Re: [PATCH 2/3] i2c: bcm2835: Add support for combined write-read transfer

From: Martin Sperl
Date: Tue Sep 20 2016 - 06:15:17 EST

On 20.09.2016 10:41, Noralf TrÃnnes wrote:

Den 20.09.2016 09:19, skrev Martin Sperl:
Hi Noralf!

On 19.09.2016 17:26, Noralf TrÃnnes 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 TrÃnnes <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)
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
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.
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.