[Buildroot] Buildroot: enable lib/firmware/brcm for rpi0w

[Andreas Ziegler]

Hello Peter,

On 2021-03-25 21:20, Peter Seiderer wrote:
> 
> Many thanks for doing the tests!
> 

Not at all; after getting confused with different versions and 
repositories, I wanted to understand the reason behind this.

Short summary of my findings; I may state the obvious here, but it was 
new for me and maybe it helps someone with the same hazy knowledge about 
hardware engineering as myself:

Broadcom sold its RF division to Cypress in 2016. Cypress now supports 
the legacy wireless /bluetooth /radio hardware from Broadcom; that is 
the reason why BCM* and CYW* versions of firmware exist, and 
linux-firmware removed the original Broadcom supplied binaries.

Cypress does not directly sell to end customers, but provides silicon to 
hardware partners (AzureWave, Murata, ...), which in turn sell RF 
solutions. Cypress distributes firmware for their silicon chips 
(probably to same hardware partners).

End product is some type of SOC (subsystem on chip); the benefit is that 
tuning and regulatory issues have been taken care of. The SOC 
manufacturer is responsible for supplying bluetooth patch (HCD), 
regulatory data (CLM) and firmware configuration (TXT).

That means the firmware binary (BIN) can be obtained from any source, 
the other items (HCD, CLM, TXT) need to be sourced from the implementer.

>> The LibreELEC repository (rpi-wifi-firmware) seems to have adopted 
>> this
>> version in commit 7dbd877545ae15069f2bd0e73893af942500e23d on Jan. 25.
> 
> So a good reason to stay with LibreELEC (and fix the firmware package 
> to
> provide raspberrypi,4-model-b.txt as a link or copy, do you know if 
> missing
> it has some disadvantages or produces only the firmware load warning or 
> is
> there a fallback to plain brcmfmac43455-sdio.txt?), keeping up with
> RPi-Distro...

Yes, also my opinion to stay with LibreELEC. The repository is small, is 
updated in a timely manner, and is (slightly) better documented than 
RPi-Distro.

There is one thing (apart from providing the missing link) I would 
propose: merge rpi-bt-firmware and rpi-wifi-firmware into 
rpi-rf-firmware, combining the code from both .mk and Config.in files. 
Functionality would be identical, but one less firmware package. If 
anyone is interested, I could provide a patch.

In recent kernels, firmware load tries board-specific path first, then 
falls back to firmware derived path [1]. If I remember correctly, in 
kernels < 5.0 the firmware load failed.

>> There seem to be some bigger changes in the pipeline: linux-firmware
>> removed the Broadcom redistributed binaries in January [4] for the
>> 20210208 release; instead Cypress firmware should be used. The 
>> upstream
>> repository for this firmware seems to be murata-wireless /
>> cyw-fmac-fw [5]
>> 
>> I need to look into this in detail at some other time ...
> 
> Seems kind of (another) firmware mess...., different versions at 
> different
> repositories...., all with the same name (instead of clear version 
> string)...

There are at least three different versions available in Buildroot:

(available in Buildroot, repo 'master' version listed respectively)
   libreELEC		build 2021-01-04	version 7.45.229
   linux-firmware	build 2020-09-18	version 7.45.221
   murata-wireless	build 2020-09-18	version 7.45.221
(not in buildroot)
   rpi-distro		build 2021-01-04	version 7.45.229

> Regards,
> Peter

Kind regards,
Andreas

[1] 
https://elixir.bootlin.com/linux/v5.10.21/source/drivers/net/wireless/broadcom/brcm80211/brcmfmac/firmware.c#L616