Re: [PATCH/RFC] i2c: rcar: Support ACK by HW auto restart after NACK

[Wolfram Sang <wsa@the-dreams.de>]

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On Fri, Mar 06, 2015 at 11:18:09PM +0100, Wolfram Sang wrote:
> 
> > > >Even if R-Car I2C received NACK, after that it might receive ACK
> > > >by HW auto restart. In case of that, driver would continue process.
> > > >If R-Car I2C didn't receive ACK, the driver would detect timeout
> > > >and would report NACK as -ENXIO.
> > > >
> > > >Signed-off-by: Ryo Kataoka <ryo.kataoka.wt@renesas.com>
> > > >Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
> > > 
> > > Excuse me, but what exactly is HW auto restart in this case? Is it a feature
> > > of the I2C slave?
> > 
> > I asked Kataoka-san about this and his response was as follows:
> > 
> >     It is a feature of the i2c-rcar(H/W) master.
> > 
> >     If system(CPU) is busy, NACK procedure may have interrupt latency.
> >     Since the clear of ICMCR.ESG bit is delayed, i2c-rcar(H/W) may auto-restart
> >     after NACK. Please refer to ESG bit of H/W UM section 55.3.5.
> > 
> >     For example, this is I2C write transmitting.
> >     1.Start / 2.SlaveAddr,ACK / 3.RegAddr,ACK / 4.RegData,ACK / 5.Stop
> > 
> >     If No.2 has NACK and interruption has delay, this transmitting is as follows.
> >     1.Start / 2.SlaveAddr,NACK/ 1x.auto-restart / 2x.SlaveAddr,ACK
> >                                     / 3.RegAddr,ACK / 4.RegData,ACK / 5.Stop
> > 
> >     NACK of No.2 is invalidated by ACK of No.2x. It means recover.
> 
> Does this make some I2C device work which did not work before?
> 
> Most I2C devices always ack their address, so NACK very often means
> "nothing is there". I think it makes sense that the rcar driver returns
> ENXIO in this case which is documented to be used for NACK after address
> phase. Then, the i2c client driver should know if this means "not there"
> or "currently busy". And it should know when is a good time for another
> try. As I read the patch, the driver would use the auto-restart feature
> until the timeout is reached. That would make bus scanning pretty slow,
> too.

Hi,

any news on this one?

Kind regards,

   Wolfram


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