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

[Jonas Karlman]

Hi,

On 3/24/2026 6:52 AM, Balaji Selvanathan wrote:
> 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;

Have you tried to configure SYS_MMC_MAX_BLK_COUNT=16384 for your
platform, or otherwise set b_max in your mmc driver?

That should cause similar max blocks handling at a the mmc-uclass level.

Regards,
Jonas

>  
> -		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 */
>