Patchwork i2c-omap: always send stop after nack

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Submitter Felipe Balbi
Date July 16, 2013, 11:27 a.m.
Message ID <20130716112746.GN8880@arwen.pp.htv.fi>
Download mbox | patch
Permalink /patch/259390/
State Not Applicable
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Felipe Balbi - July 16, 2013, 11:27 a.m.
Hi,

On Tue, Jul 16, 2013 at 02:01:11PM +0300, Grygorii Strashko wrote:
> >>>>On a OMAP4460, i2c-bus-3:
> >>>>
> >>>>A driver (lm75) is causing many 'timeout waiting for bus ready' errors.
> >>>>SDA remains high (as it should), but SCL remains low after a NACK.
> >>>>The bus becomes _unusable for other clients_.
> >>>>
> >>>>While probing, "lm75" writes a command, followed by a read + stop,
> >>>>but the write command is NACK'd. The chip does accept other writes/reads,
> >>>>it just refuses to ack invalid commands.
> >>>>
> >>>>Can you tell me if the patch below would make any sense? Or is it the
> >>>>responsibility of the client to reset the i2c_smbus?
> >>>patch below breaks repeated start.
> Felipe, I'd very appreciate if you'd be able to provide the use case
> which will fail with such solution?

can't you see how this would fail ?

assume omap_i2c_xfer() is called with its last argument (num) being
greater than one and you get the NAK before the last transfer.

Will you not be breaking a possible repeated start for the following
transfer ?

> >>No, after the NACK, no more commands are being processed,
> >>including a repeated start. omap_i2c_xfer() returns -EREMOTEIO
> >>without ever freeing the bus.
> >>
> >>The bus is left in an impossible state with SCL constantly low
> >>and all next commands (to different chips) will therefore get
> >>a -ETIMEDOUT
> >>
> >>With this patch, the bus will become idle again and new commands
> >>can be processed normally
> I think, this is valid fix, but it was done here already:)
> http://patchwork.ozlabs.org/patch/249790/
> "i2c: omap: query STP always when NACK is received"
> 
> And nacked in the same way :(
> But! I've back-ported my patch on TI Android product kernel 3.4, did
> sanity test and I didn't see any issues with my patch :))

that's because you don't care about repeated start, but that's a valid
bus signal which needs to be supported.

> >but you mentioned that if you have IGNORE_NAK set, everything is fine,
> >since lm75 will get a return value of 0 and things will work just fine,
> >right ?
> >
> >Also, you also said that the chip 'refuses to ack invalid commands', why
> >are you sending invalid commands to start with ? This could be a bug in
> >i2c-omap.c, sure, but let's try to figure out why IGNORE_NAK helps and
> >why is lm75 driver sending invalid commands.
> >
> 
> The problem is, that lm75 device is SmBus compatible and its driver has
> .detect() function implemented. During detection it tries to scan some
> registers which might be not present in current device - in my case
> it's tmp105.
> 
> For example to read regA in tmp105 following is done:
> 1) do write in "Index" register (val RegA index) (I2C 1st message)
> 2) do read (I2C 2d message)
> the message 1 is Nacked by device in case if register index is wrong,
> but i2c-omap don't send NACK (or STP). As result, bus stack in Bus
> Busy state.

wait a minute, it's not i2c-omap which needs to send NAK, it's LM75,
and it does the NAK. The handling for NAK in the i2c framework is to
return -EREMOTEIO as we do. If our last message got a NAK, we send STOP
because there will be no other transfers following this one, namely, the
for loop in omap_i2c_xfer() will be finished.

> For SMBus devices the specification states (http://smbus.org/specs/)
> "4.2.Acknowledge (ACK) and not acknowledge (NACK)":
> - "The slave device detects an invalid command or invalid data. In this
> case the slave device must not acknowledge the received byte. The
> master upon detection of this condition must generate a STOP condition
> and retry the transaction"

hmm, but that's something that the OMAP I2C controller doesn't support
and is emulated by the i2c framework, right ?

If you look into the I2C specification, the one the OMAP controller is
compliant to, you'll see e.g. in Figure 13 that a repeated start is a
valid condition after a NAK.

Also it states that:

"This is indicated by the slave generating the not-acknowledge on the
first byte to follow. The slave leaves the data line HIGH and the master
generates a STOP or a repeated START condition."

Because the OMAP I2C controller is compliant to the I2C specification,
not the SMBus specification, we must follow through with the loop and
let the next message try to send a repeated start.

