The shown speed is inversely proportional to the data size.
See the output:

  spi-nand: spi_nand nand@0: Micron SPI NAND was found.
  spi-nand: spi_nand nand@0: 256 MiB, block size: 128 KiB, page size: 2048, OOB 
size: 128
  ...
  => mtd read.benchmark spi-nand0 $loadaddr 0 0x40000
  Reading 262144 byte(s) (128 page(s)) at offset 0x00000000
  Read speed: 63kiB/s
  => mtd read.benchmark spi-nand0 $loadaddr 0 0x20000
  Reading 131072 byte(s) (64 page(s)) at offset 0x00000000
  Read speed: 127kiB/s
  => mtd read.benchmark spi-nand0 $loadaddr 0 0x10000
  Reading 65536 byte(s) (32 page(s)) at offset 0x00000000
  Read speed: 254kiB/s

In the spi-nand case 'io_op.len' is not always the same as 'len', thus
we are using the wrong amount of data to derive the speed.

Also make sure we are using 64-bit calculation to get a more precise
results.

Fixes: d246e70cf81d0 ("cmd: mtd: Enable speed benchmarking")
Signed-off-by: Mikhail Kshevetskiy <[email protected]>
Reviewed-by: Miquel Raynal <[email protected]>
---
 cmd/mtd.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/cmd/mtd.c b/cmd/mtd.c
index 689d8d11655..34870fd3835 100644
--- a/cmd/mtd.c
+++ b/cmd/mtd.c
@@ -10,6 +10,7 @@
 
 #include <command.h>
 #include <console.h>
+#include <div64.h>
 #include <led.h>
 #if CONFIG_IS_ENABLED(CMD_MTD_OTP)
 #include <hexdump.h>
@@ -469,7 +470,7 @@ static int do_mtd_io(struct cmd_tbl *cmdtp, int flag, int 
argc,
                     char *const argv[])
 {
        bool dump, read, raw, woob, benchmark, write_empty_pages, has_pages = 
false;
-       u64 start_off, off, len, remaining, default_len;
+       u64 start_off, off, len, remaining, default_len, speed;
        unsigned long bench_start, bench_end;
        struct mtd_oob_ops io_op = {};
        uint user_addr = 0, npages;
@@ -595,9 +596,10 @@ static int do_mtd_io(struct cmd_tbl *cmdtp, int flag, int 
argc,
 
        if (benchmark && bench_start) {
                bench_end = timer_get_us();
+               speed = lldiv(len * 1000000, bench_end - bench_start);
                printf("%s speed: %lukiB/s\n",
                       read ? "Read" : "Write",
-                      ((io_op.len * 1000000) / (bench_end - bench_start)) / 
1024);
+                      (unsigned long)(speed / 1024));
        }
 
        led_activity_off();
-- 
2.51.0

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