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

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Date:   Wed, 1 Jul 2020 18:13:46 +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 30 June 2020 at 14:31 Peter Ujfalusi <peter.ujfalusi@...com> wrote:
> 
> 
> 
> 
> On 29/06/2020 18.18, Thomas Ruf wrote:
> > 
> >> On 26 June 2020 at 12:29 Peter Ujfalusi <peter.ujfalusi@...com> wrote:
> >>
> >> On 24/06/2020 16.58, Thomas Ruf wrote:
> >>>
> >>>> 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.
> >>
> >> 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.
> > 
> > Sorry, not sure if i understand you right?
> > You suggest that in case DMA_SG gets revived we should restrict the support to symmetric sg_lists?
> 
> No, not at all. That would not make much sense.

Glad that this was just a misunderstanding.

> > Just had a glance at the deleted code and the *_prep_dma_sg of these drivers had code to support asymmetric lists and by that "unaligend" memory (relative to page start):
> > at_hdmac.c         
> > dmaengine.c        
> > dmatest.c          
> > fsldma.c           
> > mv_xor.c           
> > nbpfaxi.c          
> > ste_dma40.c        
> > xgene-dma.c        
> > xilinx/zynqmp_dma.c
> > 
> > Why not just revive that and keep this nice functionality? ;-)
> 
> What I'm saying is that the drivers (at least at_hdmac) in essence
> creates aligned sg_list out from the received non aligned ones.
> It does this w/o actually creating the sg_list itself, but that's just a
> small detail.
> 
> In a longer run what might make sense is to have a helper function to
> convert two non symmetric sg_list into two symmetric ones so drivers
> will not have to re-implement the same code and they will only need to
> care about symmetric sg lists.

Sounds like a superb idea!

> Note, some DMAs can actually handle non symmetric src and dst lists, but
> I believe it is rare.

So i was a bit lucky that the zynqmp_dma is one of them.
 
> >> 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?
> > 
> > Sounds good to me, my proxy driver implementation could be a good start for that, too!
> 
> It needs to find it's place as well... I'm not sure where that would be.
> Simple block-copy offload, sg copy offload, interleaved offload (frame
> extraction) offload, dmabuf copy offload comes to mind as candidates.

And who would decide that...

> >> 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.
> > 
> > Fully agree with that!
> > I was also unsure how my approach will perform but in our case the latency was increased by ~20%, cpu load roughly stayed the same, of course this was the benchmark from user memory to user memory.
> > From uncached to user memory the DMA was around 15 times faster.
> 
> It depends on the size of the transfer. Lots of small individual
> transfers might be worst via DMA do the the setup time, completion
> handling, etc.

Yes, exactly.

Thanks again for your great input!

best regards,
Thomas

PS: I am on vacation for the next two weaks and probably will not check this mailing list till 20.7. But will fetch later.