Writing PCI driver for DMA transfer on Qemu - linux-device-driver

Question

I am writing a PCI device on Qemu and driver(LKM) in the guest OS. While Qemu provides an example PCI device, edu(edu.txt and edu.c) with it's distribution, I am having trouble writing the kernel module to do DMA transfer. A basic driver has been covered here but it does not support DMA.
I am following the implementation of the link and this. I tried to transmit buffer to the PCI device from the IRQ handler. The device can read the data (pci_dma_read) but the I am not getting the correct data that I am supposed to receive. Here is the code segment that is doing DMA transfer:
static int write_to_HyPerf(void *dev, void* addr, uint32_t size)
{
/* ----------------------------------------------------------------------------
* PCI address 'addr':
* addr -> DMA source address
* 0x40000 -> DMA destination address
* 100 -> DMA transfer count
* 1 -> DMA command register
* while (DMA command register & 1)
*--------------------------------------------------------------------------------
*/
iowrite32((u32 *)dma_handle_to_device, mmio + IO_DMA_SRC);
iowrite32(DMA_START, mmio + IO_DMA_DST);
iowrite32((u32 *)size, mmio + IO_DMA_XCNT);
iowrite32(DMA_CMD | DMA_IRQ, mmio + IO_DMA_CMD);
}
I also have setup coherent mapping using dma_alloc_coherent.
vaddr_to_device = dma_alloc_coherent(&(dev->dev), DMA_SIZE, &dma_handle_to_device, GFP_ATOMIC);
The complete code is available here. What am I doing wrong?

Answer 1

Could be that you having problem in your driver.
In this case you can use this:
https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/kernel_modules/qemu_edu.c
and you can use dd command as in this script to write and read from your device:
https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/rootfs_overlay/lkmc/qemu_edu.sh
Then all you need is to write the wanted dma values to the correct address
like in the edu.c code:
case 0x80:
dma_rw(edu, false, &val, &edu->dma.src, false);
break;
case 0x88:
dma_rw(edu, false, &val, &edu->dma.dst, false);
break;
case 0x90:
dma_rw(edu, false, &val, &edu->dma.cnt, false);
break;
case 0x98:
dma_rw(edu, false, &val, &edu->dma.cmd, false);
break;