From mboxrd@z Thu Jan 1 00:00:00 1970 From: Grygorii Strashko Subject: Re: [2/5] i2c: davinci: query STP always when NACK is received Date: Mon, 24 Nov 2014 15:34:35 +0200 Message-ID: <547333EB.5050403@ti.com> References: <1416477788-5544-3-git-send-email-grygorii.strashko@ti.com> <20141120221953.GI27002@pengutronix.de> <546F34B9.1000206@ti.com> <20141121131008.GQ27002@pengutronix.de> <546F5B51.1030006@ti.com> <20141123203314.GF4431@pengutronix.de> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20141123203314.GF4431-bIcnvbaLZ9MEGnE8C9+IrQ@public.gmane.org> Sender: linux-i2c-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: =?windows-1252?Q?Uwe_Kleine-K=F6nig?= Cc: Wolfram Sang , linux-i2c-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Sekhar Nori , Kevin Hilman , Santosh Shilimkar , Murali Karicheri List-Id: linux-i2c@vger.kernel.org Hi Uwe, On 11/23/2014 10:33 PM, Uwe Kleine-K=F6nig wrote: > On Fri, Nov 21, 2014 at 05:33:37PM +0200, Grygorii Strashko wrote: >> On 11/21/2014 03:10 PM, Uwe Kleine-K=F6nig wrote: >>> On Fri, Nov 21, 2014 at 02:48:57PM +0200, Grygorii Strashko wrote: >>>> On 11/21/2014 12:19 AM, Uwe Kleine-K=F6nig wrote: >>>>>> diff --git a/drivers/i2c/busses/i2c-davinci.c b/drivers/i2c/buss= es/i2c-davinci.c >>>>>> index 9bbfb8f..2cef115 100644 >>>>>> --- a/drivers/i2c/busses/i2c-davinci.c >>>>>> +++ b/drivers/i2c/busses/i2c-davinci.c >>>>>> @@ -411,11 +411,9 @@ i2c_davinci_xfer_msg(struct i2c_adapter *ad= ap, struct i2c_msg *msg, int stop) >>>>>> if (dev->cmd_err & DAVINCI_I2C_STR_NACK) { >>>>>> if (msg->flags & I2C_M_IGNORE_NAK) >>>>>> return msg->len; >>>>>> - if (stop) { >>>>>> - w =3D davinci_i2c_read_reg(dev, DAVINCI_I2C_MDR_REG); >>>>>> - w |=3D DAVINCI_I2C_MDR_STP; >>>>>> - davinci_i2c_write_reg(dev, DAVINCI_I2C_MDR_REG, w); >>>>>> - } >>>>>> + w =3D davinci_i2c_read_reg(dev, DAVINCI_I2C_MDR_REG); >>>>>> + w |=3D DAVINCI_I2C_MDR_STP; >>>>>> + davinci_i2c_write_reg(dev, DAVINCI_I2C_MDR_REG, w); >>>>> I think this is a good change, but I wonder if the handling of >>>>> I2C_M_IGNORE_NAK is correct here. If the controller reports a NAC= K say >>>>> for the 2nd byte of a 5-byte-message, the transfer supposed to >>>>> continue, right? (Hmm, maybe the framework handle this and restar= ts the >>>>> transfer with I2C_M_NOSTART but the davinci driver doesn't seem t= o >>>>> handle this flag?) >>>> >>>> Have nothing to say about handling of I2C_M_IGNORE_NAK. I'm not go= ing to >>>> change current behavior - davinci driver will interrupt transfer o= f i2c_msg always >>>> in case of NACK and start transfer of the next i2c_msg (if exis= t). >>>> In my opinion, Above question is out of scope of this patch. >>> Yeah right, that's exactly what I thought. >>> >>> Thinking again I wonder if with your change handling is correct whe= n the >>> sender wants to do a repeated start. That would need a more detaile= d >>> look into the driver. >> >> Davinci driver will always abort transfer with error -EREMOTEIO in c= ase if >> NACK received from I2C slave device. And the next omap_i2c_xfer() ca= ll may >> be *not* targeted to the same I2C slave device. >> ^ if !I2C_M_IGNORE_NAK > Does this resolve my concern? I think it doesn't. Also a Sr might wel= l > address another device, doesn't it? >=20 > A call to .master_xfer with a message sequence implicitly expects ACK= s > 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 o= r > Sr. Sry, but what should be done is defined by I2C/SMbus specs? Does it? =46or 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= =20 case the slave device must not acknowledge the received byte. The mas= ter upon detection of this condition must generate a STOP condition and retry the transaction" =46or I2C devices, the specification states [http://www.nxp.com/documen= ts/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 a= s the Not Acknowledge signal. The master can then generate either a STOP conditio= n to abort the transfer, or a repeated START condition to start a new transf= er." 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 =20 So, there is no guarantee that next xfer will address the same I2C clie= nt 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). Small summary, I2C core + Davinci I2C driver provide ability to use rep= eated start (Sr) only within one I2C transaction - which is a number of write= /read operations specified by i2c_msg array. NACK always interrupts transacti= on with -EREMOTEIO. Also, the I2C core doesn't provide ability to manually send P. regards, -grygorii