Mapping off-board memory over a PCIe interface as a tmpfs drive.

Hi all. I'm interested in discussing ways to map off-board memory to a locally mapped file system.
To explain: I have access to off-board memory, and a kernel module supplied by Xilinx called xdma that is up and running nicely. I define the DMA channel, pass it an address and offset, and I can read/write over the PCIe interface just fine, using interrupts or just polling.

Now I was thinking I could write a block kernel module to interface with the DMA engine, then map the block device as a drive (like a hard drive), and assign a file system to it, and use it like a hard drive.

I'd like to discuss this approach with someone more familiar with linux than myself.

You have it right. Implementing a block driver gets you all the benefits of the standard machinery for dealing with disks and storage. The O'Reilley "Linux Device Drivers" has a chapter on block device drivers, but is not updated since 2005, so some details may have wandered a little. Not sure if there is a better reference.

Basically, you have to implement a request queue for handling the async I/O requests to your device, and some of the calls on the block_device_operations struct (open/close/ioctl). Look at an existing driver for reference. The hard disk (sd.c) and floppy disk (floppy.c) are both pretty big to deal with all the hardware complications, but yours can take a lot of shortcuts.

Awesome. That's what I was looking for. I read the section from O-Reilley https://www.oreilly.com/openbook/lin.../book/ch16.pdf and it seemed appropriate. You're right, it's antiquated.
I'll have a look at sd.c and floppy.c...

Is ioctl still used, or has this been deprecated now? I thought I read somewhere there's another preferred method?

Anyway, thank you for confirming this approach.

Some userspace code might expect certain ioctls, but I don't think they are absolutely necessary. I'm thinking of equally deprecated software like fdisk. It's simpler to use the /sys interface for new code.

Ok, I'll update later when I get things working correctly.
Thanks again!
-Dave