fs: ext4l: Support mounting a filesystem read-only

Fix several issues preventing ext4_fill_super() from completing:

- Fix shrinker_alloc() to return a dummy shrinker instead of NULL
- Fix getblk_unmovable() to use sb_getblk() instead of returning NULL
- Fix mb_cache_create() to allocate a cache structure
- Implement d_make_root() to create root dentry for mount
- Add ext4_init_system_zone() call for block validity checking
- Add global task_struct for journal_info consistency
- Store super_block pointer after successful mount

The filesystem now mounts successfully in read-only mode.

Series-to: concept
Series-links: 87
Cover-letter:
fs: ext4l: Add support for mounting ext4 filesystems (part G)
This series adds the ext4l filesystem driver, which is a port of the
Linux ext4 driver to U-Boot. It allows mounting ext4 filesystems in
read-only mode.

The driver uses Linux kernel code directly where possible, with
compatibility shims to handle differences between Linux and U-Boot.
This approach makes it easier to keep the driver up to date with
upstream changes.

Key features:
- Read-only mounting of ext4 filesystems
- Support for extents, flex_bg, and other ext4 features
- Buffer cache for improved performance
- CRC32C checksums for metadata verification
END

Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

fs/ext4l/ext4_uboot.h

@@ -58,16 +58,14 @@ struct timespec64 {
 };
 
 /*
- * ktime_t, sector_t are in linux/types.h
- * atomic_t, atomic64_t are in asm-generic/atomic.h
- * MAX_JIFFY_OFFSET is in linux/jiffies.h
- * BDEVNAME_SIZE is in linux/blkdev.h
+ * ktime_t, atomic_t, atomic64_t, sector_t are now in linux/types.h
+ * MAX_JIFFY_OFFSET is now in linux/jiffies.h
+ * BDEVNAME_SIZE is now in linux/blkdev.h
  */
-#include <asm-generic/atomic.h>
 #include <linux/jiffies.h>
 #include <linux/blkdev.h>
 
-/* Extra atomic operations not in asm-generic/atomic.h */
+/* Extra atomic operation not in linux/types.h */
 #define atomic_dec_if_positive(v)	(--(v)->counter)
 
 /* SMP stubs - U-Boot is single-threaded */
@@ -1138,7 +1136,10 @@ struct shrinker {
 static inline struct shrinker *shrinker_alloc(unsigned int flags,
 					      const char *fmt, ...)
 {
-	return NULL;
+	/* Return static dummy - U-Boot doesn't need memory reclamation */
+	static struct shrinker dummy_shrinker;
+
+	return &dummy_shrinker;
 }
 
 static inline void shrinker_register(struct shrinker *s)
@@ -1506,7 +1507,7 @@ static inline char *d_path(const struct path *path, char *buf, int buflen)
 #define filemap_splice_read(i, p, pi, l, f)	({ (void)(i); (void)(p); (void)(pi); (void)(l); (void)(f); 0L; })
 
 /* Buffer operations - additional */
-#define getblk_unmovable(bd, b, s)		((struct buffer_head *)NULL)
+#define getblk_unmovable(bdev, block, size)	sb_getblk(bdev->bd_super, block)
 #define create_empty_buffers(f, s, flags)	({ (void)(f); (void)(s); (void)(flags); (struct buffer_head *)NULL; })
 #define bh_offset(bh)				(0UL)
 #define block_invalidate_folio(f, o, l)		do { } while (0)
@@ -2314,8 +2315,8 @@ struct mb_cache_entry {
 /* MB cache flags */
 #define MBE_REUSABLE_B	0
 
-#define mb_cache_create(bits)			((struct mb_cache *)NULL)
-#define mb_cache_destroy(cache)			do { (void)(cache); } while (0)
+#define mb_cache_create(bits)			kzalloc(sizeof(struct mb_cache), GFP_KERNEL)
+#define mb_cache_destroy(cache)			do { kfree(cache); } while (0)
 #define mb_cache_entry_find_first(c, h)		((struct mb_cache_entry *)NULL)
 #define mb_cache_entry_find_next(c, e)		((struct mb_cache_entry *)NULL)
 #define mb_cache_entry_delete_or_get(c, k, v)	((struct mb_cache_entry *)NULL)

fs/ext4l/interface.c

@@ -28,6 +28,9 @@ static struct blk_desc *ext4l_blk_dev;
 static struct disk_partition ext4l_partition;
 static int ext4l_mounted;
 
