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

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Date:   Wed, 24 Jun 2020 15:07:54 +0300
From:   Peter Ujfalusi <peter.ujfalusi@...com>
To:     Vinod Koul <vkoul@...nel.org>, Thomas Ruf <freelancer@...usul.de>
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 24/06/2020 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?).

>> - 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.

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.

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.

- Péter

Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki