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

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Message-ID: <CAFLxGvyNazbyBpySuNA6SU-8tcvqYWKHsAHuhTLsofR=VoiEwQ@mail.gmail.com>
Date:   Mon, 22 Jun 2020 14:27:12 +0200
From:   Richard Weinberger <richard.weinberger@...il.com>
To:     Thomas Ruf <freelancer@...usul.de>
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 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.

> 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?

-- 
Thanks,
//richard