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Date:   Mon, 22 Jun 2020 14:01:12 +0200 (CEST)
From:   Thomas Ruf <freelancer@rufusul.de>
Reply-To: Thomas Ruf <freelancer@rufusul.de>
To:     Vinod Koul <vkoul@kernel.org>,
        Federico Vaga <federico.vaga@cern.ch>
Cc:     Dave Jiang <dave.jiang@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        dmaengine@vger.kernel.org, linux-kernel@vger.kernel.org
Message-ID: <419762761.402939.1592827272368@mailbusiness.ionos.de>
In-Reply-To: <20200622044733.GB2324254@vkoul-mobl>
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 <20200621203634.y3tejmh6j4knf5iz@cwe-513-vol689.cern.ch>
 <20200622044733.GB2324254@vkoul-mobl>
Subject: Re: DMA Engine: Transfer From Userspace
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> 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

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;

best regards,
Thomas