DMA-heap driver hints

[Christian König]

Hi guys,

this is just an RFC! The last time we discussed the DMA-buf coherency
problem [1] we concluded that DMA-heap first needs a better way to
communicate to userspace which heap to use for a certain device.

As far as I know userspace currently just hard codes that information
which is certainly not desirable considering that we should have this
inside the kernel as well.

So what those two patches here do is to first add some
dma_heap_create_device_link() and  dma_heap_remove_device_link()
function and then demonstrating the functionality with uvcvideo
driver.

The preferred DMA-heap is represented with a symlink in sysfs between
the device and the virtual DMA-heap device node.

What's still missing is certainly matching userspace for this since I
wanted to discuss the initial kernel approach first.

Please take a look and comment.

Thanks,
Christian.

[1] https://lore.kernel.org/all/11a6f97c-e45f-f24b-8a73-48d5a388a2cc@gmail.com/T/