I went down the eeprom rabbithole a bit. To access and manipulate RPI4's
eeprom directly,
Plan 9 would need to support a device like /dev/vchiq (VideoCore interface)
on Linux:

https://github.com/raspberrypi/linux/blob/rpi-3.2.27/drivers/misc/vc04_services/interface/vchiq_arm/vchiq_arm.c#L334

The rest of the tools (e.g. rpi-eeprom-update) eventually use vcgencmd
which ioctl into /dev/vchiq.
My /dev/mmcblk comment was wrong.

On Sat, Jan 16, 2021 at 2:15 PM Bakul Shah <bakul@iitbombay.org> wrote:

> I believe all it takes is the presence of recover.bin file on the dos
> partition.
> You still need an updated eeprom image for it to flash the real eeprom with
>  but preparing this image doesn't require any h/w access. Unless things
> have changed in the last year.
>
> On Jan 16, 2021, at 1:24 PM, Skip Tavakkolian <skip.tavakkolian@gmail.com>
> wrote:
>
> regarding /dev/serial, this should be helpful for anyone wanting to set up
> netboot.
>
> If you have a number of RPI's that netboot, the way that the common
> config.txt is segmented for each board is by using the '[serial number in
> hex]' section headers. Unfortunately, netboot also requires changing the
> BOOT_ORDER parameter in EEPROM, which requires access to /dev/mmcblk* and
> some way of generating the proper .bin format. So Linux is still needed for
> the initial setup.
>
> On Sat, Jan 16, 2021 at 9:13 AM Richard Miller <9fans@hamnavoe.com> wrote:
>
>> > I didn't realise the 9pi image was not up to date.
>> > I'll put a new one on 9p.io today.
>> 
>> I've done that. Sorry for letting it get out of sync with the kernel
>> source.
>> 
>> The new contrib/miller/9pi.img.gz has
>> - the latest firmware files from the raspberry pi github
>> - the update to initialise xhci firmware for pi400 and recent pi4 boards
>> - a tweak to the pi4 gigabit ethernet driver to allow jumbo packets
>> - a new /dev/serial device from which you can read the board serial
>> number (I forget why)
>> 
>> Nothing outside the kernel has changed from the previous image.
>> 
>
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