How do I change the SD card controller that's used by U-boot for the Raspberry Pi 3 Model B+?

The Raspberry Pi 3 Model B+ has two SD card controllers.

There is custom controller called sdhost as shown by the device tree node:

/* SDHOST is used to drive the SD card */
&sdhost {
	pinctrl-names = "default";
	pinctrl-0 = <&sdhost_gpio48>;
	status = "okay";
	bus-width = <4>;
};

And an SDHCI-compliant controller as shown by the following device tree node:

/* SDHCI is used to control the SDIO for wireless */
&sdhci {
	#address-cells = <1>;
	#size-cells = <0>;
	pinctrl-names = "default";
	pinctrl-0 = <&emmc_gpio34>;
	status = "okay";
	bus-width = <4>;
	non-removable;
	mmc-pwrseq = <&wifi_pwrseq>;

	brcmf: wifi@1 {
		reg = <1>;
		compatible = "brcm,bcm4329-fmac";
	};
};

As referenced by the U-boot comments, U-boot is using SDHOST as the SD card controller and SDHCI as the WiFi controller.

/* SDHOST is used to drive the SD card */
...
/* SDHCI is used to control the SDIO for wireless */

The problem I'm facing is that I'm unable to run this SD card driver (source code here) via U-boot, which uses the Pi's SDHCI controller to interact with SD cards.

The SD card driver runs successfully bare-metal via the Pi's existing bootloader as shown by the output below:

EMMC: GPIO set up
EMMC: reset OK
sd_clk divisor 00000068, shift 00000006
EMMC: Sending command 00000000 arg 00000000
EMMC: Sending command 08020000 arg 000001AA
EMMC: Sending command 37000000 arg 00000000
EMMC: Sending command 29020000 arg 51FF8000
EMMC: CMD_SEND_OP_COND returned VOLTAGE CCS 0000000040F98000
...

However, when I try to run the binary via U-Boot using the go command, the code is stuck during the initialisation phase of the SDHCI controller (exact line in source code here) as shown below:

EMMC: GPIO set up
EMMC: reset OK
sd_clk divisor 00000068, shift 00000006
EMMC: Sending command 00000000 arg 00000000
EMMC: Sending command 08020000 arg 000001AA
ERROR: failed to send EMMC command

My suspicion is that since U-boot has configured the SDHCI controller for Wifi (and not for SD cards), I can no longer use it as an SD card controller. However, I want to use the SDHCI controller as the SD Card controller.

What is the best way to get my SD card binary running via U-boot?

Is the only option to configure/recompile U-boot to use the SDHCI controller as the main SD card controller (and forget about WiFi)?

If so, some pointers on how to go about this would be much appreciated.

Alternatively, is there a U-boot shell command (like some sort of reset command) that give binaries a runtime environment similar to a bare-metal environment that has been un-modified by U-boot?

Edit:

Here is my progress so far. I've figured out how to exclude the custom SDHOST driver from being initialised via two methods:

1. Modifying the DTS file by replacing status = "okay" with status = "disabled" for the custom SDHOST driver.
2. Adding CONFIG_MMC_BCM2835=n to U-Boot's configs/rpi_3_b_plus_defconfig file.

When I launch U-boot, and run mmc list, I see only the SDHCI controller, which is great:

U-Boot> mmc list
mmc@7e300000: 0

I know the above is the SDHCI controller because its device registers are mapped to the physical address of 7e300000 as shown in arch/arm/dts/bcm283x.dtsi below:

sdhci: mmc@7e300000 {
	compatible = "brcm,bcm2835-sdhci";
	reg = <0x7e300000 0x100>;
	interrupts = <2 30>;
	clocks = <&clocks BCM2835_CLOCK_EMMC>;
	status = "disabled";
};

However, as shown in the following boot logs, U-boot is still not happy to use this controller as the SD card controller:

MMC:   mmc@7e300000: 0
Loading Environment from FAT... Card did not respond to voltage select! : -110
** Bad device specification mmc 0 **

Below is the entire boot log:

U-Boot 2023.01-00790-g8c80619653-dirty (Feb 11 2023 - 10:04:49 +1100)

DRAM:  948 MiB
RPI 3 Model B+ (0xa020d3)
Core:  63 devices, 14 uclasses, devicetree: embed
MMC:   mmc@7e300000: 0
Loading Environment from FAT... Card did not respond to voltage select! : -110
** Bad device specification mmc 0 **
In:    serial
Out:   vidconsole
Err:   vidconsole
Net:   No ethernet found.
starting USB...
Bus usb@7e980000: USB DWC2
scanning bus usb@7e980000 for devices... 4 USB Device(s) found
       scanning usb for storage devices... 0 Storage Device(s) found
Hit any key to stop autoboot:  0
U-Boot> mmc list
mmc@7e300000: 0

My suspicion is that the SDHCI controller is still being treated by U-Boot as a WiFi controller (I can't tell for sure). What are some troubleshooting steps I can take to figure out what is going on?

My solution is to change the following line in the sdhci node of the DT located at arch/arm/dts/bcm2837-rpi-3-b-plus.dts from

pinctrl-0 = <&emmc_gpio34>;

to

pinctrl-0 = <&sdhost_gpio48>;

Then I recompile U-boot and use the resultant U-boot binary as the Pi’s bootloader.

The final and desired result is that:

1. U-boot can still use the sdhost driver and controller to access the SD card when it is running (which is U-boot’s default preference).
2. The binary run by U-boot (via the go command) can also manipulate the sdhci controller’s device registers to interact with the SD card.

For additional context around the solution, the sdhost_gpio48 reference above is located in this DTSI file located at arch/arm/dts/bcm283x.dtsi:

sdhost_gpio48: sdhost_gpio48 {
	brcm,pins = <48 49 50 51 52 53>;
	brcm,function = <BCM2835_FSEL_ALT0>;
};

The pinctrl-0 parameter we have changed refers to “Pin Control” and detailed documentation about this parameter can be found here.

From my understanding, our change specifies the same GPIO pins that are being used for the SDHOST controller for the SDHCI controller, which enables us to use the SDHCI controller as an SD card controller when the binary is running.