device_tree.md

Naming conventions

• DT

  Device Tree.

• FDT

  Flat Device Tree.

Dump from skiboot

Hostboot prepares HDAT for skiboot, which turns it into a DT that we want to dump here. The idea is to make skiboot build FDT, print out its location and size and then dump it from memory using pdbg while skiboot is halted.

Patching skiboot

Find skiboot sources in your locally built talos-op-build. It lies in output/build/skiboot-*/. Apply the following patch to the sources:

 core/init.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/core/init.c b/core/init.c
index 0fe6c168..9da8c3b6 100644
--- a/core/init.c
+++ b/core/init.c
@@ -1015,8 +1015,15 @@ void __noreturn __nomcount main_cpu_entry(const void *fdt)
 		if (parse_hdat(true) < 0)
 			abort();
 	} else if (fdt == NULL) {
+		void *tmp_fdt;
+
 		if (parse_hdat(false) < 0)
 			abort();
+
+		tmp_fdt = create_dtb(dt_root, false);
+		printf("FDT is located at 0x%8p, its size is 0x%08x\n",
+			   tmp_fdt, fdt_totalsize(tmp_fdt));
+		abort();
 	} else {
 		dt_expand(fdt);
 	}

Updating skiboot

Rebuild flash image:

export KERNEL_BITS=64
. op-build-env
op-build -j7 skiboot-rebuild openpower-pnor-rebuild

Possibly extract just the PAYLOAD section and update only it (its size is 1 MiB):

fcp file:output/images/talos.pnor:PAYLOAD - -o 0 -R > PAYLOAD

Obtaining the dump

Update flash image, start the machine and wait for it to hang in this state:

[   36.253046600,5] FDT is located at 0x30423a00, it's size is 0x0001728f
[   36.253141301,0] Aborting!
CPU 0010 Backtrace:
 S: 0000000031c83d10 R: 0000000030013840   .backtrace+0x48
 S: 0000000031c83db0 R: 000000003001b2c4   ._abort+0x4c
 S: 0000000031c83e30 R: 0000000030014c84   .main_cpu_entry+0x13c
 S: 0000000031c83f00 R: 0000000030002730   boot_entry+0x1c0

Finally, dump DT in binary format (update address and size in the command!):

ssh root@talos pdbg -p0 -c1 -t0 getmem 0x30423a00 0x0001728f > hb-skiboot.dtb
# convert to text
dtc -I dtb -O dts < hb-skiboot.dtb > hb-skiboot.dts