What you need here is a way to discriminate between SMBus message and
normal I2C message, that way you could have something like:


and, btw, this also means that I2C_M_IGNORE_NAK is invalid during SMBus
transfers, so you might want to patch the framework to prevent that case
altogether.
Grygorii Strashko - July 16, 2013, 12:08 p.m.
Hi Felipe,
On 07/16/2013 02:27 PM, Felipe Balbi wrote:
> Hi,
>
> On Tue, Jul 16, 2013 at 02:01:11PM +0300, Grygorii Strashko wrote:
>>>>>> On a OMAP4460, i2c-bus-3:
>>>>>>
>>>>>> A driver (lm75) is causing many 'timeout waiting for bus ready' errors.
>>>>>> SDA remains high (as it should), but SCL remains low after a NACK.
>>>>>> The bus becomes _unusable for other clients_.
>>>>>>
>>>>>> While probing, "lm75" writes a command, followed by a read + stop,
>>>>>> but the write command is NACK'd. The chip does accept other writes/reads,
>>>>>> it just refuses to ack invalid commands.
>>>>>>
>>>>>> Can you tell me if the patch below would make any sense? Or is it the
>>>>>> responsibility of the client to reset the i2c_smbus?
>>>>> patch below breaks repeated start.
>> Felipe, I'd very appreciate if you'd be able to provide the use case
>> which will fail with such solution?
>
> can't you see how this would fail ?
>
> assume omap_i2c_xfer() is called with its last argument (num) being
> greater than one and you get the NAK before the last transfer.
That's our case - NACK from slave before last transfer
>
> Will you not be breaking a possible repeated start for the following
> transfer ?
Sorry, but in this case omap_i2c_xfer() will be aborted and there would 
be no transfers until next call to omap_i2c_xfer().
Which, in turn, may address another device!?


>
>>>> No, after the NACK, no more commands are being processed,
>>>> including a repeated start. omap_i2c_xfer() returns -EREMOTEIO
>>>> without ever freeing the bus.
>>>>
>>>> The bus is left in an impossible state with SCL constantly low
>>>> and all next commands (to different chips) will therefore get
>>>> a -ETIMEDOUT
>>>>
>>>> With this patch, the bus will become idle again and new commands
>>>> can be processed normally
>> I think, this is valid fix, but it was done here already:)
>> http://patchwork.ozlabs.org/patch/249790/
>> "i2c: omap: query STP always when NACK is received"
>>
>> And nacked in the same way :(
>> But! I've back-ported my patch on TI Android product kernel 3.4, did
>> sanity test and I didn't see any issues with my patch :))
>
> that's because you don't care about repeated start, but that's a valid
> bus signal which needs to be supported.

>
>>> but you mentioned that if you have IGNORE_NAK set, everything is fine,
>>> since lm75 will get a return value of 0 and things will work just fine,
>>> right ?
>>>
>>> Also, you also said that the chip 'refuses to ack invalid commands', why
>>> are you sending invalid commands to start with ? This could be a bug in
>>> i2c-omap.c, sure, but let's try to figure out why IGNORE_NAK helps and
>>> why is lm75 driver sending invalid commands.
>>>
>>
>> The problem is, that lm75 device is SmBus compatible and its driver has
>> .detect() function implemented. During detection it tries to scan some
>> registers which might be not present in current device - in my case
>> it's tmp105.
>>
>> For example to read regA in tmp105 following is done:
>> 1) do write in "Index" register (val RegA index) (I2C 1st message)
>> 2) do read (I2C 2d message)
>> the message 1 is Nacked by device in case if register index is wrong,
>> but i2c-omap don't send NACK (or STP). As result, bus stack in Bus
>> Busy state.
>
> wait a minute, it's not i2c-omap which needs to send NAK, it's LM75,
> and it does the NAK. The handling for NAK in the i2c framework is to
> return -EREMOTEIO as we do. If our last message got a NAK, we send STOP
> because there will be no other transfers following this one, namely, the
> for loop in omap_i2c_xfer() will be finished.
Sorry, wrong descr, my bad - slave sends NACK (lm75), master (OMAP i2)
should send STP.
But, you *can* send STT if you wish to continue with next
message to the *same* device - which is not true for OMAP i2c, because 
OMAP I2C driver always interrupts transfer with error -EREMOTEIO!!
And, again:), next call of omap_i2c_xfer() may be *not* to the same
slave I2C device.

