Driver to allow DMA from user space

[Greg KH]

On Sat, Dec 24, 2016 at 05:47:19PM +0200, Ran Shalit wrote:
> 
> 
> On Tue, Dec 20, 2016 at 12:26 PM, Greg KH <greg at kroah.com> wrote:
> 
>     On Tue, Dec 20, 2016 at 12:15:22PM +0200, Ran Shalit wrote:
>     > 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 ?
> 
>     I think you should use the UIO driver api, as that's exactly what it was
>     written for.  Have you looked at it yet?  It handles your interrupt
>     logic for you.
> 
> 
> Hello,
> 
> If I may please ask, I made some reading about uio, but didn't yet understand
> what's the benefit of using uio instead of creating a character device ? 

It's a lot less work than writing a custom char driver that will not be
accepted upstream because you are not using the expected UIO api
interface :)

Writing a UIO driver should be very simple, and very small, all of the
framework is already done, in a correct way, why would you _not_ want to
use the UIO interface?

thanks,

greg k-h