Get Linux MMC transfer speeds

test.sh

# Background: https://github.com/ThomasKaiser/Knowledge/blob/master/articles/A1_and_A2_rated_SD_cards.md
# Symptoms: Write speed seems to be capped at ~ 20MByte/s if controller does only support 3.3V
# To see if your controller supports higher speeds, enable these debug message:
echo "func mmc_set_ios +p" | sudo tee /sys/kernel/debug/dynamic_debug/control
echo "func mmc_set_initial_signal_voltage +p" | sudo tee /sys/kernel/debug/dynamic_debug/control
# Verify:
sudo grep mmc.*=p /sys/kernel/debug/dynamic_debug/control
# (this assumes debugfs under /sys/kernel/debug)

# On the next card insert, you should see power/speed debug messages in dmesg