What does mkknlimg exactly do, and what is a trailer?

Question

I recomplie the Raspbian kernel for Cirrus Logic audio card, following this description.

https://github.com/CirrusLogic/rpi-linux/wiki/Building-the-code

and I wonder what does mkknlimg exaclty do.

and I found these:

"Note that DT and ATAGs are mutually exclusive. As a result, passing a DT blob to a kernel that doesn't understand it causes a boot failure. To guard against this, the loader checks kernel images for DT-compatibility, which is marked by a trailer added by the mkknlimg utility; this can be found here, or in the scripts directory of a recent kernel source tree. Any kernel without a trailer is assumed to be non-DT-capable." at https://www.raspberrypi.org/documentation/configuration/device-tree.md

What I found: From ATAGs I found that They are used to pass physical memory layouts to the kernel at booting process on ARM arch, and they are incompatible with the Device Tree blob.

My question: What is a "trailer"?

Answer 1

It's just a blob of data at the end of the kernel image. Observe:

# knlinfo /boot/kernel7.img
Kernel trailer found at 4222224/0x406d10:
  KVer: "Linux version 4.4.8-v7+ (dc4@dc4-XPS13-9333) (gcc version 4.9.3 (crosstool-NG crosstool-ng-1.22.0-88-g8460611) ) #880 SMP Fri Apr 22 21:55:04 BST 2016"
  DTOK: y
  DDTK: y
  270X: 01000000
  283X: 00000000
  283x: n

Note how the blob is starting at offset 4222224. Compare the above with what's there:

# dd if=/boot/kernel7.img bs=1 skip=4222224 | hd 
00000000  00 00 00 00 00 00 00 00  00 00 00 00 04 00 00 00  |................|
00000010  32 38 33 78 00 00 00 00  04 00 00 00 32 38 33 58  |283x........283X|
00000020  01 00 00 00 04 00 00 00  32 37 30 58 01 00 00 00  |........270X....|
00000030  04 00 00 00 44 44 54 4b  01 00 00 00 04 00 00 00  |....DDTK........|
00000040  44 54 4f 4b 4c 69 6e 75  78 20 76 65 72 73 69 6f  |DTOKLinux versio|
00000050  6e 20 34 2e 34 2e 38 2d  76 37 2b 20 28 64 63 34  |n 4.4.8-v7+ (dc4|
00000060  40 64 63 34 2d 58 50 53  31 33 2d 39 33 33 33 29  |@dc4-XPS13-9333)|
00000070  20 28 67 63 63 20 76 65  72 73 69 6f 6e 20 34 2e  | (gcc version 4.|
00000080  39 2e 33 20 28 63 72 6f  73 73 74 6f 6f 6c 2d 4e  |9.3 (crosstool-N|
00000090  47 20 63 72 6f 73 73 74  6f 6f 6c 2d 6e 67 2d 31  |G crosstool-ng-1|
000000a0  2e 32 32 2e 30 2d 38 38  2d 67 38 34 36 30 36 31  |.22.0-88-g846061|
000000b0  31 29 20 29 20 23 38 38  30 20 53 4d 50 20 46 72  |1) ) #880 SMP Fr|
000000c0  69 20 41 70 72 20 32 32  20 32 31 3a 35 35 3a 30  |i Apr 22 21:55:0|
000000d0  34 20 42 53 54 20 32 30  31 36 00 00 96 00 00 00  |4 BST 2016......|
000000e0  4b 56 65 72 f0 00 00 00  04 00 00 00 52 50 54 4c  |KVer........RPTL|
000000f0

Cute, huh? :)

/ji

PS: I'm also futzing around with a cirrus logic card; I got it to work without any problems, just cross-compiling.