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