[openwrt/openwrt] ramips: Fix root volume for tplink-er605-v2

[LEDE Commits]

hauke pushed a commit to openwrt/openwrt.git, branch master:
https://git.openwrt.org/62dbcb8305d7d28393e1d7157680f9c6dbfbed63

commit 62dbcb8305d7d28393e1d7157680f9c6dbfbed63
Author: Nikolay Martynov <mar.kolya at gmail.com>
AuthorDate: Fri Jan 6 22:44:10 2023 -0500

    ramips: Fix root volume for tplink-er605-v2
    
    This device has two sets of volumes: main ones (`kernel`, `rootfs`, etc) and
    'backup' (`kernel.b`, `rootfs.b`, etc). Bootloader tries to determine which set of
    volumes to use by looking at contens of `extra-para` and `extra-para.b` volumes.
    These volumes contain JSON that looks like this:
    
    ```
    {
            "dbootFlag": "1",
            "integerFlag": "1",
            "fwFlag": "GOOD",
            "score":1
    }
    ```
    
    It looks like the bootloader looks for `"fwFlag": "GOOD"` (as opposed to `BAD`)
    then it compares `score` field - whichever 'good' volume has bigger score wins.
    This determines which set of volumes to use to boot.
    
    So for example if `extra-para` is good and has bigger score then `kernel`,
    `rootfs`, etc volumes are used. This means bootloader needs to explain to the
    kernel which volume to use for the rootfs. After looking at bootloader code with
    disassembler I think it contains a bug. Relevant part of code looks something
    like this:
    
    ```
      if (image_id == 0) {
        rootfs_volume_id = 8;
        rootfs_volume_name = "rootfs";
      }
      else {
        rootfs_volume_id = 0xf;
        rootfs_volume_name = "rootfs.b";
      }
      sprintf(
        &buffer,
        0x800,
        "console=ttyS0,115200 noinitrd ubi.mtd=3,2048 ubi.block=0,%s
        root=/dev/ubiblock0_%d DKMGT_IMAGE_ID=%d DKMGT_IMAGE_TYPE=ubi",
        rootfs_volume_name,
        rootfs_volume_id,
        image_id
        );
    ```
    
    Where `image_id == 0` if 'normal' (not '*.b' set of volumes is used).
    However from device dumps we know that from the factory `rootfs.b` has id 8 and
    `rootfs` has id 15.
    
    So from above we can see that ids and names of rootfs volumes do not match. More
    over - they are hardcoded in the bootloader.
    
    Both things are problematic for OpwnWRT which completely removes volumes on
    update meaning that volume ids may actually change.
    
    So instead of relying on bootloader to provide the kernel with root device this
    patch forces kernel to determine root automatically - and it defaults to
    `rootfs` volume which is correct for our purposes.
    
    Overall this makes image boot fine from flash after sysupgrade from inirams.
    assuming `extra-para*` volumes make bootloader use non-'*.b' set of volumes.
    
    Signed-off-by: Nikolay Martynov <mar.kolya at gmail.com>
---
 target/linux/ramips/dts/mt7621_tplink_er605-v2.dts | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/target/linux/ramips/dts/mt7621_tplink_er605-v2.dts b/target/linux/ramips/dts/mt7621_tplink_er605-v2.dts
index 3381e598bd..04935e3e99 100644
--- a/target/linux/ramips/dts/mt7621_tplink_er605-v2.dts
+++ b/target/linux/ramips/dts/mt7621_tplink_er605-v2.dts
@@ -12,6 +12,9 @@
 
 	chosen {
 		bootargs = "console=ttyS0,115200 noinitrd";
+		// Override bootargs because u-boot passes wrong root parameter.
+		// Instead allow kernel determine root automatically by looking for rootfs volume
+		bootargs-override = "console=ttyS0,115200 noinitrd ubi.mtd=3,2048";
 	};
 
 	aliases {