Re: DMA Engine: Transfer From Userspace

[Thomas Ruf <freelancer@rufusul.de>]

> On 22 June 2020 at 14:30 Federico Vaga <federico.vaga@cern.ch> wrote:
> 
> 
> On Mon, Jun 22, 2020 at 02:01:12PM +0200, Thomas Ruf wrote:
> >> On 22 June 2020 at 06:47 Vinod Koul <vkoul@kernel.org> wrote:
> >>
> >> On 21-06-20, 22:36, Federico Vaga wrote:
> >> > On Sun, Jun 21, 2020 at 12:54:57PM +0530, Vinod Koul wrote:
> >> > > On 19-06-20, 16:31, Dave Jiang wrote:
> >> > > >
> >> > > >
> >> > > > On 6/19/2020 3:47 PM, Federico Vaga wrote:
> >> > > > > Hello,
> >> > > > >
> >> > > > > is there the possibility of using a DMA engine channel from userspace?
> >> > > > >
> >> > > > > Something like:
> >> > > > > - configure DMA using ioctl() (or whatever configuration mechanism)
> >> > > > > - read() or write() to trigger the transfer
> >> > > > >
> >> > > >
> >> > > > I may have supposedly promised Vinod to look into possibly providing
> >> > > > something like this in the future. But I have not gotten around to do that
> >> > > > yet. Currently, no such support.
> >> > >
> >> > > And I do still have serious reservations about this topic :) Opening up
> >> > > userspace access to DMA does not sound very great from security point of
> >> > > view.
> >> >
> >> > I was thinking about a dedicated module, and not something that the DMA engine
> >> > offers directly. You load the module only if you need it (like the test module)
> >>
> >> But loading that module would expose dma to userspace.
> >> >
> >> > > Federico, what use case do you have in mind?
> >> >
> >> > Userspace drivers
> >>
> >> more the reason not do do so, why cant a kernel driver be added for your
> >> usage?
> >
> >by chance i have written a driver allowing dma from user space using a memcpy like interface ;-)
> >now i am trying to get this code upstream but was hit by the fact that DMA_SG is gone since Aug 2017 :-(
> 
> Not sure to get what you mean by "DMA_SG is gone". Can I have a reference?

here the link to the mailinglist when DMA_SG was removed:
https://www.spinics.net/lists/dmaengine/msg13778.html

> >just let me introduce myself and the project:
> >- coding in C since '91
> >- coding in C++ since '98
> >- a lot of stuff not relevant for this ;-)
> >- working as a freelancer since Nov '19
> >- implemented a "dma-sg-proxy" driver for my client in Mar/Apr '20 to copy camera frames from uncached memory to cached memory using a second dma on a Zynq platform
> >- last week we figured out that we can not upgrade from "Xilinx 2019.2" (kernel 4.19.x) to "2020.1" (kernel 5.4.x) because the DMA_SG interface is gone
> >- subscribed to dmaengine on friday, saw the start of this discussion on saturday
> >- talked to my client today if it is ok to try to revive DMA_SG and get our driver upstream to avoid such problems in future
> >
> >here the struct for the ioctl:
> >
> >typedef struct {
> >  unsigned int struct_size;
> >  const void *src_user_ptr;
> >  void *dst_user_ptr;
> >  unsigned long length;
> >  unsigned int timeout_in_ms;
> >} dma_sg_proxy_arg_t;
> 
> Yes, roughly this is what I was thinking about

cool, i really hope i get my stuff upstream!

Best regards,
Thomas