[PATCH 3/3] fs: fat: Limit transfer size to prevent SDHCI controller timeout

[Balaji Selvanathan]

Some SDHCI controllers have limitations on maximum transfer sizes
and can timeout on large block operations.

Break large read and write operations into chunks of 16384 blocks
to stay within controller limits. This prevents timeout errors during
large file transfers on FAT filesystems while maintaining
compatibility with all platforms.

This patch builds on top of the work from:
https://lore.kernel.org/u-boot/20260224035000.1617869-1-varadarajan.narayanan@oss.qualcomm.com/

Signed-off-by: Balaji Selvanathan <balaji.selvanathan at oss.qualcomm.com>
---
 fs/fat/fat.c | 29 ++++++++++++++++++++---------
 1 file changed, 20 insertions(+), 9 deletions(-)

diff --git a/fs/fat/fat.c b/fs/fat/fat.c
index 85b511f75af..6ec241af584 100644
--- a/fs/fat/fat.c
+++ b/fs/fat/fat.c
@@ -30,6 +30,9 @@
 /* maximum number of clusters for FAT12 */
 #define MAX_FAT12	0xFF4
 
+/* maximum blocks per read/write to avoid SDHCI timeout */
+#define MAX_BLOCKS_PER_TRANSFER 16384
+
 /*
  * Convert a string to lowercase.  Converts at most 'len' characters,
  * 'len' may be larger than the length of 'str' if 'str' is NULL
@@ -136,18 +139,26 @@ static int disk_rw(__u32 sect, __u32 nr_sect, void *buf, bool read)
 	if (rem > blksz) {
 		n = rem / blksz;
 
-		if (read)
-			ret = blk_dread(cur_dev, start + s, n, buf);
-		else
-			ret = blk_dwrite(cur_dev, start + s, n, buf);
+		/* Break large reads/writes into chunks */
+		while (n > 0) {
+			__u32 transfer_count = (n > MAX_BLOCKS_PER_TRANSFER) ? MAX_BLOCKS_PER_TRANSFER : n;
 
-		if (ret != n) {
-			ret = -1;
-			goto exit;
+			if (read)
+				ret = blk_dread(cur_dev, start + s, transfer_count, buf);
+			else
+				ret = blk_dwrite(cur_dev, start + s, transfer_count, buf);
+
+			if (ret != transfer_count) {
+				ret = -1;
+				goto exit;
+			}
+
+			buf += transfer_count * blksz;
+			s += transfer_count;
+			n -= transfer_count;
 		}
-		buf += n * blksz;
+
 		rem = rem % blksz;
-		s += n;
 	}
 
 	/* Do part 3, read a block and copy the trailing sectors */

-- 
2.34.1