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

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Date:   Mon, 22 Jun 2020 14:30:19 +0200
From:   Federico Vaga <federico.vaga@...n.ch>
To:     Thomas Ruf <freelancer@...usul.de>
CC:     Vinod Koul <vkoul@...nel.org>, Dave Jiang <dave.jiang@...el.com>,
        "Dan Williams" <dan.j.williams@...el.com>,
        <dmaengine@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: DMA Engine: Transfer From Userspace

On Mon, Jun 22, 2020 at 02:01:12PM +0200, Thomas Ruf wrote:
>> On 22 June 2020 at 06:47 Vinod Koul <vkoul@...nel.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?

>
>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

>best regards,
>Thomas