Driver to allow DMA from user space

[Ran Shalit]

On Sun, Dec 25, 2016 at 2:37 PM, Greg KH <greg at kroah.com> wrote:
> 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?
>
Hi,

UIO drivers seems like a good choice in my case, I am familiar with
generic-uio interface in devicetree.
Just for my understanding, I am trying to understand the difference between
writing a small character device which notifies the interrupt, to using uio
interface.
Is there any advantage of using uio over the the small chracter device?(I
am sure there is. I just do not know it yet)
Another question, in performance terms which is better: uio driver or
kernel driver?

Thank you,
Ran

> thanks,
>
> greg k-h
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