The DE10-Nano board ships with a MicroSD card that is loaded with Angstrom Linux. However, you can also download a couple of other MicroSD images with different Linux flavors from the Terasic website. Since I planned to use OpenCV to capture and stitch images, I downloaded the MicroSD image with LXDE desktop, which handily already has OpenCV and a few OpenCV example programs on it.
The DE10 Nano uses an application called U-Boot to load Linux for the ARM cores to run and to load an image onto the FPGA. I don't know exactly how it works, but I've been able to get it to do my bidding with a bit of trial and error. The first thing I attempted to do was set aside some space in the DDR3 memory for the FPGA to write to, so the ARM wouldn't attempt to use that space for Linux memory. This turned out to be fairly simple, as I was able to follow the example "Nios II Access HPS DDR3" in the DE10 Nano User Manual. This example shows how to communicate with U-Boot and Linux on the DE10 using PuTTY on your PC, and set the boot command in U-Boot to allow Linux to only use the lower 512MB of the DDR3. Here's the pertinent snippet that you enter into U-Boot before it runs Linux:
setenv mmcboot "setenv bootargs console=ttyS0,115200 root=/dev/mmcblk0p2 rw \ rootwait mem=512M;bootz 0x8000 - 0x00000100"
My next task was a bit more difficult. I also needed my FPGA to be able to access the DDR3 to write images for my OpenCV program in Linux to access. This required the use of a boot script that tells U-Boot to load the FPGA image from the SD card and then enable the FPGA-to-DDR3 bridge so it can be used.
I used the helpful instructions on this page to create my boot script:
fatload mmc 0:1 $fpgadata soc_system.rbf; fpga load 0 $fpgadata $filesize; setenv fpga2sdram_handoff 0x1ff; run bridge_enable_handoff; run mmcload; run mmcboot;
And that did it! I had my FPGA writing directly to the DDR3. The next step was to actually get pictures from the cameras...