[OpenWrt-Devel] ipq40xx: bootarg Manipulation Failing

[Jeff Kletsky]

I'm trying to bring up an IPQ4019-based Linksys EA8300 and have a
challenge with the OEM bootargs from U-Boot. While they could be
modified once in OpenWrt, I'm hoping to provide a serial-less way
for users to easily flash OpenWrt from the OEM web interface.

The OEM boot args are

   init=/sbin/init rootfstype=ubifs ubi.mtd=NN,2048 root=ubi0:ubifs 
rootwait rw

where NN is either 11 or 13, depending on the "selected" firmware

The OpenWrt boot args that work are

   ubi.mtd=NN root=/dev/ubiblock0_0


Changing the OEM line to include `root=ubi0:rootfs` as a quick way to
check if a change of the UBI volume name would resolve the issue
failed to boot. `/dev/ubiblock0_0` was the only viable option shown.


TL;DR
=====

   * I've tried 0067-generic-Mangle-bootloader-s-kernel-arguments.patch
     It doesn't seem to be able to modify the FDT

   * Can I better confirm if atags_to_fdt() is executing?

   * If it is executing, what am I not doing properly to modify the FDT?

   * If it is not executing:
     * Is there an example of command-line modification in `init`?
     * Is there a better way, short of changing U-Boot, to make the change?


---

I'm aware of 0067-generic-Mangle-bootloader-s-kernel-arguments.patch
used by the ipq80xx devices and copied that patch over. I configured
the kernel appropriately, based on the #ifdef statements in the patch

   root at OpenWrt:~# zcat /proc/config.gz | fgrep ATAG
   # CONFIG_ATAGS is not set
   CONFIG_ARM_ATAG_DTB_COMPAT=y
   # CONFIG_ARM_ATAG_DTB_COMPAT_CMDLINE_FROM_BOOTLOADER is not set
   # CONFIG_ARM_ATAG_DTB_COMPAT_CMDLINE_EXTEND is not set
   CONFIG_ARM_ATAG_DTB_COMPAT_CMDLINE_MANGLE=y

as well as adding in the `append-rootblock` property to the DTS

         chosen {
                 // stdout-path = "serial0:115200n8";    // set in 
dk07.1.dtsi
                 bootargs = "ubi.mtd=11 root=/dev/ubiblock0_0";
                 bootargs-append = " clk_ignore_unused";
                 append-rootblock = "append_rootblock=";
                 call-count = "not";
         };

(call-count was not present in initial tests, and is discussed later)

There was no evidence of the patched code running, either in the
booted /proc/device-tree, or from early `init` messages
(`Bootloader command line not present` appears, not
  `Bootloader command line (ignored): ... `)

---

Confirming that the code was running is challenging, as it is so early
in the boot process, still in  arch/arm/boot/compressed/head.S

To try to confirm that it was running, I moved the "extend the FDT"
code up and tried to write to a new, already defined property:

int atags_to_fdt(void *atag_list, void *fdt, int total_space)
{
         struct tag *atag = atag_list;
         /* In the case of 64 bits memory size, need to reserve 2 cells for
          * address and size for each bank */
         uint32_t mem_reg_property[2 * 2 * NR_BANKS];
         int memcount = 0;
         int ret, memsize;

         /* let's give it all the room it could need */
         ret = fdt_open_into(fdt, fdt, total_space);
         if (ret < 0)
                 return ret;

         /* indicate that this has been called and how many times */
         /* for now, just say "yes" */
         setprop_string(fdt, "/chosen", "call-count", "yes");

before any of the branching.

There is no change to the call-count property; it remains "not" as set
by the DTS.

---

I've looked through the logic in arch/arm/boot/compressed/head.S
and I only see one reason why atags_to_fdt() wouldn't be called
(assuming I've followed the nested #ifdef properly)

<https://github.com/torvalds/linux/blob/v4.19/arch/arm/boot/compressed/head.S#L294>

(I previously had the same results with 4.14)

* 294: Branch to dtb_check_done (386) if there is no DTB signature

The signature looks to be there, based on hexdump (0xd00dfeed):

0021fe50  00 00 00 00 41 52 4d 20  4f 70 65 6e 57 72 74 20  |....ARM 
OpenWrt |
0021fe60  6c 69 6e 6b 73 79 73 5f  65 61 38 33 30 30 20 64 
|linksys_ea8300 d|
0021fe70  65 76 69 63 65 20 74 72  65 65 20 62 6c 6f 62 00  |evice tree 
blob.|
0021fe80  00 00 00 03 00 00 4a fe  00 00 00 1b d0 0d fe ed 
|......J.........|
                                                ===========

---

Which leaves me with the puzzle:

   * Can I better confirm if atags_to_fdt() is executing?

   * If it is executing, what am I not doing properly to modify the FDT?

   * If it is not executing:
     * Is there an example of command-line modification in `init`?
     * Is there a better way, short of changing U-Boot, to make the change?


Thanks,

Jeff


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