>
>> For SMBus devices the specification states (http://smbus.org/specs/)
>> "4.2.Acknowledge (ACK) and not acknowledge (NACK)":
>> - "The slave device detects an invalid command or invalid data. In this
>> case the slave device must not acknowledge the received byte. The
>> master upon detection of this condition must generate a STOP condition
>> and retry the transaction"
>
> hmm, but that's something that the OMAP I2C controller doesn't support
> and is emulated by the i2c framework, right ?
>
> If you look into the I2C specification, the one the OMAP controller is
> compliant to, you'll see e.g. in Figure 13 that a repeated start is a
> valid condition after a NAK.
>
> Also it states that:
>
> "This is indicated by the slave generating the not-acknowledge on the
> first byte to follow. The slave leaves the data line HIGH and the master
> generates a STOP or a repeated START condition."
>
> Because the OMAP I2C controller is compliant to the I2C specification,
> not the SMBus specification, we must follow through with the loop and
> let the next message try to send a repeated start.
>
> What you need here is a way to discriminate between SMBus message and
> normal I2C message, that way you could have something like:
I don't think that is right (my explanation above) - the same can happen 
even with pure I2C device.
>
> diff --git a/drivers/i2c/busses/i2c-omap.c b/drivers/i2c/busses/i2c-omap.c
> index 142b694d..571b160 100644
> --- a/drivers/i2c/busses/i2c-omap.c
> +++ b/drivers/i2c/busses/i2c-omap.c
> @@ -618,7 +618,7 @@ static int omap_i2c_xfer_msg(struct i2c_adapter *adap,
>          if (dev->cmd_err & OMAP_I2C_STAT_NACK) {
>                  if (msg->flags & I2C_M_IGNORE_NAK)
>                          return 0;
> -               if (stop) {
> +               if (stop || is_smbus) {
>                          w = omap_i2c_read_reg(dev, OMAP_I2C_CON_REG);
>                          w |= OMAP_I2C_CON_STP;
>                          omap_i2c_write_reg(dev, OMAP_I2C_CON_REG, w);
>
> and, btw, this also means that I2C_M_IGNORE_NAK is invalid during SMBus
> transfers, so you might want to patch the framework to prevent that case
> altogether.
>
Regards,
-grygorii
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Felipe Balbi - July 16, 2013, 1 p.m.
Hi,

(when replying, can you add some blank lines around your reply and the
previous mail, it's quite difficult to find your replies with so many
quote marks (>) around)

On Tue, Jul 16, 2013 at 03:08:04PM +0300, Grygorii Strashko wrote:
> Hi Felipe,
> On 07/16/2013 02:27 PM, Felipe Balbi wrote:
> >Hi,
> >
> >On Tue, Jul 16, 2013 at 02:01:11PM +0300, Grygorii Strashko wrote:
> >>>>>>On a OMAP4460, i2c-bus-3:
> >>>>>>
> >>>>>>A driver (lm75) is causing many 'timeout waiting for bus ready' errors.
> >>>>>>SDA remains high (as it should), but SCL remains low after a NACK.
> >>>>>>The bus becomes _unusable for other clients_.
> >>>>>>
> >>>>>>While probing, "lm75" writes a command, followed by a read + stop,
> >>>>>>but the write command is NACK'd. The chip does accept other writes/reads,
> >>>>>>it just refuses to ack invalid commands.
> >>>>>>
> >>>>>>Can you tell me if the patch below would make any sense? Or is it the
> >>>>>>responsibility of the client to reset the i2c_smbus?
> >>>>>patch below breaks repeated start.
> >>Felipe, I'd very appreciate if you'd be able to provide the use case
> >>which will fail with such solution?
> >
> >can't you see how this would fail ?
> >
> >assume omap_i2c_xfer() is called with its last argument (num) being
> >greater than one and you get the NAK before the last transfer.
> That's our case - NACK from slave before last transfer

with one difference, your device requires a STP condition because it's
SMBus, right ? Not all devices act like that :-)

But now I noticed, because of your reply, something I have been
overlooking for quite some time. As you said below, in case of NAK we
break out of the loop and don't even try the following messages.

Which means your original patch starts to make a lot more sense. I
wonder is this is really what we should be doing though - breaking out
of the loop, I mean.

It'd be cool to get an answer from I2C maintainers if we're doing the
right thing here.
Hein Tibosch - July 16, 2013, 4:17 p.m.
Hi Grygorii, Filipe,

On 7/16/2013 9:00 PM, Felipe Balbi wrote:
> On Tue, Jul 16, 2013 at 03:08:04PM +0300, Grygorii Strashko wrote:
>> Hi Felipe,
>> On 07/16/2013 02:27 PM, Felipe Balbi wrote:
>>> Hi,
>>>
>>> On Tue, Jul 16, 2013 at 02:01:11PM +0300, Grygorii Strashko wrote:
>>>>>>>> On a OMAP4460, i2c-bus-3:
>>>>>>>>
>>>>>>>> A driver (lm75) is causing many 'timeout waiting for bus ready' errors.
>>>>>>>> SDA remains high (as it should), but SCL remains low after a NACK.
>>>>>>>> The bus becomes _unusable for other clients_.
>>>>>>>>
>>>>>>>> While probing, "lm75" writes a command, followed by a read + stop,
>>>>>>>> but the write command is NACK'd. The chip does accept other writes/reads,
>>>>>>>> it just refuses to ack invalid commands.

