linux-kernel - Re: [PATCH] dmaengine: qcom_bam

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Date:	Thu, 22 May 2014 11:40:49 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Andy Gross <agross@...eaurora.org>
Cc:	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] dmaengine: qcom_bam_dma: Add descriptor flag APIs

On Thu, May 15, 2014 at 12:32:06PM -0500, Andy Gross wrote:
> On Fri, May 02, 2014 at 01:08:27PM -0500, Andy Gross wrote:
> > On Fri, May 02, 2014 at 09:58:41PM +0530, Vinod Koul wrote:
> > > On Thu, Apr 17, 2014 at 05:04:02PM -0500, Andy Gross wrote:
> > > > This patch adds APIs that allow for BAM hardware flags to be set per
> > > > descriptor.  Each one of the new flags informs the attached peripheral of a
> > > > special behavior that is required.
> > > > 
> > > > The EOT flag requests that the peripheral assert an end of transaction interrupt
> > > > when that descriptor is complete.  It also results in special signaling protocol
> > > > that is used between the attached peripheral and the core using the DMA
> > > > controller.
> > > DMA_PREP_INTERRUPT ??
> > 
> > I have 3 different IRQs that can be asserted based on the bit I set in the
> > hardware descriptor.  The normal IRQ is the INT bit.  However, in some cases the
> > peripheral protocol requires the use of the EOT or EOB interrupt instead.  The
> > DMA_PREP_INTERRUPT would only work if I had only 2 choices.
> 
> Thinking about this more, I could use the DMA_PREP_INTERRUPT to cover the EOT
> flag.  However, I might get in a bind later if I need to support the EOB (end of
> block) interrupt.
Sorry for delay in this.

I think it would make sense to use DMA_PREP_INTERRUPT for EOB interrupt. The EOT
should always be enabled for the cases where it is applicable instead of nomral
irq.

That should genrically ocvery your cases, or did we miss anything here
> 
> > 
> > > 
> > > > 
> > > > The NWD flag requests that the peripheral wait until the data has been fully
> > > > processed before signaling an interrupt.
> > > interrupt for transaction complete or DMA request?
> > 
> > This is a special signaling mechanism that holds off the DMA interrupt until the
> > peripheral actually acks that the data has been processed completely.  This is
> > required in many cases by the peripheral.  One example is the SPI controller.
> > At the end of a transaction you are supposed to set the NWD so that the chip
> > select is de-asserted.
> 
> I'm not sure what flag I could map this to... maybe DMA_CTRL_ACK?  or maybe the
> DMA_PREP_FENCE?  I don't generally like overloading the flags and slightly
> twisting their intent.  Could we add a flag to denote device ACK?
Nope lets not override these...

-- 
~Vinod
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