1======================== 2Direct filesystem lookup 3======================== 4 5Direct filesystem lookup is the most common form of firmware lookup performed 6by the kernel. The kernel looks for the firmware directly on the root 7filesystem in the paths documented in the section 'Firmware search paths'. 8The filesystem lookup is implemented in fw_get_filesystem_firmware(), it 9uses common core kernel file loader facility kernel_read_file_from_path(). 10The max path allowed is PATH_MAX -- currently this is 4096 characters. 11 12It is recommended you keep /lib/firmware paths on your root filesystem, 13avoid having a separate partition for them in order to avoid possible 14races with lookups and avoid uses of the custom fallback mechanisms 15documented below. 16 17Firmware and initramfs 18---------------------- 19 20Drivers which are built-in to the kernel should have the firmware integrated 21also as part of the initramfs used to boot the kernel given that otherwise 22a race is possible with loading the driver and the real rootfs not yet being 23available. Stuffing the firmware into initramfs resolves this race issue, 24however note that using initrd does not suffice to address the same race. 25 26There are circumstances that justify not wanting to include firmware into 27initramfs, such as dealing with large firmware files for the 28remote-proc subsystem. For such cases using a userspace fallback mechanism 29is currently the only viable solution as only userspace can know for sure 30when the real rootfs is ready and mounted. 31