DMA to Userspace

[Nikolaus Rath]

Hello,

I am trying to make a DMA transfer from userspace. My code looks like
this (data.vi_buf is a pointer to the userspace address):

    down_read(&current->mm->mmap_sem);
    rc = get_user_pages(current, current->mm, (unsigned long) data.vi_buf,
    		1, 1, 0, &acq_dev->vi_pg, NULL);
    up_read(&current->mm->mmap_sem);
    if (rc != 1) {
    	err("get_user_pages failed for VI.");
    	return -EINVAL;
    }
    acq_dev->vi_d = dma_map_page(dev, acq_dev->vi_pg, 0, PAGE_SIZE,
            		             DMA_TO_DEVICE);
    if (acq_dev->vi_d == 0) {
    	err("dma_map_page failed for VI.");
    	goto cleanup_vi_2;
    }
    info("VI bus address: 0x%0lx", (unsigned long) acq_dev->vi_d);

But when my PCI device then reads data from the bus address
acq_dev->vi_d, it just gets \0 bytes rather then the contents of the
acq_dev->userspace buffer.

I tried to debug this with an additional

    info("VI Start: 0x%0x", ((uint32_t*) page_address(acq_dev->vi_pg))[0]);

to print out the first byte of the mapped page, but this actually
results in a kernel crash:

    BUG: unable to handle kernel NULL pointer dereference at (null)


Can anyone tell why my debugging idea does not work? And does anyone
have a suggestion how I could better debug my original problem?


Thanks,

   -Nikolaus

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