On Apple platforms, firmware selection uses the following elements:

  Property         Example   Source
  ==============   =======   ========================
* Chip name        4378      Device ID
* Chip revision    B1        OTP
* Platform         shikoku   DT (ARM64) or ACPI (x86)
* Module type      RASP      OTP
* Module vendor    m         OTP
* Module version   6.11      OTP
* Antenna SKU      X3        DT (ARM64) or ACPI (x86)

In macOS, these firmwares are stored using filenames in this format
under /usr/share/firmware/wifi:

    C-4378__s-B1/P-shikoku-X3_M-RASP_V-m__m-6.11.txt

To prepare firmwares for Linux, we rename these to a scheme following
the existing brcmfmac convention:

    brcmfmac<chip><lower(rev)>-pcie.apple,<platform>-<mod_type>-\
	<mod_vendor>-<mod_version>-<antenna_sku>.txt

The NVRAM uses all the components, while the firmware and CLM blob only
use the chip/revision/platform/antenna_sku:

    brcmfmac<chip><lower(rev)>-pcie.apple,<platform>-<antenna_sku>.bin

e.g.

    brcm/brcmfmac4378b1-pcie.apple,shikoku-RASP-m-6.11-X3.txt
    brcm/brcmfmac4378b1-pcie.apple,shikoku-X3.bin

In addition, since there are over 1000 files in total, many of which are
symlinks or outright duplicates, we deduplicate and prune the firmware
tree to reduce firmware filenames to fewer dimensions. For example, the
shikoku platform (MacBook Air M1 2020) simplifies to just 4 files:

    brcm/brcmfmac4378b1-pcie.apple,shikoku.clm_blob
    brcm/brcmfmac4378b1-pcie.apple,shikoku.bin
    brcm/brcmfmac4378b1-pcie.apple,shikoku-RASP-m.txt
    brcm/brcmfmac4378b1-pcie.apple,shikoku-RASP-u.txt

This reduces the total file count to around 170, of which 75 are
symlinks and 95 are regular files: 7 firmware blobs, 27 CLM blobs, and
61 NVRAM config files. We also slightly process NVRAM files to correct
some formatting issues.

To handle this, the driver must try the following path formats when
looking for firmware files:

    brcm/brcmfmac4378b1-pcie.apple,shikoku-RASP-m-6.11-X3.txt
    brcm/brcmfmac4378b1-pcie.apple,shikoku-RASP-m-6.11.txt
    brcm/brcmfmac4378b1-pcie.apple,shikoku-RASP-m.txt
    brcm/brcmfmac4378b1-pcie.apple,shikoku-RASP.txt
    brcm/brcmfmac4378b1-pcie.apple,shikoku-X3.txt *
    brcm/brcmfmac4378b1-pcie.apple,shikoku.txt

* Not relevant for NVRAM, only for firmware/CLM.

The chip revision nominally comes from OTP on Apple platforms, but it
can be mapped to the PCI revision number, so we ignore the OTP revision
and continue to use the existing PCI revision mechanism to identify chip
revisions, as the driver already does for other chips. Unfortunately,
the mapping is not consistent between different chip types, so this has
to be determined experimentally.
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Hector Martin <marcan@marcan.st>
Reviewed-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/E1oZDns-0077aY-Qn@rmk-PC.armlinux.org.uk