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@xxxxxxxxxxx> > > > >Signed-off-by: Yoshihiro Kaneko <ykaneko0929@xxxxxxxxx> > > > > > > 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
Attachment:
signature.asc
Description: Digital signature