In case the NACK may not be ignored, I believe it is correct
to break out of the loop and send a stop for 2 reasons:

1. all chips, including the target chip, will know that the
   current transaction is finished

2. to set CLK high again, which prevents numerous timeouts
   on subsequent transactions

As a test I've set 'I2C_M_IGNORE_NAK' for all .detect messages
sent by the lm75 driver.
Now the chip (tmp105) will provide read data as expected
after the _repeated start_, even though it NACK'd the preceding
WR command.
And also the detection trick does work now, addresses 4,5,6,7 return
the same read data as was returned from the last valid address 2.

Felipe:
> Which means your original patch starts to make a lot more sense. I
> wonder is this is really what we should be doing though - breaking out
> of the loop, I mean.

So yes, we have to break the loop as the caller is not interested
in processing any further commands.

In i2c/algos/i2c-algo-bit.c, bit_xfer() works exactly the same way:
break the loop (unless IGNORE_NAK) and call unconditionally i2c_stop().

It looks like TI's i2c-davinci will have the same problem as i2c-omap,
and will need the same change.

Grygorii, if you submit the patch, please add my

Signed-off-by: Hein Tibosch <hein_tibosch@yahoo.es>

cause you were earlier to notice and fix this problem.

Hein
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Wolfram Sang - Aug. 19, 2013, 12:11 p.m.
Hi,

> > Which means your original patch starts to make a lot more sense. I
> > wonder is this is really what we should be doing though - breaking out
> > of the loop, I mean.

Yup, that is fine. I applied the old patch with Acks from Hein and
Felipe to -next. Thanks!

> It looks like TI's i2c-davinci will have the same problem as i2c-omap,
> and will need the same change.

Somebody up for this?
Felipe Balbi - Aug. 19, 2013, 1:57 p.m.
On Mon, Aug 19, 2013 at 02:11:23PM +0200, Wolfram Sang wrote:
> Hi,
> 
> > > Which means your original patch starts to make a lot more sense. I
> > > wonder is this is really what we should be doing though - breaking out
> > > of the loop, I mean.
> 
> Yup, that is fine. I applied the old patch with Acks from Hein and
> Felipe to -next. Thanks!
> 
> > It looks like TI's i2c-davinci will have the same problem as i2c-omap,
> > and will need the same change.
> 
> Somebody up for this?

I would suggest deleting i2c-davinci and making sure it can use
i2c-omap. It's the same IP anyway. Just an older version which was used
back in OMAP1 times.
Wolfram Sang - Aug. 19, 2013, 2:05 p.m.
On Mon, Aug 19, 2013 at 08:57:02AM -0500, Felipe Balbi wrote:
> On Mon, Aug 19, 2013 at 02:11:23PM +0200, Wolfram Sang wrote:
> > Hi,
> > 
> > > > Which means your original patch starts to make a lot more sense. I
> > > > wonder is this is really what we should be doing though - breaking out
> > > > of the loop, I mean.
> > 
> > Yup, that is fine. I applied the old patch with Acks from Hein and
> > Felipe to -next. Thanks!
> > 
> > > It looks like TI's i2c-davinci will have the same problem as i2c-omap,
> > > and will need the same change.
> > 
> > Somebody up for this?
> 
> I would suggest deleting i2c-davinci and making sure it can use
> i2c-omap. It's the same IP anyway. Just an older version which was used
> back in OMAP1 times.

Yay, I'd love such a patch...
Grygorii Strashko - Aug. 20, 2013, 4:39 p.m.
On 08/19/2013 03:11 PM, Wolfram Sang wrote:
> Hi,
>
>>> Which means your original patch starts to make a lot more sense. I
>>> wonder is this is really what we should be doing though - breaking out
>>> of the loop, I mean.
>
> Yup, that is fine. I applied the old patch with Acks from Hein and
> Felipe to -next. Thanks!

Thanks.

>
>> It looks like TI's i2c-davinci will have the same problem as i2c-omap,
>> and will need the same change.
>
> Somebody up for this?
>

Regards,
-grygorii
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Patch

diff --git a/drivers/i2c/busses/i2c-omap.c b/drivers/i2c/busses/i2c-omap.c
index 142b694d..571b160 100644
--- a/drivers/i2c/busses/i2c-omap.c
+++ b/drivers/i2c/busses/i2c-omap.c
@@ -618,7 +618,7 @@  static int omap_i2c_xfer_msg(struct i2c_adapter *adap,
        if (dev->cmd_err & OMAP_I2C_STAT_NACK) {
                if (msg->flags & I2C_M_IGNORE_NAK)
                        return 0;
-               if (stop) {
+               if (stop || is_smbus) {
                        w = omap_i2c_read_reg(dev, OMAP_I2C_CON_REG);
                        w |= OMAP_I2C_CON_STP;
                        omap_i2c_write_reg(dev, OMAP_I2C_CON_REG, w);