Re: [PATCH] i2c-pnx: fix setting start/stop condition

[Luotao Fu]

On Mon, Mar 01, 2010 at 07:43:24PM +0100, Kevin Wells wrote:
> Hi Luotao,
> 
> > 
> > The start/stop condtions are set in different places repetedly in the i2c-
> > pnx
> > driver.  Beside in i2c_pnx_start and i2c_pnx_stop the start/stop bit are
> > also
> > set during the transfer of a i2c message in the master_xmit/rcv calls.
> > This is
> > wrong since we can't set the start/stop condition during the transaction
> > of a
> > single message any way. As a matter of fact, the driver will sometimes set
> > both
> > the start and the stop bits at one time. This can be easily reproduced by
> > sending a simple read request like e.g
> > struct i2c_msg msgs[] = {
> > { addr, 0, 1, buf },
> > { addr, I2C_M_RD, offset, buf }
> > };
> > While processing the first message the i2c_pnx_master_xmit will set both
> > the
> > start_bit and the stop_bit, which will eventually confuse the slave.
> > 
> > Fixed by remove setting start/stop condition from the transmit routines.
> > 
> > Signed-off-by: Luotao Fu <l.fu@xxxxxxxxxxxxxx>
> > ---
> >  drivers/i2c/busses/i2c-pnx.c |   11 -----------
> >  1 files changed, 0 insertions(+), 11 deletions(-)
> > 
> > diff --git a/drivers/i2c/busses/i2c-pnx.c b/drivers/i2c/busses/i2c-pnx.c
> > index 5d1c260..92c0e46 100644
> > --- a/drivers/i2c/busses/i2c-pnx.c
> > +++ b/drivers/i2c/busses/i2c-pnx.c
> > @@ -175,12 +175,6 @@ static int i2c_pnx_master_xmit(struct i2c_adapter
> > *adap)
> >  		/* We still have something to talk about... */
> >  		val = *alg_data->mif.buf++;
> > 
> > -		if (alg_data->mif.len == 1) {
> > -			val |= stop_bit;
> > -			if (!alg_data->last)
> > -				val |= start_bit;
> 
> Removing the start bit assertion here should be ok. The initial start
> condition is setup as part of the start of the transfer when the addr
> is sent out. The stop bit needs to stay or the I2C transfer will not
> correctly terminate on the last byte out and you will end up with a
> bus busy failure (clock low, data high) on the next transfer.

I had a closer look into this and think that you are right. I was
irritated by the unregular usage of start_bit and thought the stop bit
is asserted with the i2c_pnx_stop call, which is apparently only used
for zero sized transfers.

Fixed patch is attached with this mail.

cheers
Luotao Fu
-- 
Pengutronix e.K.                           | Dipl.-Ing. Luotao Fu        |
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From 4a142700fec80489e3aa38b64d0b519d3968b0d9 Mon Sep 17 00:00:00 2001
From: Luotao Fu <l.fu@xxxxxxxxxxxxxx>
Date: Mon, 1 Mar 2010 13:00:20 +0100
Subject: [PATCH] i2c-pnx: fix setting start condition

The start condtions is set in different places repeatedly in the i2c-pnx driver.
Beside in i2c_pnx_start the start bit is also set during the transfer of a i2c
message in the master_xmit/rcv calls. This is wrong since we can't set the start
condition during the transaction of a single message any way. As a matter of
fact, the driver will sometimes set both the start and the stop bits at one
time. This can be easily reproduced by sending a simple read request like e.g
struct i2c_msg msgs[] = {
{ addr, 0, 1, buf },
{ addr, I2C_M_RD, offset, buf }
};
While processing the first message the i2c_pnx_master_xmit will set both the
start_bit and the stop_bit, which will eventually confuse the slave.

Fixed by remove setting start condition from the transmit routines.

Signed-off-by: Luotao Fu <l.fu@xxxxxxxxxxxxxx>
Acked-by: Vitaly Wool <vitalywool@xxxxxxxxx>
---
 drivers/i2c/busses/i2c-pnx.c |    9 ++-------
 1 files changed, 2 insertions(+), 7 deletions(-)

diff --git a/drivers/i2c/busses/i2c-pnx.c b/drivers/i2c/busses/i2c-pnx.c
index 5d1c260..9edbe98 100644
--- a/drivers/i2c/busses/i2c-pnx.c
+++ b/drivers/i2c/busses/i2c-pnx.c
@@ -175,11 +175,9 @@ static int i2c_pnx_master_xmit(struct i2c_adapter *adap)
 		/* We still have something to talk about... */
 		val = *alg_data->mif.buf++;
 
-		if (alg_data->mif.len == 1) {
+		/* last byte of a message */
+		if (alg_data->mif.len == 1)
 			val |= stop_bit;
-			if (!alg_data->last)
-				val |= start_bit;
-		}
 
 		alg_data->mif.len--;
 		iowrite32(val, I2C_REG_TX(alg_data));
@@ -254,9 +252,6 @@ static int i2c_pnx_master_rcv(struct i2c_adapter *adap)
 		if (alg_data->mif.len == 1) {
 			/* Last byte, do not acknowledge next rcv. */
 			val |= stop_bit;
-			if (!alg_data->last)
-				val |= start_bit;
-
 			/*
 			 * Enable interrupt RFDAIE (data in Rx fifo),
 			 * and disable DRMIE (need data for Tx)
-- 
1.7.0

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