[U-Boot] 'load mmc' speed varies significantly

[Jan Kiszka]

Hi all,

I'm getting significant speed differences while loading kernel and
initrd from an ext4 rootfs on an SD card via the 'load' command:

## Executing script at 80000000
54764 bytes read in 296 ms (180.7 KiB/s)
12359688 bytes read in 42982 ms (280.3 KiB/s)
4333462 bytes read in 441 ms (9.4 MiB/s)

The script just does

setenv bootargs root=/dev/mmcblk1p2 rw rootwait
load ${devtype} ${devnum}:2 ${fdt_addr_r} /usr/lib/linux-image-4.14.67+/${fdtfile}
load ${devtype} ${devnum}:${distro_bootpart} ${kernel_addr_r} /boot/vmlinuz-4.14.67+
load ${devtype} ${devnum}:${distro_bootpart} ${ramdisk_addr_r} /boot/initrd.img-4.14.67+
booti ${kernel_addr_r} ${ramdisk_addr_r}:${filesize} ${fdt_addr_r}

So, the kernel takes an eternity while the initrd on the very same
filesystem loads at full speed. Moreover, when I rewrite the kernel like
this

# cat /boot/vmlinuz-4.14.67+ > /boot/vmlinuz-4.14.67+.rewrite                                                                                                                                                                 
# mv /boot/vmlinuz-4.14.67+.rewrite /boot/vmlinuz-4.14.67+                                                                                                                                                                    

the transfer speed is fine for it as well.

I'm currently seeing this on an AM65xx (downstream U-boot, 2018.01-
based), but I had the very same effect on other boards (e.g. Orange Pi
Zero) with upstream U-boot as well in the past. We generate the
filesystem and the SD card image via wic - possibly with some
unfortunate parameters?

Anyone any idea?

Thanks,
Jan