On 20.09.2016 10:41, Noralf Tr?nnes wrote:

Den 20.09.2016 09:19, skrev Martin Sperl:

quoted

Hi Noralf!

On 19.09.2016 17:26, Noralf Tr?nnes wrote:

quoted

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 Tr?nnes <redacted>
---
  drivers/i2c/busses/i2c-bcm2835.c | 107
+++++++++++++++++++++++++++++++++++----
  1 file changed, 98 insertions(+), 9 deletions(-)

...

quoted

@@ -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.

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
bytes.
Refs:
http://www.circuitwizard.de/raspi-i2c-fix/raspi-i2c-fix.html
http://raspberrypi.stackexchange.com/questions/31728/has-anyone-successfully-used-i2c-repeated-starts-on-the-pi2-my-scope-says-they


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).
Refs:
https://github.com/raspberrypi/linux/issues/254#issuecomment-15254134
https://www.raspberrypi.org/forums/viewtopic.php?p=807834&sid=2b612c7209f2175bf1a266359c72ae6c#p807834


I found this answer/report by joan that the downstream combined support
isn't reliable:
http://raspberrypi.stackexchange.com/questions/31728/has-anyone-successfully-used-i2c-repeated-starts-on-the-pi2-my-scope-says-they


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.

I have not said that the approach you have taken is wrong or bad.

I was only telling you that the portion inside the bcm2835_i2c_xfer:
+	/* 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;
+	}
is very specific and maybe could be done in a "generic" manner
supporting more cases.

At least add a dev_warn_once for all num > 1 cases not handled by the
code above.

This gives people an opportunity to detect such a situation if they
find something is not working as expected.

Martin