Re: DMA Engine: Transfer From Userspace

[Vinod Koul <vkoul@kernel.org>]

On 22-06-20, 14:01, 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 :-(
> 
> 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

DMA_SG was removed as it had no users, if we have a user (in-kernel) we
can certainly revert that removal patch.
> 
> 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;

Again, am not convinced opening DMA to userspace like this is a great
idea. Why not have Xilinx camera driver invoke the dmaengine and do
DMA_SG ?

-- 
~Vinod