linux-kernel - Re: [PATCH v7 1/2] i2c: imx: add DMA support for freescale i2c driver

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Message-ID: <1411055145666.72178@freescale.com>
Date:	Thu, 18 Sep 2014 15:46:04 +0000
From:	Yao Yuan <yao.yuan@...escale.com>
To:	Marek Vasut <marex@...x.de>
CC:	"wsa@...-dreams.de" <wsa@...-dreams.de>,
	"LW@...o-electronics.de" <LW@...o-electronics.de>,
	"mark.rutland@....com" <mark.rutland@....com>,
	"fugang.duan@...escale.com" <fugang.duan@...escale.com>,
	"shawn.guo@...aro.org" <shawn.guo@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>
Subject: Re: [PATCH v7 1/2] i2c: imx: add DMA support for freescale i2c driver


Marek Vasut wrote:
> On Wednesday, September 17, 2014 at 04:50:34 PM, Yao Yuan wrote:
> [...]
> > > > > Would that mean that the "crashed" DMA would be running until the
> > > > > next transmission is scheduled ?
> > > >
> > > > [Yuan Yao] No, In fact any DMA timeout will result the failure of I2C
> > > > transmission and then it will turn to report the exception and wait
> > > > for next transmission.
> > >
> > > Can you tell when the next transmission will happen? What if I issue a
> > > single transmission and that one fails ? Will the DMA run until who knows
> > > when ?
> >
> > [Yuan Yao]
> > Sorry for my unclear description. In fact, During the DMA transmission  if
> > an error happened or time out, DMA will stop at once and be disabled.
> > I just continue to route the TX and RX request to signal the DMA
> > controller. Because the DMA is disabled, it will ignore those signals.
> >
> > In a word, I just want to block the I2C TX, RX and interrupt signal when
> > DMA mode failed until the next I2C transmission start.

> So the I2C block is in error state until you clean it up upon next transmission?
[Yuan Yao]
No, just DMAEN bit.
Other I2C error state will clean soon.

> > In fact, the bit "I2CR_DMAEN" is a switch which decide whether I2C route
> > the TX, RX and interrupt signal to DMA controller.
> >
> > > The only thing I worried about is I2C may still receive some feedbacks
> > > after DMA timeout. In this case the feedbacks may lead to abnormal
> > > state in PIO mode.But it will be ignored in DMA model.
> > > That's why I tend to delay force-disable DMA until the next
> > > transmission begin. Could you please give me some suggestion?
> >
> > > No, this design just seems flawed to me. You should stop the DMA
> > > immediatelly if there is an error to avoid wasting resources and prevent
> > > possible other adverse effects.
> >
> > [Yuan Yao]
> > Yes, I have stopped the DMA immediately. However I keep the I2C DMA
> > single route.
> >
> > I don't have the exact evidence to prove that my design is acceptable.
> > So if you are sure it's flawed, I will change it in the next version(V8).

> I'm just trying to understand it.

[Yuan Yao]
Both of us know that we should stop DMA immediately when issue happened.
The only argument is that I want to set the DMAEN bit just before the next 
transmission starts. But you think when I stop the DMA I should set it at the same
time. The bit is the switch which is used to decide whether Rx and Tx signal can
be routed to DMA. In fact, I deeply think about what is the difference between
our arguments for those days. I think the two ways are almost the same.
Your way is more acceptable because people tend to clear the DMA status
just after stopping it. So I think your way is better.

Best regards,
Yuan Yao
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