linux-kernel - Re: [2/5] i2c: davinci: query STP always when NACK is received

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Date:	Mon, 24 Nov 2014 21:02:35 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Grygorii Strashko <grygorii.strashko@...com>,
	Wolfram Sang <wsa@...-dreams.de>
Cc:	linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
	Sekhar Nori <nsekhar@...com>,
	Kevin Hilman <khilman@...prootsystems.com>,
	Santosh Shilimkar <ssantosh@...nel.org>,
	Murali Karicheri <m-karicheri2@...com>
Subject: Re: [2/5] i2c: davinci: query STP always when NACK is received

Hello Grygorii,

On Mon, Nov 24, 2014 at 03:34:35PM +0200, Grygorii Strashko wrote:
> On 11/23/2014 10:33 PM, Uwe Kleine-König wrote:
> > A call to .master_xfer with a message sequence implicitly expects ACKs
> > from the slave and doesn't tell anything about what should be done on a
> > NAK. So IMHO you must not send a P when the slave responds with a NAK,
> > but error out and let the sender decide if it wants to reply with P or
> > Sr.
> 
> Sry, but what should be done is defined by I2C/SMbus specs? Does it?
> For SMBus devices, the specification states (http://smbus.org/specs/)
> "4.2.Acknowledge (ACK) and not acknowledge (NACK)":
> - "The slave device detects an invalid command or invalid data. In this 
>   case the slave device must not acknowledge the received byte. The master
>   upon detection of this condition must generate a STOP condition and
>   retry the transaction"
> For I2C devices, the specification states [http://www.nxp.com/documents/user_manual/UM10204.pdf]:
> "3.1.6 Acknowledge (ACK) and Not Acknowledge (NACK)"
> "When SDA remains HIGH during this ninth clock pulse, this is defined as the Not
> Acknowledge signal. The master can then generate either a STOP condition to
> abort the transfer, or a repeated START condition to start a new transfer."
Yes, that's exactly what I meant. The master has the choice, and the
driver should not eliminate options here.
 
> Let take a look on i2c/smbus xfer:
> i2c_lock_adapter(adap)
>  adap->algo->master_xfer/smbus_xfer()
> i2c_unlock_adapter(adap);
>  |- rt_mutex_unlock(&adapter->bus_lock);
>     |- task switch
>  
> So, there is no guarantee that next xfer will address the same I2C client device,
> which, in turn, may lead to BB detection (will lead to BB detection if previous
> transfer has been not acknowledged by SMbus client device).
That's a valid concern.
 
> Small summary, I2C core + Davinci I2C driver provide ability to use repeated
> start (Sr) only within one I2C transaction - which is a number of write/read
> operations specified by i2c_msg array. NACK always interrupts transaction
> with -EREMOTEIO.
> 
> Also, the I2C core doesn't provide ability to manually send P.
Hmm, Wolfram, what do you think?

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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