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7.3.2.1. Sysfs
The sysfs filesystem was mentioned briefly above. One may wonder how sysfs knows about the devices present on a system and what device numbers should be used for them. Drivers that have been compiled into the kernel directly register their objects with a sysfs (devtmpfs internally) as they are detected by the kernel. For drivers compiled as modules, this registration will happen when the module is loaded. Once the sysfs filesystem is mounted (on /sys), data which the drivers register with sysfs are available to userspace processes and to udevd for processing (including modifications to device nodes).