I got a new (to me) computer. I installed Slackware, built a new kernel on the machine. It fails to detect any USB device, which includes my only keyboards and mice. I figured this out by putting lsusb (and dmesg and lsmod) in rc.M, saving the output to a file. lsusb lists only the hubs, no attached devices. I forced loading of the relevant modules; they load, but do nothing because they see no devices. When I boot from a USB drive they work. What is failing to happen?

There is a comprehensive list of functionalities for USB devices in kernel configuration, also for HID. You want us to guess what you have missed (I will not) or you will drop your kernel conf into a pastebin site for us to look at it?

I started with the same kernel config that works on this computer with the same keyboard, mouse, and usb drives. Then I enabled every item that deals with usb keyboard and mouse input and built a clean kernel. It misses my bluetooth adaptor and smartphone, too, every device I have. The BIOS finds them. They worked in a Win 95 partition that I wiped out when I formatted the disk. When I boot from the USB drive it sees the keyboard (but lacks a mouse driver module) and drives.

The dmesg from when it doesn't work finds every USB hub but no connected device. I think there's something more basic wrong. I wouldn't think of subjecting others to reading my kernel config, but if anyone wants to https://pastebin.com/tLUPCLgP

I built the kernel with a bunch of USB facilities built in, rather than as modules (I prefer to make everything a module) and building some new facilities which I don't know I have, and it works. I'd still like to know what makes the difference.

I narrowed it down to:

I haven't tried to narrow it down further. The BIOS sees these devices before I load the kernel; why can't Linux? I have no UHCI devices, so it's probably irrelevant. My laptop has EHCI but no XHCI. I build EHCI as modules on it, and they work. So maybe it's just XHCI. Is there a problem with expecting their modules to load?