Den 20.09.2016 09:19, skrev Martin Sperl:
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.
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 <>
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)
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.

Apparently it's a known problem that the i2c controller doesn't support
Repeated Start. It will always issue a Stop when it has transferred DLEN

UNLESS: a Start Transfer (ST) is issued after Transfer Active (TA) is set
and before DONE is set (or the last byte is shifted, I don't know excatly).

I found this answer/report by joan that the downstream combined support
isn't reliable:

My implementation differs from downstream in that I use local_irq_save()
to protect the polling loop. But that only protects from missing the TA
(downstream can miss the TA and issue a Stop).

So currently in mainline we have a driver that says it support the standard
(I2C_FUNC_I2C), but it really only supports one message transfers since it
can't do ReStart.

What I have done in this patch is to support ReStart for transfers with
2 messages: first write, then read. But maybe a better solution is to just
leave this alone if it is flaky and use bitbanging instead. I don't know.


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] <>->flags <> &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)

The corresponding device driver (or userspace) will need to set the flag correctly.


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