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.
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@xxxxxxxxxxx>
---
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.
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.
Noralf.
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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