Feature Suggestion: "config.txt"-based control for inc/exclusion of /boot/firmware files

[britkig]

Can I suggest a feature for achieving even-smaller resultant images, by allowing a user to supply a "config.txt" file, which could be read to manipulate the inclusion/exclusion of files that normally end up in /boot/firmware? For example, lets say a user has this in their config.txt file...
...against this text string (for example, destined for a headless Compute Module 4 with on-board storage, but no WiFi/Bluetooth module)...

#arch=v8+
#model=bcm2711-rpi-cm4
disable_overscan=1
disable_splash=1
dtoverlay=disable-bt
dtoverlay=disable-wifi
dtoverlay=i2c-fan,emc2301,i2c_csi_dsi
dtparam=watchdog=on
gpu_mem=16

This could instruct to add the bcm2711-rpi-cm4.dtb device tree base, the latest Pi arm64 v8+ kernel8.img, and, through via a match-all regex expression with the multi-line flag...

^dtoverlay=(.+?)[,\n$]

...to include the disable-bt, disable-wifi and i2c-fan device tree overlays. Another possibility is, by searching for gpu_mem=16, to determine whether or not to use the smaller fixup4cd.dat and start4cd.elf firmware files over the full size fixup4.dat and start.elf ones (subject to if the intended Pi board/CPU model uses them).

[roliver-rpi]

(@tdewey-rpi for visibility)

Hi @britkig , thank you for opening this issue.

Device tree blobs/overlay end up being a relatively small part of the generated images:

$ find . -name '*.dtb*' -exec du -ch {} + | tail -1
2.7M	total

The same cannot be said for the various firmware files:

$ find . -name '*.dat' -o -name '*.elf' -exec du -ch {} + | tail -1
22M	total

Removing non-'cd' FW files alone would represent a ~20M saving in the gpu_mem=16 case.

[britkig]

In regards to dtoverlays, yes, but maybe that can be opt-in as a "comfort" feature for those seeking a maximum reduction.

In the same vein, the main DTB files, and as of writing there's 26 of them, make up ~1.22MiB, and yet only one will ever get used once-installed. Maybe the detection can be achieved with one-or-more always-commented #model=bcm2711-rpi-cm4 using the regex...

^#model=(.+)$

This part can be reduced to less than 85KB.

Then, there's the kernel, five of those in the official firmware repository amounting to ~41MB, only need one of them in a deployment install, so another average ~30MB in savings can be achieved there by using just the one; could be fished out with:

^#arch=(.+)$

The result is below 10MB as of kernel version 6.6.74.

Since opening this issue thread, I have spotted another avenue for kernel-linked reductions, but it's outside of /boot/firmware, so I'll explore in opening a new issue on that once I've an A-to-B way on it.