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

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Message-ID: <1666251320.1024432.1593007095381@mailbusiness.ionos.de>
Date:   Wed, 24 Jun 2020 15:58:15 +0200 (CEST)
From:   Thomas Ruf <freelancer@...usul.de>
To:     Peter Ujfalusi <peter.ujfalusi@...com>,
        Vinod Koul <vkoul@...nel.org>
Cc:     Federico Vaga <federico.vaga@...n.ch>,
        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 24 June 2020 at 14:07 Peter Ujfalusi <peter.ujfalusi@...com> wrote:
> On 24/06/2020 12.38, Vinod Koul wrote:
> > On 24-06-20, 11:30, Thomas Ruf wrote:
> > 
> >> To make it short - i have two questions:
> >> - what are the chances to revive DMA_SG?
> > 
> > 100%, if we have a in-kernel user
> 
> Most DMAs can not handle differently provisioned sg_list for src and dst.
> Even if they could handle non symmetric SG setup it requires entirely
> different setup (two independent channels sending the data to each
> other, one reads, the other writes?).

Ok, i implemented that using zynqmp_dma on a Xilinx Zynq platform (obviously ;-) and it works nicely for us.
Don't think that it uses two channels from what a saw in their implementation.
Of course that was on kernel 4.19.x where DMA_SG was still available.

> >> - what are the chances to get my driver for memcpy like transfers from
> >> user space using DMA_SG upstream? ("dma-sg-proxy")
> > 
> > pretty bleak IMHO.
> 
> fwiw, I also get requests time-to-time to DMA memcpy support from user
> space from companies trying to move from bare-metal code to Linux.
> 
> What could be plausible is a generic dmabuf-to-dmabuf copy driver (V4L2
> can provide dma-buf, DRM can also).
> If there is a DMA memcpy channel available, use that, otherwise use some
> method to do the copy, user space should not care how it is done.

Yes, i'm using it together with a v4l2 capture driver and also saw the dma-buf thing but did not find a way how to bring this together with "ordinary user memory". For me the root of my problem seems to be that dma_alloc_coherent leads to uncached memory on ARM platforms. But maybe i am doing it all wrong ;-)

> Where things are going to get a bit more trickier is when the copy needs
> to be triggered by other DMA channel (completion of a frame reception
> triggering an interleaved sub-frame extraction copy).
> You don't want to extract from a buffer which can be modified while the
> other channel is writing to it.

I think that would be no problem in case of our v4l2 capture driver doing both DMAs:
Framebuffer DMA for streaming and Zynqmp DMA (using DMA_SG) to get it to "ordinary user memory".
But as i wrote before i prefer to do the "logic and management" in userspace so the capture driver is just using the first DMA and the "dma-sg-proxy" driver is only used as a memcpy replacement.
As said this is all working fine with kernel 4.19.x but now we are stuck :-(

> In Linux the DMA is used for kernel and user space can only use it
> implicitly via standard subsystems.
> Misused DMA can be very dangerous and giving full access to program a
> transfer can open a can of worms.

Fully understand that!
But i also hope you understand that we are developing a "closed system" and do not have a problem with that at all.
We are also willing to bring that driver upstream for anyone doing the same but of course this should not affect security of any desktop or server systems.
Maybe we just need the right place for that driver?!
Not sure if staging would change your concerns.

Thanks and best regards,
Thomas