Hi,
I have been trying recently to make a usb-based-webcam device to work with
Linux. The entire scheme is a bit complex:
UVC gadget <--User Pointer--> User-Space Daemon <-- MMAP --> V4L2 capture
device.
The UVC gadget is internally a v4l2 based device supporting VB2_VMALLOC
operations,
whereas the V4L2 capture device supports VB2_DMA_CONTIG operations.
The application (user-space daemon), is responsible for getting memory allocated
from the V4L2 capture device via REQBUF calls. The V4L2 capture side exposes a
MMAP IO method, whereas the UVC gadget can get a USERPTR to the buffer filled
with
video data from the V4L2 capture device and then send the same on a USB bus.
This scheme works absolutely fine on an architecture having a MMU, but when I
try
the same on a NOMMU arch, I see MMAP calls from the user-space daemon failing.
I have implemented a .get_unmapped_area callback in my V4L2 capture driver using
the blackfin video capture driver as a reference (see [1]).
I make a MMAP call from the user-space application in a sequence like this
(pretty similar to the standard capture.c example, see[2]):
static void init_mmap (void)
{
struct v4l2_requestbuffers req;
CLEAR (req);
req.count = 4;
req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
req.memory = V4L2_MEMORY_MMAP;
if (-1 == xioctl (fd, VIDIOC_REQBUFS, &req))
{
if (EINVAL == errno)
{
fprintf (stderr, "%s does not support memory
mapping\n", dev_name);
exit (EXIT_FAILURE);
}
else
{
errno_exit ("VIDIOC_REQBUFS");
}
}
if (req.count < 2)
{
fprintf (stderr, "Insufficient buffer memory on %s\n",dev_name);
exit (EXIT_FAILURE);
}
buffers = (buffer*) calloc (req.count, sizeof (*buffers));
if (!buffers)
{
fprintf (stderr, "Out of memory\n");
exit (EXIT_FAILURE);
}
for (n_buffers = 0; n_buffers < req.count; ++n_buffers)
{
struct v4l2_buffer buf;
CLEAR (buf);
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
buf.index = n_buffers;
if (-1 == xioctl (fd, VIDIOC_QUERYBUF, &buf))
errno_exit ("VIDIOC_QUERYBUF");
buffers[n_buffers].length = buf.length;
buffers[n_buffers].start =
mmap (NULL /* start anywhere */,
buf.length,
PROT_READ | PROT_WRITE /* required */,
MAP_SHARED /* recommended */,
fd, buf.m.offset);
if (MAP_FAILED == buffers[n_buffers].start)
errno_exit ("mmap");
}
}
Now, I see that the requested videobuffers are correctly allocated via
'vb2_dma_contig_alloc'
call (see [3] for reference). But the MMAP call fails in 'vb2_dma_contig_alloc'
function
in mm/nommu.c (see [4] for reference) when it tries to make the following check:
if (addr != (pfn << PAGE_SHIFT))
return -EINVAL;
I address Scott also, as I see that he has worked on the Blackfin v4l2 capture
driver using
DMA contiguous method and may have seen this issue (on a NOMMU system) with a
v4l2 application
performing a MMAP operation.
Any comments on what I could be doing wrong here?
References:
[1] Blackfin capture driver,
http://lxr.linux.no/linux+v3.5.3/drivers/media/video/blackfin/bfin_capture.c#L243
[2] capture.c, http://linuxtv.org/downloads/v4l-dvb-apis/capture-example.html
[3] vb2_dma_contig_alloc,
http://lxr.linux.no/linux+v3.5.3/drivers/media/video/videobuf2-dma-contig.c#L37
[4] remap_pfn_range, http://lxr.linux.no/linux+v3.5.3/mm/nommu.c#L1819
Regards,
Bhupesh
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