Compiled a custom 3.15.5. kernel for an Optiplex 745, running Debian Wheezy:

make proper
make allmodconfig
make localyesconfig

Checked it for any obvious missing/incorrect stuff and compiled, then copied the kernel etc. to /boot, installed modules (such as existed), and updated GRUB2.

It does not boot. Complains that it cannot mount the root filesystem "null" or unknown-block(0,0) and panics, no matter what I specify for root partition. This suggests to me that it is not seeing the partitions at all; however, the necessary ATA drivers are all definitely compiled into the kernel, and the required partition type is enabled in the config. So I have no idea what's going on.

Likewise, the necessary filesystem drivers are compiled in, and the SATA, PATA, and SCSI hard disk drivers.

Did support for this computer's ATA controller get dropped from the kernel, or something? What is happening here? I've never seen a kernel panic like this when all necessary drivers are configured.

Edit: to add insult to injury, the kernel does in fact see the partitions - it prints a list of them when it panics. It appears to be just ignoring the root= directive. How can I make it not ignore that?

Just to be sure: did you built-in the driver for root's (/) file system, or included it in an initrd?

Yup. Root is ext4, ext4 drivers are compiled into the kernel.

I also just tried recompiling without initrd support, having read in one thread on one forum somewhere that enabling initrd makes the kernel ignore the root directive. No go. It continues to ignore it.

I think maybe the issue was the "internal kernel command line" option being enabled, set to defaults, and overriding the specified parameters... Tried recompiling with that disabled, but wound up with all debug options disabled, and a kernel that traces itself more and more slowly until it panics.

My bad. It looks like allmodconfig does not produce a usable kernel, and stripping it down with localyesconfig does nothing to change that; I will try oldconfig instead.

Maybe we'll be able to help you better if you state your aim in compiling a custom kernel.

Nevertheless, I'd recommend that you take as a basis your running kernel's config file (or one provided by your distribution), do a "make oldconfig" then possibly "make menuconfig" (replace "menu" with "n", "x" or "g" if you prefer), to tweak the configuration to your liking.

I have done that any times. I was hoping that the new 'localyesconfig' option would allow me to automatically generate stripped-down GrSecurity kernels from a base config.

But it clearly is not adequate on its own. This time I got a kernel that booted... And did not include keyboard and mouse drivers. Yay.

I'm marking this as [solved] for now, since what I was trying to do is clearly not supported at all.