# What I should check if kernel outputs nothing to console

**URL:** https://mangoh.discourse.group/t/what-i-should-check-if-kernel-outputs-nothing-to-console/10065
**Category:** mangOH Yellow
**Created:** [July 27, 2022, 11:41pm UTC](https://mangoh.discourse.group/t/what-i-should-check-if-kernel-outputs-nothing-to-console/10065 "2022-07-27T23:41:28Z")
**Posts on this page:** 1
**Page:** 1

<div class="post-metadata">

### Author: ![uchida.s](https://avatars.discourse-cdn.com/v4/letter/u/b9bd4f/32.png) [@uchida.s](https://mangoh.discourse.group/u/uchida.s)
#### Post date: [July 27, 2022, 11:41pm UTC](https://mangoh.discourse.group/t/what-i-should-check-if-kernel-outputs-nothing-to-console/10065/1 "2022-07-27T23:41:28Z")

</div>

I found a Linux kernel I had rebuilt does not output anything.  
What should I check?

What I did:

1. I rebuilt a Linux kernel with GCOV enabled.
2. I checked if the flash area is sufficient to store the kernel, by executing `cat /proc/mtd` on a mangOH Yellow and `swicwe` on a laptop.
3. I wrote the kernel module to WP7605 flash on the mangOH Yellow by `swiflash`, which rebooted the mangOH Yellow after I finished writing the module.

What I observed:

- The output from CON(a serial console) when executing the rebuilt module was the same as that when extending the original one.
- The LED on the mangOH Yellow was not yellow but green, which means the kernel boot procedures did not finish properly.
- An Ethernet over USB connection between the mangOH Yellow and my Ubuntu computer was not established.

Details:  
_GCOV enabled_ means the following configuration variables are set to `y`:

- `CONFIG_GCOV_KERNEL`
- `CONFIG_GCOV_PROFILE_ALL`

**Flash area versus kernel size**  
`cat /proc/mtd`

```auto
dev: size erasesize name
mtd0: 00280000 00040000 "sbl"
mtd1: 00280000 00040000 "mibib"
mtd2: 00680000 00040000 "backup"
mtd3: 00100000 00040000 "security"
mtd4: 00200000 00040000 "persist"
mtd5: 01180000 00040000 "efs2"
mtd6: 04f40000 00040000 "swifota"
mtd7: 00180000 00040000 "tz"
mtd8: 000c0000 00040000 "devcfg"
mtd9: 000c0000 00040000 "rpm"
mtd10: 02000000 00040000 "modem"
mtd11: 00100000 00040000 "aboot"
mtd12: 00f00000 00040000 "boot"
mtd13: 01e00000 00040000 "system"
mtd14: 008c0000 00040000 "lefwkro"
mtd15: 01900000 00040000 "swirw"
mtd16: 08300000 00040000 "userapp"
mtd17: 03680000 00040000 "reserved"
mtd18: 02180000 00040000 "slot_2"
mtd19: 02180000 00040000 "slot_1"
mtd20: 02180000 00040000 "slot_0"

```

`swicwe -P yocto_wp76xx_with_coverage.4k.cwe`

```auto
 Type Size(Exclude Header) Product Compress Version
*APPL 39843984 Y921 0 Mon Mar 14 07:04:56 UTC 2022
 SYST 25427968 9X28 0 Mon Mar 14 07:04:56 UTC 2022
 APPS 14415216 9X28 0 3.18.131 20220314070454

```

**Size comparison**

- The size of mtd13 (0x01e00000 = 31,457,280) is bigger than that of SYST (25,427,968).
- The size of mtd12 (0x00f00000 = 15,728,640) is bigger than that of APPS (14,415,216).

Therefore the flash area is sufficient to store the kernel.