+/* Global super_block pointer for filesystem operations */
+static struct super_block *ext4l_sb;
+
 /**
  * ext4l_get_blk_dev() - Get the current block device
  * Return: Block device descriptor or NULL if not mounted
@@ -89,9 +92,7 @@ int ext4l_probe(struct blk_desc *fs_dev_desc,
 	if (!fs_dev_desc)
 		return -EINVAL;
 
-	/* Set up block device for buffer I/O */
-	ext4l_set_blk_dev(fs_dev_desc, fs_partition);
-
+	/* Initialise CRC32C table for checksum verification */
 	ext4l_crc32c_init();
 
 	/* Initialise journal subsystem if enabled */
@@ -113,6 +114,11 @@ int ext4l_probe(struct blk_desc *fs_dev_desc,
 	if (ret)
 		return ret;
 
+	/* Initialise system zone for block validity checking */
+	ret = ext4_init_system_zone();
+	if (ret)
+		goto err_exit_es;
+
 	/* Allocate super_block */
 	sb = kzalloc(sizeof(struct super_block), GFP_KERNEL);
 	if (!sb) {
@@ -193,11 +199,18 @@ int ext4l_probe(struct blk_desc *fs_dev_desc,
 	if (fs_partition)
 		memcpy(&ext4l_part, fs_partition, sizeof(ext4l_part));
 
+	/* Set block device for buffer I/O */
+	ext4l_set_blk_dev(fs_dev_desc, fs_partition);
+
 	/* Mount the filesystem */
 	ret = ext4_fill_super(sb, fc);
-	if (ret)
+	if (ret) {
+		printf("ext4l: ext4_fill_super failed: %d\n", ret);
 		goto err_free_ctx;
+	}
 
+	/* Store super_block for later operations */
+	ext4l_sb = sb;
 	return 0;
 
 err_free_buf:
@@ -219,6 +232,7 @@ err_exit_es:
 
 void ext4l_close(void)
 {
-	ext4l_clear_blk_dev();
 	ext4l_dev_desc = NULL;
+	ext4l_sb = NULL;
+	ext4l_clear_blk_dev();
 }

fs/ext4l/stub.c

@@ -561,9 +561,30 @@ void generic_set_sb_d_ops(struct super_block *sb)
 {
 }
 
+/**
+ * d_make_root() - Create a root dentry for an inode
+ * @inode: Inode to create dentry for
+ * Return: Allocated dentry or NULL on failure
+ */
 struct dentry *d_make_root(struct inode *inode)
 {
-	return NULL;
+	struct dentry *de;
+
+	if (!inode)
+		return NULL;
+
+	de = kzalloc(sizeof(struct dentry), GFP_KERNEL);
+	if (!de) {
+		iput(inode);
+		return NULL;
+	}
+
+	de->d_inode = inode;
+	de->d_sb = inode->i_sb;
+	de->d_name.name = "/";
+	de->d_name.len = 1;
+
+	return de;
 }
 
 /* percpu init rwsem */

fs/ext4l/support.c

@@ -19,6 +19,13 @@
 #include "ext4_uboot.h"
 #include "ext4.h"
 
+/*
+ * Global task_struct for U-Boot.
+ * This must be a single global instance shared across all translation units,
+ * so that journal_info remains consistent.
+ */
+struct task_struct ext4l_current_task = { .comm = "u-boot", .pid = 1 };
+
 /*
  * CRC32C support - uses Castagnoli polynomial 0x82F63B78
  * Table is initialised on first mount

include/linux/types.h

@@ -165,6 +165,28 @@ typedef s64	ktime_t;
 typedef u64 sector_t;
 typedef u64 blkcnt_t;
 
+/* Atomic types - stubs for single-threaded U-Boot */
+typedef struct {
+	int counter;
+} atomic_t;
+
+#ifdef CONFIG_64BIT
+typedef struct {
+	s64 counter;
+} atomic64_t;
+#else
+typedef struct {
+	long counter;
+} atomic64_t;
+#endif
+
+#define atomic_read(v)		((v)->counter)
+#define atomic_set(v, i)	((v)->counter = (i))
+#define atomic_inc(v)		((v)->counter++)
+#define atomic_dec(v)		((v)->counter--)
+#define atomic64_read(v)	((v)->counter)
+#define atomic64_set(v, i)	((v)->counter = (i))
+
 #ifdef __linux__
 struct ustat {
 	__kernel_daddr_t	f_tfree;