Pinephone Pro - Booting an OS off SPI vs eMMC

I finally got a chance to pick the Pinephone Pro back up and play with it some more.

I was able to charge up the battery, and boot the phone and verify that Tow-Boot was installed on it properly. That was my first step. I believe I verified this by holding the volume down button, and waiting for the light to turn aqua (note, it may have been volume up, I should check this for correctness).

Next, I rebooted the phone,  and it booted into the Manjaro OS which is installed on the eMMC drive of the phone.

Next, I put the PostMarketOS into the microSD card slot, and booted the phone. Apparently Tow-Boot uses the following boot order:

1. SPI - more on this in a bit, I had to learn what this is
   
2. microSD Card
3. eMMC (which has Manjaro on it)

I didn't get a Boot Menu - but maybe a key sequence (volume up?) would give me such a menu. It booted straight into the PostMarket OS. 

I proceeded to experiment with PostMarket OS, and did a complete update of all of the packages on it.

Next, I wondered how I could "replace" the default Manjaro with the PostMarket OS, which was newer than Manjaro, such that it would boot PostMarket OS on the eMMC, allowing me recycle the microSD card for perhaps another OS distribution I could take a look at later. 

It turns out, that there is a PostMarketOS "on-disk installer".  It is called pmbootstrap.

THIS is where I had to learn about SPI. Because there is a warning about over-writing your Tow-Boot installation, if Tow-Boot was not installed on SPI. 

so...what is SPI? (more search required)

SPI Flash is a type of non-volatile memory that uses the Serial Peripheral Interface (SPI) protocol for communication. It is commonly used in embedded systems for data storage and transfer, allowing devices to retain information even when powered off. 

Apparently it is a newer (or improved, perhaps) concept, found on phones with System-On-A-Chip (SOC) architectures. 

so...how do you know if you even have SPI?

Answer: I had to figure out which version of Pinephone Pro I have. 

I finally learned that there is a Developer Edition of the Pinephone Pro, and there is a Explorer Edition. The Explorer Edition supposedly has the SPI. 

But what confused me, is that it said the phone supporting SPI had the Rockchip RK3399S SoC. And when I went into the terminal on the phone and ran "lscpu", it said I had an ARM Cortex A-53 chip. 

so...now I am thoroughly confused.

Well, I finally learned, that the Rockchip RK3399S SoC combines four Cortex-A53 cores with two Cortex-A72 cores.

hmmm, I did not see the 72 in the lscpu command I ran - but, it does look like I have the SPI.

but, how do I know that Tow-Boot was installed on the SPI, versus the eMMC? Because if I have this wrong, I can't boot an OS as there would be no bootloader partition.

I think the SPI is mmcblk1 device. And /boot is on mmcblk1p1 partition of that device.

The Manjaro (previous installation) is definitely on the eMMC, which is on mmcblk2 device, which has two partitions on it, one of them being /root.