linux-kernel - RE: [PATCH V7 3/6] i2c: qup: Transfer each i2c_msg in i2c

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Date:	Thu, 28 Jan 2016 10:27:05 +0530
From:	"Sricharan" <sricharan@...eaurora.org>
To:	"'Wolfram Sang'" <wsa@...-dreams.de>
Cc:	<devicetree@...r.kernel.org>, <linux-arm-msm@...r.kernel.org>,
	<agross@...eaurora.org>, <linux-kernel@...r.kernel.org>,
	<linux-i2c@...r.kernel.org>, <iivanov@...sol.com>,
	<galak@...eaurora.org>, <dmaengine@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <andy.gross@...aro.org>,
	<ntelkar@...eaurora.org>, <architt@...eaurora.org>
Subject: RE: [PATCH V7 3/6] i2c: qup: Transfer each i2c_msg in i2c_msgs without a stop bit

Hi Wolfram,

> -----Original Message-----
> From: Wolfram Sang [mailto:wsa@...-dreams.de]
> Sent: Sunday, January 24, 2016 4:59 PM
> To: Sricharan R
> Cc: devicetree@...r.kernel.org; linux-arm-msm@...r.kernel.org;
> agross@...eaurora.org; linux-kernel@...r.kernel.org; linux-
> i2c@...r.kernel.org; iivanov@...sol.com; galak@...eaurora.org;
> dmaengine@...r.kernel.org; linux-arm-kernel@...ts.infradead.org;
> andy.gross@...aro.org; ntelkar@...eaurora.org; architt@...eaurora.org
> Subject: Re: [PATCH V7 3/6] i2c: qup: Transfer each i2c_msg in i2c_msgs
> without a stop bit
> 
> Hi,
> 
> > "If this is the last message in a group, it is followed by a STOP.
> > Otherwise it is followed by the next @i2c_msg transaction segment,
> > beginning with a (repeated) START"
> 
> This is correct.
> 
> > So the expectation is that there is no 'STOP' bit inbetween individual
> > i2c_msg segments with repeated 'START'. The QUP i2c hardware has no
> > way to inform that there should not be a 'STOP' at the end of
transaction.
> > The only way to implement this is to coalesce all the i2c_msg in
> > i2c_msgs in to one transaction and transfer them. Adding the support for
> the same.
> 
> So, there will not be a REP_START condition on the bus? I am sorry to say
that
> I can't accept this. A REP_START is a REP_START and nothing else. There
are
> devices which will get confused if there is no real REP_START condition.
> 
> Without knowing the HW in detail, can't you implement I2C_M_NOSTART
> and let the touchscreen driver use it via regmap? That would be the proper
> way (from what I understand).


Ah, so what I meant above is there is no 'STOP' bit between each msg in
i2c_msgs, 
but 'REAPEATED_START' still holds true.  We are sending 'START' bit for each
msg.
So these is how each msg in i2c_msg is sent,

|------MSG1--------|-----MSG2---------|------MSG3------------|

  |START|DATA|------|START|DATA|---|START|DATA|STOP|      

If my commit text does not make this clear, I  can reword that ?

Regards,
  Sricharan