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

From: Noralf TrÃnnes
Date: Tue Sep 20 2016 - 06:57:11 EST

Den 20.09.2016 12:15, skrev Martin Sperl:

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 didn't take it as such, I'm just not sure what's the best approach here,
so I added and looked up some more information

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.

It has to be specific when it comes to number of messages. We can only
support ReStart after the first message unless we use polling for the
whole transfer. And in that case we can't disable interrupts for such
a long period and we will end up sometimes loosing Transfer Active,
resulting in Stop Condition between the messages.
So we can only do transfers with 2 messages if we want Restart.

It is possible to support more than 16 bytes for the first message,
filling the FIFO after polling TA, but I'm not sure that is common.
Mostly it's 1 or 2 bytes to set a register.
The write-read restriction isn't absolutely necessary either, but it's the
most common case I think. So it was about reusing bcm2835_i2c_xfer_msg().
A less restrictive approach would require a dedicated function I think.

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.

I agree.

After reading joan's report I wonder if it would be best to add a module
parameter like downstream has, so it can be disabled. What do you think?