Driver to allow DMA from user space

[Ran Shalit]

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 ?

Thanks,
Ran


thanks,
>
> greg k-h
>
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