linux-kernel - Re: [PATCH 2/4] dmaengine: vdma: Add support for mulit-channel dma mode

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Date:	Tue, 21 Jun 2016 22:03:41 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Appana Durga Kedareswara Rao <appana.durga.rao@...inx.com>
Cc:	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	"pawel.moll@....com" <pawel.moll@....com>,
	"mark.rutland@....com" <mark.rutland@....com>,
	"ijc+devicetree@...lion.org.uk" <ijc+devicetree@...lion.org.uk>,
	"galak@...eaurora.org" <galak@...eaurora.org>,
	Michal Simek <michals@...inx.com>,
	Soren Brinkmann <sorenb@...inx.com>,
	"dan.j.williams@...el.com" <dan.j.williams@...el.com>,
	"moritz.fischer@...us.com" <moritz.fischer@...us.com>,
	"laurent.pinchart@...asonboard.com" 
	<laurent.pinchart@...asonboard.com>,
	"luis@...ethencourt.com" <luis@...ethencourt.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>
Subject: Re: [PATCH 2/4] dmaengine: vdma: Add support for mulit-channel dma
 mode

On Tue, Jun 21, 2016 at 04:02:05PM +0000, Appana Durga Kedareswara Rao wrote:
> Hi Vinod,
> 
> 	Thanks for the review...
> 
> > 
> > On Fri, Jun 10, 2016 at 02:42:32PM +0530, Kedareswara rao Appana wrote:
> > > This patch adds support for AXI DMA multi-channel dma mode
> > > Multichannel mode enables DMA to connect to multiple masters And
> > > slaves on the streaming side.
> > > In Multichannel mode AXI DMA supports 2D transfers.
> > 
> > Funny formatting!
> 
> Will fix in next version...
> 
> > 
> > Can you elobrate what you meant by Multichannel mode? This patch seems to
> > do two things, one is to add interleaved dma support and something else. Can
> > you explain the latter part?
> 
> AXI DMA has two Stream interfaces (Memory to Stream MM2S and Stream to Memory S2MM)

what is a stream in this context?

> In Multi-Channel dma mode each stream interface can be configured up to 16 channels.
> In Multi-channel DMA mode IP supports only interleaved transfers (2-D transfers).


> 
> > 
> > >  /**
> > > + * struct xilinx_mcdma_config - DMA Multi channel configuration
> > > +structure
> > > + * @tdest: Channel to operate on
> > > + * @tid:   Channel configuration
> > > + * @tuser: Tuser configuration
> > > + * @ax_user: ax_user value
> > > + * @ax_cache: ax_cache value
> > > + */
> > > +struct xilinx_mcdma_config {
> > > +	u8 tdest;
> > > +	u8 tid;
> > > +	u8 tuser;
> > > +	u8 ax_user;
> > > +	u8 ax_cache;
> > 
> > can you describe these in details, what do these do, what are the values to be
> > programmed?
> 
> As said above In Multi-Channel Mode each Stream interface can be 
> Configured up to 16 channels each channel is differentiated based on the tdest and tid values.

Then why are you not registering 16 channels for this? That should give you
channel to operate on!

> 
> tdest:
> TDEST provides routing information for the data stream.

pls elobrate

> TDEST values are static for the entire packet.

what do these mean

> 
> tid:
> Provides a stream identifier. TID values are static for entire packet.
> TID values provided in the TX descriptor field are presented on
> TID signals of the streaming side.

what is this used for?

> 
> tuser:
> Sideband signals used for user-defined information.
> TUSER values are static for entire packet. TUSER values provided in the TX
> Descriptor field are presented on TUSER signals of streaming side.

what is this used for?

> 
> ax_user: 
> Sideband signals used for user-defined information
> ARUSER values and their interpretations are user-defined

How is it used by client?

> 
> ax_cache:
> Cache type this signal provides additional information about the cacheable
> Characteristics of the transfer.

and what are those...

> 
> In the above parameters tdest and tid are mandatory to differentiate b/w different channels
> Other parameters are optional please let me know if you don't want me to 
> Include other optional parameters will remove in the next version.

dmaengine provides channel as an argument so it is quite odd that you need
to differentiate b/w different channels. This only makes me worry that
things are not standard and fishy

-- 
~Vinod