R-Car/Use-MMC-block-as-rootfs-on-v5-10

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Overview

Linux kernel v5.10-rc1 has the following commit which applied into R-Car SDHI driver.

7320915 mmc: Set PROBE_PREFER_ASYNCHRONOUS for drivers that existed in v4.14

Since probing the SDHI driver is in asynchronous on v5.10-rc1, the order of /dev/mmcblkN will be unstable. So, we are hard to use an mmc block device as a rootfs by using "root=" on kernel command line. To resolve this issue, we have some solutions.

  • device tree alias
  • UUID
  • PARTUUID

How to use?

Device tree aliases

Linux kernel v5.10-rc1 also has the following commit.

fa2d0aa mmc: core: Allow setting slot index via device tree alias

So, we can add the following aliases to fix the /dev/mmcblkN order.

	aliases {
		mmc0 = &sdhi0;
		mmc1 = &sdhi1;
		mmc2 = &sdhi2;
		mmc3 = &sdhi3;
       };

To add such aliases, we have 2 options.

  • device tree in Linux kernel
  • Update on a bootloader at runtime

However, unfotunately, adding aliases into upstream has been rejected[1]. So, updating such aliases on a bootloader (e.g. U-Boot) is better like below.

=> tftpboot 0x4af00000 /r8a77951-salvator-xs.dtb
=> fdt addr 0x4af00000
=> fdt resize
=> fdt set /aliases mmc0 '/soc/mmc@ee100000'
=> fdt set /aliases mmc1 '/soc/mmc@ee120000'
=> fdt set /aliases mmc2 '/soc/mmc@ee140000'
=> fdt set /aliases mmc3 '/soc/mmc@ee160000'
=> fdt print /aliases
aliases {
        mmc3 = "/soc/mmc@ee160000";
        mmc2 = "/soc/mmc@ee140000";
        mmc1 = "/soc/mmc@ee120000";
        mmc0 = "/soc/mmc@ee100000";
        i2c0 = "/soc/i2c@e6500000";
        i2c1 = "/soc/i2c@e6508000";
        i2c2 = "/soc/i2c@e6510000";
        i2c3 = "/soc/i2c@e66d0000";
        i2c4 = "/soc/i2c@e66d8000";
        i2c5 = "/soc/i2c@e66e0000";
        i2c6 = "/soc/i2c@e66e8000";
        i2c7 = "/soc/i2c@e60b0000";
        serial0 = "/soc/serial@e6e88000";
        serial1 = "/soc/serial@e6550000";
        ethernet0 = "/soc/ethernet@e6800000";
};
=> tftpboot 0x48080000 /Image
=> booti 0x48080000 - 0x4af00000

UUID

UUID is in a filesystem. But, Linux kernel doesn't support UUID as root filesystem directly. So, we have to use it via initramfs.

PARTUUID

PARTUUID is in a partition which GUID Partition Table (GPT) is used. And, Linux kernel supports "root=PARTUUID=..." on the Kernel command line. To create a GPT, we have to use gdisk instead of fdisk like below.

# gdisk /dev/mmcblk0
GPT fdisk (gdisk) version 1.0.5

Partition table scan:
  MBR: not present
  BSD: not present
  APM: not present
  GPT: not present

Creating new GPT entries in memory.

Command (? for help): n
Partition number (1-128, default 1):
First sector (34-61071326, default = 2048) or {+-}size{KMGTP}:
Last sector (2048-61071326, default = 61071326) or {+-}size{KMGTP}:
Current type is 8300 (Linux filesystem)
Hex code or GUID (L to show codes, Enter = 8300):
Changed type of partition to 'Linux filesystem'

Command (? for help): p
Disk /dev/mmcblk0: 61071360 sectors, 29.1 GiB
Sector size (logical/physical): 512/512 bytes
Disk identifier (GUID): 343D8BBD-BED8-485D-A98B-D1700B9C9B52
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 61071326
Partitions will be aligned on 2048-sector boundaries
Total free space is 2014 sectors (1007.0 KiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048        61071326   29.1 GiB    8300  Linux filesystem

Command (? for help): i
Using 1
Partition GUID code: 0FC63DAF-8483-4772-8E79-3D69D8477DE4 (Linux filesystem)
Partition unique GUID: CC7E6949-2881-4770-833C-3ADB27B110D8
First sector: 2048 (at 1024.0 KiB)
Last sector: 61071326 (at 29.1 GiB)
Partition size: 61069279 sectors (29.1 GiB)
Attribute flags: 0000000000000000
Partition name: 'Linux filesystem'

Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): y
OK; writing new GUID partition table (GPT) to /dev/mmcblk0.

And then, after the partition had a root filesystem, we can use the "Partition unique GUID" as a PARTUUID like below.

=> setenv bootargs 'rw root=PARTUUID=CC7E6949-2881-4770-833C-3ADB27B110D8'

References