mmap physically contiguous memory

Question

I might have some misconceptions here, so bear with me.

I wrote a program that captures images from a camera. I am sharing the memory between the camera and my application with mmap as I found in the V4L2 documentation. This works great. Now my processor (it's TI's DM3730) also has a DSP. I want to use the DSP, but it requires physical contiguous memory. TI provides drivers to allocate the memory. My problem is that right now I lose a lot of time to copy the mmap'ed memory into the physical contiguous memory. Is there a way to tell mmap that it should not allocate memory itself, but that I tell mmap to use memory that I allocate.

To give you an idea of what I am doing (There is a lot of code missing of course, but I stuck very close to the V4L2 documentation. I hope this is enough to understand my problem):

//reserve physical contiguous memory
dsp_buffer      = Memory_alloc(buffer_length, &myParams); 

...
//reserve memory for buffer, but not contiguous
buffers[n_buffers].start =
     mmap (NULL ,                    /* start anywhere */
     buf.length,
     PROT_READ | PROT_WRITE ,  /* required */                               
     MAP_SHARED ,              /* recommended */
     fd, buf.m.offset);

After that I copy the memory out of the non-contiguous memory into the contiguous memory, whenever a frame is ready.

...
//wait until frame is ready in memory
r = select (fd + 1, &fds, NULL, NULL, &tv); 
...
//copy the memory over to the physically contiguous memory
memcpy(dsp_buffer,buffers[buf.index].start,size); 
...

How could I get the frames into the physical contiguous memory right away?

Answer 1

If you cannot pass the result of Memory_alloc() as first argument to your mmap() (for example, if it also uses mmap() that would make it impossible to map to that memory again), you probably should use another streaming I/O method from the given example - IO_METHOD_USERPTR variation. It uses the same ioctl as IO_METHOD_MMAP to capture frames and should provide the similar efficiency.

Answer 2

You would need driver support from the camera driver. mmap gets the physical pages it maps from whatever driver it's mapping - the camera in this case. You cannot tell mmap to use some pre-allocated pages, because the underlying driver would have to be told to use these pre-allocated pages.