return PTR_ERR(root);
 	sb->s_root = d_make_root(root);
@@ -347,6 +394,92 @@ static int cramfs_fill_super(struct super_block *sb, void *data, int silent)
 	return 0;
 }
 
+static int cramfs_blkdev_fill_super(struct super_block *sb, void *data, int silent)
+{
+	struct cramfs_sb_info *sbi;
+	struct cramfs_super super;
+	int i, err;
+
+	sbi = kzalloc(sizeof(struct cramfs_sb_info), GFP_KERNEL);
+	if (!sbi)
+		return -ENOMEM;
+	sb->s_fs_info = sbi;
+
+	/* Invalidate the read buffers on mount: think disk change.. */
+	for (i = 0; i < READ_BUFFERS; i++)
+		buffer_blocknr[i] = -1;
+
+	err = cramfs_read_super(sb, &super, silent);
+	if (err)
+		return err;
+	return cramfs_finalize_super(sb, &super.root);
+}
+
+static int cramfs_physmem_fill_super(struct super_block *sb, void *data, int silent)
+{
+	struct cramfs_sb_info *sbi;
+	struct cramfs_super super;
+	char *p;
+	int err;
+
+	sbi = kzalloc(sizeof(struct cramfs_sb_info), GFP_KERNEL);
+	if (!sbi)
+		return -ENOMEM;
+	sb->s_fs_info = sbi;
+
+	/*
+	 * The physical location of the cramfs image is specified as
+	 * a mount parameter.  This parameter is mandatory for obvious
+	 * reasons.  Some validation is made on the phys address but this
+	 * is not exhaustive and we count on the fact that someone using
+	 * this feature is supposed to know what he/she's doing.
+	 */
+	if (!data || !(p = strstr((char *)data, "physaddr="))) {
+		pr_err("unknown physical address for linear cramfs image\n");
+		return -EINVAL;
+	}
+	sbi->linear_phys_addr = memparse(p + 9, NULL);
+	if (!sbi->linear_phys_addr) {
+		pr_err("bad value for cramfs image physical address\n");
+		return -EINVAL;
+	}
+	if (sbi->linear_phys_addr & (PAGE_SIZE-1)) {
+		pr_err("physical address %pap for linear cramfs isn't aligned to a page boundary\n",
+			&sbi->linear_phys_addr);
+		return -EINVAL;
+	}
+
+	/*
+	 * Map only one page for now.  Will remap it when fs size is known.
+	 * Although we'll only read from it, we want the CPU cache to
+	 * kick in for the higher throughput it provides, hence MEMREMAP_WB.
+	 */
+	pr_info("checking physical address %pap for linear cramfs image\n", &sbi->linear_phys_addr);
+	sbi->linear_virt_addr = memremap(sbi->linear_phys_addr, PAGE_SIZE,
+					 MEMREMAP_WB);
+	if (!sbi->linear_virt_addr) {
+		pr_err("ioremap of the linear cramfs image failed\n");
+		return -ENOMEM;
+	}
+
+	err = cramfs_read_super(sb, &super, silent);
+	if (err)
+		return err;
+
+	/* Remap the whole filesystem now */
+	pr_info("linear cramfs image appears to be %lu KB in size\n",
+		sbi->size/1024);
+	memunmap(sbi->linear_virt_addr);
+	sbi->linear_virt_addr = memremap(sbi->linear_phys_addr, sbi->size,
+					 MEMREMAP_WB);
+	if (!sbi->linear_virt_addr) {
+		pr_err("ioremap of the linear cramfs image failed\n");
+		return -ENOMEM;
+	}
+
+	return cramfs_finalize_super(sb, &super.root);
+}
+
 static int cramfs_statfs(struct dentry *dentry, struct kstatfs *buf)
 {
 	struct super_block *sb = dentry->d_sb;
@@ -573,38 +706,67 @@ static const struct super_operations cramfs_ops = {
 	.statfs		= cramfs_statfs,
 };
 
-static struct dentry *cramfs_mount(struct file_system_type *fs_type,
-	int flags, const char *dev_name, void *data)
+static struct dentry *cramfs_blkdev_mount(struct file_system_type *fs_type,
+				int flags, const char *dev_name, void *data)
+{
+	return mount_bdev(fs_type, flags, dev_name, data, cramfs_blkdev_fill_super);
+}
+
+static struct dentry *cramfs_physmem_mount(struct file_system_type *fs_type,
+				int flags, const char *dev_name, void *data)
 {
-	return mount_bdev(fs_type, flags, dev_name, data, cramfs_fill_super);
+	return mount_nodev(fs_type, flags, data, cramfs_physmem_fill_super);
 }
 
 static struct file_system_type cramfs_fs_type = {
 	.owner		= THIS_MODULE,
 	.name		= "cramfs",
-	.mount		= cramfs_mount,
-	.kill_sb	= cramfs_kill_sb,
+	.mount		= cramfs_blkdev_mount,
+	.kill_sb	= cramfs_blkdev_kill_sb,
 	.fs_flags	= FS_REQUIRES_DEV,
 };
+
+static struct file_system_type cramfs_physmem_fs_type = {
+	.owner		= THIS_MODULE,
+	.name		= "cramfs_physmem",
+	.mount		= cramfs_physmem_mount,
+	.kill_sb	= cramfs_physmem_kill_sb,
+};
+
+#ifdef CONFIG_CRAMFS_BLOCKDEV
 MODULE_ALIAS_FS("cramfs");
+#endif
+#ifdef CONFIG_CRAMFS_PHYSMEM
+MODULE_ALIAS_FS("cramfs_physmem");
+#endif
 
 static int __init init_cramfs_fs(void)
 {
 	int rv;
 
-	rv = cramfs_uncompress_init();
-	if (rv < 0)
-		return rv;
-	rv = register_filesystem(&cramfs_fs_type);
-	if (rv < 0)
-		cramfs_uncompress_exit();
-	return rv;
+	if ((rv = cramfs_uncompress_init()) < 0)
+		goto err0;
+	if (IS_ENABLED(CONFIG_CRAMFS_BLOCKDEV) &&
+	    (rv = register_filesystem(&cramfs_fs_type)) < 0)
+		goto err1;
+	if (IS_ENABLED(CONFIG_CRAMFS_PHYSMEM) &&
+	    (rv = register_filesystem(&cramfs_physmem_fs_type)) < 0)
+		goto err2;
+	return 0;
+
+err2:	if (IS_ENABLED(CONFIG_CRAMFS_BLOCKDEV))
+		unregister_filesystem(&cramfs_fs_type);
+err1:	cramfs_uncompress_exit();
+err0:	return rv;
 }
 
 static void __exit exit_cramfs_fs(void)
 {
 	cramfs_uncompress_exit();
-	unregister_filesystem(&cramfs_fs_type);
+	if (IS_ENABLED(CONFIG_CRAMFS_BLOCKDEV))
+		unregister_filesystem(&cramfs_fs_type);
+	if (IS_ENABLED(CONFIG_CRAMFS_PHYSMEM))
+		unregister_filesystem(&cramfs_physmem_fs_type);
 }
 
 module_init(init_cramfs_fs)
-- 
2.9.5