Driver to allow DMA from user space

[Ran Shalit]

On Tue, Dec 20, 2016 at 11:38 AM, Greg KH <greg at kroah.com> wrote:
> On Mon, Dec 19, 2016 at 06:08:47PM +0200, Ran Shalit wrote:
>> Hello,
>>
>> I want to use DMA from userspace.
>
> Why?

Hi Greg,

We want that a userspace layer (a library) will do some HW related issues.
We have a memory mapped space (from FPGA), so we think it will be
easier, and I think also more correct way , that we create the driver,
and interact hadrware using the mapped memory space, and also do the
protocols in userspace. The only thing that is less easy in userspace
is using interrupt, and dma. but that is also possible if we just wrap
the dma, and interrupt in a character device (or use uio as you
suggested below).
>
>> I already use dma in kernel, and now I want can create a character
>> device which will be responsible for this.
>
> Why?
>
>> The only problem is that I want to use the same memory which was
>> allocated in kernel with dma_alloc_coherent.
>
> Why?

in kernel we use dma_alloc_coherent, which returns contiguous memory.
As I understand, we can mmap in userspace the returned  physical
address, and use the returned virtual address in userspace.


>
>> Is it correct to use mmap in order to use the phsyical memory which
>> was allocated with dma_alloc_coherent ?
>>
>> If it's that simple it can be surely helpful, and the simple driver
>> which wraps dma_alloc_coherent can do the job for dmaing from
>> userspace.
>
> Have you looked at the uio driver interface?
>
> But again, why?  What problem are you trying to solve here?

We need to do some interaction with HW , but since most of the HW is
mapped to physical address (FPGA), it seem simpler to do that in
userspace (HW library), instead of doing this in kernel. What do you
think ?


>
> thanks,
>
> greg k-h