Messages in this thread | | | Subject | Re: [PATCH 2/3] i2c: bcm2835: Add support for combined write-read transfer | From | Martin Sperl <> | Date | Tue, 20 Sep 2016 09:19:13 +0200 |
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Hi Noralf!
On 19.09.2016 17:26, Noralf Trønnes wrote: > Some SMBus protocols use Repeated Start Condition to switch from write > mode to read mode. Devices like MMA8451 won't work without it. > > When downstream implemented support for this in i2c-bcm2708, it broke > support for some devices, so a module parameter was added and combined > transfer was disabled by default. > See https://github.com/raspberrypi/linux/issues/599 > It doesn't seem to have been any investigation into what the problem > really was. Later there was added a timeout on the polling loop. > > One of the devices mentioned to partially stop working was DS1307. > > I have run thousands of transfers to a DS1307 (rtc), MMA8451 (accel) > and AT24C32 (eeprom) in parallel without problems. > > Signed-off-by: Noralf Trønnes <noralf@tronnes.org> > --- > drivers/i2c/busses/i2c-bcm2835.c | 107 +++++++++++++++++++++++++++++++++++---- > 1 file changed, 98 insertions(+), 9 deletions(-) ... > @@ -209,8 +289,17 @@ static int bcm2835_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg msgs[], > int i; > int ret = 0; > > + /* Combined write-read to the same address (smbus) */ > + if (num == 2 && (msgs[0].addr == msgs[1].addr) && > + !(msgs[0].flags & I2C_M_RD) && (msgs[1].flags & I2C_M_RD) && > + (msgs[0].len <= 16)) { > + ret = bcm2835_i2c_xfer_msg(i2c_dev, &msgs[0], &msgs[1]); > + > + return ret ? ret : 2; > + } > + > for (i = 0; i < num; i++) { > - ret = bcm2835_i2c_xfer_msg(i2c_dev, &msgs[i]); > + ret = bcm2835_i2c_xfer_msg(i2c_dev, &msgs[i], NULL); > if (ret) > break; > } This does not seem to implement the i2c_msg api correctly.
As per comments in include/uapi/linux/i2c.h on line 58 only the last message in a group should - by default - send a STOP.
As far as I understand you would need to implement the I2C_M_STOP flag (by exposing I2C_FUNC_PROTOCOL_MANGLING in bcm2835_i2c_func) to make this work correctly:
for (i = 0; i < num; i++) { + bool send_stop = (i == num - 1) ||msgs[i] <http://lxr.free-electrons.com/ident?i=msg>->flags <http://lxr.free-electrons.com/ident?i=flags> &I2C_M_STOP <http://lxr.free-electrons.com/ident?i=I2C_M_STOP>; - ret = bcm2835_i2c_xfer_msg(i2c_dev, &msgs[i]); + ret = bcm2835_i2c_xfer_msg(i2c_dev, &msgs[i], send_stop); if (ret) break; }
The corresponding device driver (or userspace) will need to set the flag correctly.
Martin
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