fs/fat/fat.c: Do not perform zero block reads if there are no blocks left

v2: Fix indentation and remove unneeded braces.

While using u-boot with qemu's virtio driver I stumbled across a
problem reading files less than sector size.  On the real hardware the
block reader seems ok with reading zero blocks, and while we could fix
the virtio host side of qemu to deal with a zero block read instead of
crashing, the u-boot fat driver should not be doing zero block reads
in the first place.  If you ask hardware to read zero blocks you are
just going to get zero data.  There may also be other hardware that
responds similarly to the virtio interface so this is worth fixing.

Without the patch I get the following and have to restart qemu because
it dies.
---------------------------------
=> fatls virtio 0:1
       30   cmdline.txt
=> fatload virtio 0:1 ${loadaddr} cmdline.txt
qemu-system-aarch64: virtio: zero sized buffers are not allowed
---------------------------------

With the patch I get the expected results.
---------------------------------
=> fatls virtio 0:1
       30   cmdline.txt
=> fatload virtio 0:1 ${loadaddr} cmdline.txt
30 bytes read in 11 ms (2 KiB/s)
=> md.b ${loadaddr} 0x1E
40080000: 64 77 63 5f 6f 74 67 2e 6c 70 6d 5f 65 6e 61 62    dwc_otg.lpm_enab
40080010: 6c 65 3d 30 20 72 6f 6f 74 77 61 69 74 0a          le=0 rootwait.

---------------------------------

Signed-off-by: Jason Wessel <jason.wessel at windriver.com>
---
 fs/fat/fat.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

On Fri, Jun 19, 2020 at 10:12:20AM -0700, Jason Wessel wrote:

> v2: Fix indentation and remove unneeded braces.

Series notes go below the '--- '

> 
> While using u-boot with qemu's virtio driver I stumbled across a
> problem reading files less than sector size.  On the real hardware the
> block reader seems ok with reading zero blocks, and while we could fix
> the virtio host side of qemu to deal with a zero block read instead of
> crashing, the u-boot fat driver should not be doing zero block reads
> in the first place.  If you ask hardware to read zero blocks you are
> just going to get zero data.  There may also be other hardware that
> responds similarly to the virtio interface so this is worth fixing.
> 
> Without the patch I get the following and have to restart qemu because
> it dies.
> ---------------------------------
> => fatls virtio 0:1
>        30   cmdline.txt
> => fatload virtio 0:1 ${loadaddr} cmdline.txt
> qemu-system-aarch64: virtio: zero sized buffers are not allowed
> ---------------------------------
> 
> With the patch I get the expected results.
> ---------------------------------
> => fatls virtio 0:1
>        30   cmdline.txt
> => fatload virtio 0:1 ${loadaddr} cmdline.txt
> 30 bytes read in 11 ms (2 KiB/s)
> => md.b ${loadaddr} 0x1E
> 40080000: 64 77 63 5f 6f 74 67 2e 6c 70 6d 5f 65 6e 61 62    dwc_otg.lpm_enab
> 40080010: 6c 65 3d 30 20 72 6f 6f 74 77 61 69 74 0a          le=0 rootwait.
> 
> ---------------------------------
> 
> Signed-off-by: Jason Wessel <jason.wessel at windriver.com>
> ---
>  fs/fat/fat.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/fat/fat.c b/fs/fat/fat.c
> index 7fd29470c1..8233a74620 100644
> --- a/fs/fat/fat.c
> +++ b/fs/fat/fat.c
> @@ -278,7 +278,11 @@ get_cluster(fsdata *mydata, __u32 clustnum, __u8 *buffer, unsigned long size)
>  		}
>  	} else {
>  		idx = size / mydata->sect_size;
> -		ret = disk_read(startsect, idx, buffer);
> +		if (idx == 0) {
> +		  ret = 0;
> +		} else {
> +		  ret = disk_read(startsect, idx, buffer);
> +		}
>  		if (ret != idx) {
>  			debug("Error reading data (got %d)\n", ret);
>  			return -1;

Spacing is still off and still extra braces.  Code looks fine tho,
thanks!