linux-kernel - Re: DMA Engine: Transfer From Userspace

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Date:   Mon, 22 Jun 2020 16:01:24 +0200 (CEST)
From:   Thomas Ruf <freelancer@...usul.de>
To:     Richard Weinberger <richard.weinberger@...il.com>
Cc:     Vinod Koul <vkoul@...nel.org>,
        Federico Vaga <federico.vaga@...n.ch>,
        Dave Jiang <dave.jiang@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        dmaengine@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: DMA Engine: Transfer From Userspace

> On 22 June 2020 at 14:27 Richard Weinberger <richard.weinberger@...il.com> wrote:
> 
> 
> On Mon, Jun 22, 2020 at 2:02 PM Thomas Ruf <freelancer@...usul.de> wrote:
> > > 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
> 
> How do you preserve bounds? This is the main reason why vfio requires an iommu.

Depends where the pointer "points to", i can detect:
- virtually allocated user memory, the generated scatterlist is slit on page bounderies
- contiguous pyhsical memory, in our case allocated by v4l2 (based on a dma without SG support), the generated scallterlist has just one entry

sorry, i am not really familar with vfio :-(

> > 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;
> 
> Is this on top of uio or a complete new subsystem?

Completely independent, just my own idea for a simple uapi.

Best regards,
Thomas