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How can I find what chip I have (what version of raspberry pi) with Ubuntu 18.04 server for arm64? What file can I check or what command can I run? /proc/cpuinfo does not have useful information, just some generic details without mention of the pi.

At least on Ubuntu 18.04 server for arm64 and with a Raspberry Pi 3 B, the following command gives the board, including revision:

lshw

A less verbose output that's easier to read is given by:

lshw -short

That gave me my board as a "Raspberry Pi 3 B Rev 1.2", which is exactly what was needed in this case.

The command lscpu is what you are looking for. Here's an example output of the command (taken on my Raspberry Pi 3B+):

lscpu

which produces the following output:

Architecture:          armv7l

Byte Order:            Little Endian

CPU(s):                4

On-line CPU(s) list:   0-3

Thread(s) per core:    1

Core(s) per socket:    4

Socket(s):             1

Model:                 4

Model name:            ARMv7 Processor rev 4 (v7l)

CPU max MHz:           1200,0000

CPU min MHz:           600,0000

BogoMIPS:              38.40

Flags:                 half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt vfpd32 lpae evtstrm crc32

Another option is the inxi command (you might have to install it if not present with sudo apt install inxi). Use the flag -C as follows:

inxi -C

which produces the following output:

CPU:       Quad core ARMv7 rev 4 (v7l) (-MCP-) (ARM) 

           clock speeds: max: 1200 MHz 1: 1200 MHz 2: 1200 MHz 3: 1200 MHz 4: 1200 MHz

As for identifying which chip your board has, you can check the chip on the board as well the manufacturers website for info and as well as the different selling outlets for getting technical details.

I attempted to port pigpio to Debian arm64, in the end my attempt failed because I discovererd that the Debian arm64 kernel doesn't support the userspace mailbox interface that pigpio relies on, but in doing so I did some research on how to detect Pis while running Debian arm64 kernels. I suspect this will also work for ubuntu arm64.

As you have discovered /proc/cpuinfo only has CPU core information on these kernels. Fortunately the information can be found elsewhere in /proc

Firstly to check if the device is a Pi or not, I checked /proc/device-tree/model , this has a text string describing the device, so false positives are unlikely.

To get the revision code I used /proc/device-tree/system/linux,revision , this contains the revision code as a big-endian binary integer. So it needs to be read out of the file as a binary integer, then converted to little-endian (I used ntohl for this).

You can see my code at https://github.com/joan2937/pigpio/pull/255/commits/2e229d667fde8a2a881d5aa8482b2bb936b09f26