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SubjectAlmost certainly confirmed, sun7i-a20 does not support 8 bit mmc.
Hey all,

After experimenting with a Micron eMMC chip, I made some interesting
finds I wanted to share with you all.

For a while now, some had hopes or guessed that the A20 could have
support for 8 bit wide bused on the SDC2. Reason for thinking this was,
because the sun7i uses the same IP as the sun5i which does support 8 bit
wide eMMC. The usermanual does mention 1/4/8 bit data buses and jedec
4.3 support of the MMC spec. Unfortunately so far, it appears that the
extra data pins have not been muxed out to the PC pads.

I used the following code to enable the extra pins on the A20:
mmc2_pins_a: mmc2@0 {
- allwinner,pins = "PC6", "PC7", "PC8",
- "PC9", "PC10", "PC11";
+ allwinner,pins = "PC6", "PC7",
+ "PC8", "PC9", "PC10",
"PC11",
+ "PC12", "PC13", "PC14",
"PC15";

and
SUNXI_PIN(SUNXI_PINCTRL_PIN(C, 12),
SUNXI_FUNCTION(0x0, "gpio_in"),
SUNXI_FUNCTION(0x1, "gpio_out"),
- SUNXI_FUNCTION(0x2, "nand0")), /* NDQ4 */
+ SUNXI_FUNCTION(0x2, "nand0"), /* NDQ4 */
+ SUNXI_FUNCTION(0x3, "mmc2")), /* D4 */
SUNXI_PIN(SUNXI_PINCTRL_PIN(C, 13),
SUNXI_FUNCTION(0x0, "gpio_in"),
SUNXI_FUNCTION(0x1, "gpio_out"),
- SUNXI_FUNCTION(0x2, "nand0")), /* NDQ5 */
+ SUNXI_FUNCTION(0x2, "nand0"), /* NDQ5 */
+ SUNXI_FUNCTION(0x3, "mmc2")), /* D5 */
SUNXI_PIN(SUNXI_PINCTRL_PIN(C, 14),
SUNXI_FUNCTION(0x0, "gpio_in"),
SUNXI_FUNCTION(0x1, "gpio_out"),
- SUNXI_FUNCTION(0x2, "nand0")), /* NDQ6 */
+ SUNXI_FUNCTION(0x2, "nand0"), /* NDQ6 */
+ SUNXI_FUNCTION(0x3, "mmc2")), /* D6 */
SUNXI_PIN(SUNXI_PINCTRL_PIN(C, 15),
SUNXI_FUNCTION(0x0, "gpio_in"),
SUNXI_FUNCTION(0x1, "gpio_out"),
- SUNXI_FUNCTION(0x2, "nand0")), /* NDQ7 */
+ SUNXI_FUNCTION(0x2, "nand0"), /* NDQ7 */
+ SUNXI_FUNCTION(0x3, "mmc2")), /* D7 */

which I think should be enough? Unfortunatly after booting i get the
following:

[ 0.971642] sunxi-mmc 1c11000.mmc: smc 1 err, cmd 8, RTO !!
[ 0.981346] sunxi-mmc 1c11000.mmc: smc 1 err, cmd 55, RTO !!
[ 0.987873] sunxi-mmc 1c11000.mmc: smc 1 err, cmd 55, RTO !!
[ 0.988704] sunxi-mmc 1c11000.mmc: smc 1 err, cmd 55, RTO !!
[ 0.989531] sunxi-mmc 1c11000.mmc: smc 1 err, cmd 55, RTO !!
[ 1.020574] mmc1: MAN_BKOPS_EN bit is not set
(This I always get for some reason btw, so if somebody has a thought
that'd be appreciated)

[ 1.024234] sunxi-mmc 1c11000.mmc: smc 1 err, cmd 8, RD EBE !!
[ 1.024265] sunxi-mmc 1c11000.mmc: data error, sending stop command
[ 1.024276] sunxi-mmc 1c11000.mmc: send stop command failed
[ 1.024295] mmc1: switch to bus width 2 failed
[ 1.028642] mmc1: new high speed MMC card at address 0001
[ 1.029133] mmcblk1: mmc1:0001 P1XXXX 3.60 GiB
[ 1.031071] mmcblk1boot0: mmc1:0001 P1XXXX partition 1 16.0 MiB
[ 1.031207] mmcblk1boot1: mmc1:0001 P1XXXX partition 2 16.0 MiB
[ 1.032305] mmcblk1: p1 p2 p3

Hence, the controller fails to enable 8 bit mode. mmc1's ios confirms
this afterwards:
/sys/kernel/debug/mmc1# cat ios
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: 1 (mmc high-speed)
signal voltage: 0 (3.30 V)

On the upside, performance is pretty good however. Using a sequential dd
test on the whole 4GB of storage to /dev/null and reading to a 3GB file
from /dev/zero, I'm hitting 22 MB/s read and 13 MB/s write. The
datasheet of the eMMC chip obviously has exotic values in the 75MB/s
range, but compared to ~12 MB/s read/write from a regular 8gb 'class 10'
SD card, and 4-6 MB/s read/write from the regular onboard NAND flash,
this is pretty decent.

A photograph of the soldering handy work (not by me) can be seen at the
linux-sunxi wiki [0].

Olliver

[0] http://linux-sunxi.org/File:Lime2_emmc.jpg


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