Re: DMA Engine: Transfer From Userspace

[Peter Ujfalusi]

On 24/06/2020 16.58, Thomas Ruf wrote:
> 
>> On 24 June 2020 at 14:07 Peter Ujfalusi <peter.ujfalusi@xxxxxx> 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.

I see, if the HW does not support it then something along the lines of
what the atc_prep_dma_sg did can be implemented for most engines.

In essence: create a new set of sg_list which is symmetric.

> Don't think that it uses two channels from what a saw in their implementation.

I believe it was breaking it up like atc_prep_dma_sg did.

> 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".

One of the aim of dma-buf is to share buffers between drivers and user
space (among drivers and/or drivers and userspace), but I might be
missing something.

> For me the root of my problem seems to be that dma_alloc_coherent leads to uncached memory on ARM platforms.

It depends, but in most cases that is true.

> 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?!

What might be plausible is to introduce hw offloading support for memcpy
type of operations in a similar fashion how for example crypto does it?

The issue with a user space implemented logic is that it is not portable
between systems with different DMAs. It might be that on one DMA the
setup takes longer than do a CPU copy of X bytes, on the other DMA it
might be significantly less or higher.

Using CPU vs DMA for a copy in certain lengths and setups should not be
a concern of the user space.
Yes, you have a closed system with controlled parameters, but a generic
mem2mem_offload framework should be usable on other setups and the same
binary should be working on different DMAs where one is not efficient
for <512 bytes, the other shows benefits under 128bytes.

> Not sure if staging would change your concerns.
> 
> Thanks and best regards,
> Thomas
> 

- PÃter

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