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