linux-kernel - Re: [PATCH] dma: Add Keystone Packet DMA Engine driver

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Message-Id: <201403181740.11693.arnd@arndb.de>
Date:	Tue, 18 Mar 2014 17:40:11 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Santosh Shilimkar <santosh.shilimkar@...com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Vinod Koul <vinod.koul@...el.com>,
	Mark Rutland <mark.rutland@....com>,
	devicetree@...r.kernel.org, Russell King <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org, Sandeep Nair <sandeep_n@...com>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>, dmaengine@...r.kernel.org
Subject: Re: [PATCH] dma: Add Keystone Packet DMA Engine driver

On Tuesday 18 March 2014, Santosh Shilimkar wrote:
> 
> On Tuesday 18 March 2014 11:38 AM, Arnd Bergmann wrote:
> > On Tuesday 18 March 2014 20:54:44 Vinod Koul wrote:
> >> On Mon, Mar 17, 2014 at 03:37:47PM -0400, Santosh Shilimkar wrote:
> >>>>> To simplify this bit more, you can think of this as DMA channels, flows
> >>>>> are allocated and DMA channels are enabled by DMA engine and they remains
> >>>>> enabled always as long as the channel in use. Enablling dma channel
> >>>>> actually don't start the DMA transfer but just sets up the connection/pipe
> >>>>> with peripheral and memory and vice a versa.
> >>>>>
> >>>>> All the descriptor management, triggering, sending completion interrupt or
> >>>>> hardware signal to DMAEngine all managed by centralised QMSS.
> >>>>>
> >>>>> Actual copy of data is still done by DMA hardware but its completely
> >>>>> transparent to software. DMAEngine hardware takes care of that in the
> >>>>> backyard.
> >>>> So you will use the dmaengine just for setting up the controller. Not for actual
> >>>> transfers. Those would be governed by the QMSS, right?
> >>>>
> >>> Correct.
> >>>  
> >>>> This means that someone expecting to use dmaengine API will get confused about
> >>>> this and doing part (alloc) thru dmaengine and rest (transfers) using some other
> >>>> API. This brings to me the design approach, does it really make sense creating
> >>>> dmaengine driver for this when we are not fully complying to the API
> >>>>
> >>> Thats fair. The rationale behind usage of DMEngine was that its the closest
> >>> available subsystem which can be leveraged for this hardware. We can
> >>> pretty much use all the standard DMAEngine device tree parsing as well as
> >>> the config API to setup DMAs. 
> >>>
> >>> I think you made your stand clear, just to confirm, you don't prefer this
> >>> driver to be a DMAEngine driver considering it doesn't fully complying to
> >>> the APIs. We could document the deviation of 'transfer' handling to avoid
> >>> any confusion.
> >> Yup, a user will just get confused as the driver doenst conform the dmaengine
> >> API. Unless someone comes up witha  strong argument on why it should be
> >> dmaengine driver and what befits we see form such a model, i would like a
> >> damengine driver to comply to standard API and usage.
> > 
> > I think it would be possible to turn the QMSS driver into a library and have
> > the packet DMA code use the proper dmaengine API by calling into that code.
> > 
> > The main user of packet DMA (the ethernet driver) would however still have
> > to call into QMSS directly, so I'm not sure if it's worth the effort.
> > 
> Its not. Am going to move this driver along with QMSS which is one
> of the options we discussed.

Ok, fair enough. Looking forward to the patches.

	Arnd
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