GitHub - Zain-Naqvi-tech/linux-pci-dma-driver

QEMU EDU Linux Kernel PCI Driver

Platform: QEMU EDU virtual PCI device (1234:11e8), x86-64 guest, Linux 6.8 Toolchain: Kbuild out-of-tree module, gcc for userspace, WSL2 host running a QEMU guest Language: C, kernel space and userspace

Overview

An out-of-tree Linux kernel PCI driver written from scratch against QEMU’s EDU device. It enumerates the device, maps its registers, exposes it to userspace as a character device, handles MSI interrupts, and runs DMA transfers into and out of the device’s internal buffer. On top of that sits a benchmark that compares programmed I/O against DMA across eleven transfer sizes.

The reason I picked this instead of another bare-metal board project: kernel space is a different set of rules. Nothing I had written before had to worry about which context a function runs in, whether a pointer is safe to dereference, or what a device is allowed to do to system memory without asking. I wanted to find out what actually changes when the code you write is the thing standing between userspace and the hardware.

Everything runs inside a VM. When the driver takes down the kernel, and it did, the recovery is a five second reboot instead of a wrecked laptop.

The short version

• A PCI driver that binds by vendor and device ID, claims BAR 0, and maps it with ioremap.
• A misc character device at /dev/edu with read, write, and ioctl.
• MSI interrupt-driven reads. The device signals when it is done and the reading process sleeps until then, instead of spinning on a status bit.
• 4 KB coherent DMA in both directions, driven from userspace through ioctl with a shared ABI header that both sides compile.
• A benchmark harness that runs eleven sizes in both directions, sorts the samples, and writes a CSV.
• One debugging session that ate most of a day and came down to a single missing line.

The stack, top to bottom

1. Userspace test app and benchmark harness. open, read, write, ioctl, timing, CSV output.