Boot Debian and Gentoo with Grub. Kernel Panic!

Question

I've a Debian in my computer and now I want to add a Gentoo distro. I've installed Gentoo by terminal in Debian and chrooted to Gentoo in order to emerge all packages.

Now, I added Gentoo to Grub (installed on Debian) and for that I used the command:

root@reborn:/home/jota# grub-mkconfig -o /boot/grub/grub.cfg
Generating grub configuration file ...
Found background image: /usr/share/images/desktop-base/desktop-grub.png
Encontrada imagen de linux: /boot/vmlinuz-3.16.0-4-amd64
Encontrada imagen de memoria inicial: /boot/initrd.img-3.16.0-4-amd64
Encontrado Gentoo Base System release 2.3 en /dev/sda3
hecho
root@reborn:/home/jota#

So it works ok. When I reboot I have the new entry of Gentoo but when is booting the system halts with kernel panic error

Kernel panic - not synching:VFS:Unable to mount root fs on unknown-block(0,0)

The part of grub.cfg seems ok,

menuentry 'Gentoo Base System release 2.3 (en /dev/sda3)' --class gnu-linux --class gnu --class os $menuentry_id_option 'osprober-gnulinux-simple-f40f428c-f76d-489f-9d97-a259f34ed457' {
    insmod part_msdos
    insmod ext2
    set root='hd0,msdos3'
    if [ x$feature_platform_search_hint = xy ]; then
      search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos3 --hint-efi=hd0,msdos3 --hint-baremetal=ahci0,msdos3  f40f428c-f76d-489f-9d97-a259f34ed457
    else
      search --no-floppy --fs-uuid --set=root f40f428c-f76d-489f-9d97-a259f34ed457
    fi
    linux /boot/vmlinuz-4.4.39-gentoo root=/dev/sda3 ro
}

fdisk output is:

Device     Boot     Start       End   Sectors  Size Id Type
/dev/sda1            2048  97656831  97654784 46,6G 83 Linux
/dev/sda2        97656832 116088831  18432000  8,8G 82 Linux swap / Solaris
/dev/sda3  *    116088832 218488831 102400000 48,8G 83 Linux

Where sda1 is Debian /, sda2 is swap for both Debian and Gentoo, and sda3 is Gentoo /

I've tried to change the boot flag trough sda1 and sda3 with no changes.

And the fstab file of Gentoo is:

/dev/sda3       /       ext4        errors=remount-ro   0 1
/dev/sda2       none        swap        sw          0 0

Edit: As mentioned in comments, Gentoo doesn't generate initrd.img file, I've never needed it before.

root@reborn:/boot# ls /boot/
config-3.16.0-4-amd64  grub  initrd.img-3.16.0-4-amd64  System.map-3.16.0-4-amd64  vmlinuz-3.16.0-4-amd64
root@reborn:/boot# ls /mnt/gentoo/boot/
config-4.4.39-gentoo  config-4.4.39-gentoo.old  grub  System.map-4.4.39-gentoo  System.map-4.4.39-gentoo.old  vmlinuz-4.4.39-gentoo  vmlinuz-4.4.39-gentoo.old
root@reborn:/boot# 

Answer 1

It was kernel driver problem. Need to add a driver for PATA controller.

CONFIG_PATA_ATIIXP=y

I found it by lshw command

*-ide
     description: IDE interface
     product: SB7x0/SB8x0/SB9x0 IDE Controller
     vendor: Advanced Micro Devices, Inc. [AMD/ATI]
     physical id: 14.1
     bus info: pci@0000:00:14.1
     version: 40
     width: 32 bits
     clock: 66MHz
     capabilities: ide bus_master
     configuration: driver=**pata_atiixp** latency=32