I found a file
/sys/class/firmware/timeout, and this file contains just one word 60. Is this timeout related to
GRUB timeout? What is the practical use of it?
It is not related to any bootloader at all.
When a driver uses the kernel's common firmware loading infrastructure to load a firmware file, the kernel can either load the file directly from the standard
/lib/firmware directory tree, or it can optionally start an user-space process to handle the firmware load.
This user-space process used to be part of the
hotplug subsystem, then a part of
udev, but I think it's currently deprecated and the in-kernel version is the primary means of loading firmware for any drivers.
If the entire firmware load operation takes more than the number of seconds specified in
/sys/class/firmware/timeout, the operation will be considered failed. As a result, the driver that requested the firmware will most likely fail too.
More details can be found in the Documentation/firmware_class directory of the Linux kernel source code package.