[meta-xilinx] Really bad SD/eMMC performance after upgrading 2018.1 to 2018.3

[Mike Looijmans]

The SD and eMMC controller has become terribly slow in 2018.3:

# echo 3 > /proc/sys/vm/drop_caches

# dd if=/dev/mmcblk0 of=/dev/null bs=1M count=20
20+0 records in
20+0 records out
20971520 bytes (20.0MB) copied, 4.972666 seconds, 4.0MB/s

# dd if=/dev/mmcblk1 of=/dev/null bs=1M count=20
20+0 records in
20+0 records out
20971520 bytes (20.0MB) copied, 38.899436 seconds, 526.5KB/s


This used to be over 160MB/s for the eMMC (mmcblk0) and over 20 MB/s for the 
SD card (mmcblk0) with the 2018.1 kernel and bootloader.

This is only in Linux, the bootloader still reads at about 10MB/s from the SD 
card. So apparently the clocks are set up okay.

According to the kernel, the clocks and interface are running at full speed:

# cat /sys/kernel/debug/mmc0/ios
clock:          200000000 Hz
actual clock:   200000000 Hz
vdd:            21 (3.3 ~ 3.4 V)
bus mode:       2 (push-pull)
chip select:    0 (don't care)
power mode:     2 (on)
bus width:      3 (8 bits)
timing spec:    9 (mmc HS200)
signal voltage: 1 (1.80 V)
driver type:    0 (driver type B)

# cat /sys/kernel/debug/mmc1/ios
clock:          50000000 Hz
actual clock:   50000000 Hz
vdd:            21 (3.3 ~ 3.4 V)
bus mode:       2 (push-pull)
chip select:    0 (don't care)
power mode:     2 (on)
bus width:      2 (4 bits)
timing spec:    2 (sd high-speed)
signal voltage: 0 (3.30 V)
driver type:    0 (driver type B)


Any insights?