Problem with multiple i2c multiplexers on the same bus

Problem with multiple i2c multiplexers on the same bus

[Martin Belanger]

This is regarding a series of emails between Guenter Roeck and Jean
Delvare titled "Problem with multiple i2c multiplexers on one bus, and
mux bus naming" sent in November 2013. Ref:
http://thread.gmane.org/gmane.linux.drivers.i2c/16980

I'm having the same problem with multiple PCA954x multiplexers on the
same bus and there is no way to tell them apart just by looking at the
"name" file.

There was a suggestion to change the name from "i2c-N-mux (chan_id M)"
to "i2c-N-mux-XX (chan_id M)" or even "i2c-N-mux-i2c-XX (chan_id M)",
where XX is the multiplexer's i2c address. That would solve my
problem, but unfortunately it looks like Guenter never submitted the
patch (or maybe it was rejected?).

I would like to submit a similar change, but I was thinking of adding
a module parameter so that the change is not the default behavior.
The idea is to preserve backward compatibility for applications that
don't require this fix. For example, modprobe i2c-dev
explicit_mux_id=1 would use i2c-N-mux-i2c-XX (chan_id M), whereas
modprobe i2c-dev would default to the current behavior: i.e. i2c-N-mux
(chan_id M).

Any thoughts, comments?

Regards,
Martin

Re: Problem with multiple i2c multiplexers on the same bus

[Wolfram Sang]

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Hi,

On Mon, Oct 27, 2014 at 11:46:01AM -0700, Martin Belanger wrote:
> This is regarding a series of emails between Guenter Roeck and Jean
> Delvare titled "Problem with multiple i2c multiplexers on one bus, and
> mux bus naming" sent in November 2013. Ref:
> http://thread.gmane.org/gmane.linux.drivers.i2c/16980

Please CC those people then, too. That helps getting their attention.
I've done this now.

> I'm having the same problem with multiple PCA954x multiplexers on the
> same bus and there is no way to tell them apart just by looking at the
> "name" file.
> 
> There was a suggestion to change the name from "i2c-N-mux (chan_id M)"
> to "i2c-N-mux-XX (chan_id M)" or even "i2c-N-mux-i2c-XX (chan_id M)",
> where XX is the multiplexer's i2c address. That would solve my
> problem, but unfortunately it looks like Guenter never submitted the
> patch (or maybe it was rejected?).

It just dropped off :( But you guys have my attention now, let's fix
this issue for 3.19! I am just reading through the old mails and will
think about it. Input is welcome.

> I would like to submit a similar change, but I was thinking of adding
> a module parameter so that the change is not the default behavior.
> The idea is to preserve backward compatibility for applications that
> don't require this fix. For example, modprobe i2c-dev
> explicit_mux_id=1 would use i2c-N-mux-i2c-XX (chan_id M), whereas
> modprobe i2c-dev would default to the current behavior: i.e. i2c-N-mux
> (chan_id M).

I don't like the need to set a module parameter to fix a flaw. I do
consider changing the ABI to have better strings in "name". But as said,
I need to think about it a little more...

Thanks,

   Wolfram


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Re: Problem with multiple i2c multiplexers on the same bus

[Guenter Roeck]

On 10/31/2014 02:03 PM, Wolfram Sang wrote:
> Hi,
>
> On Mon, Oct 27, 2014 at 11:46:01AM -0700, Martin Belanger wrote:
>> This is regarding a series of emails between Guenter Roeck and Jean
>> Delvare titled "Problem with multiple i2c multiplexers on one bus, and
>> mux bus naming" sent in November 2013. Ref:
>> http://thread.gmane.org/gmane.linux.drivers.i2c/16980
>
> Please CC those people then, too. That helps getting their attention.
> I've done this now.
>
>> I'm having the same problem with multiple PCA954x multiplexers on the
>> same bus and there is no way to tell them apart just by looking at the
>> "name" file.
>>
>> There was a suggestion to change the name from "i2c-N-mux (chan_id M)"
>> to "i2c-N-mux-XX (chan_id M)" or even "i2c-N-mux-i2c-XX (chan_id M)",
>> where XX is the multiplexer's i2c address. That would solve my
>> problem, but unfortunately it looks like Guenter never submitted the
>> patch (or maybe it was rejected?).
>
> It just dropped off :( But you guys have my attention now, let's fix
> this issue for 3.19! I am just reading through the old mails and will
> think about it. Input is welcome.
>
I didn't follow up on the issue since it was not an immediate concern,
and my proposed solution had some problems. If I remember correctly,
one of the problems was that the multiplexer does not have to be an
i2c chip. In that case XX would be unknown and/or have to be omitted.

Guenter

>> I would like to submit a similar change, but I was thinking of adding
>> a module parameter so that the change is not the default behavior.
>> The idea is to preserve backward compatibility for applications that
>> don't require this fix. For example, modprobe i2c-dev
>> explicit_mux_id=1 would use i2c-N-mux-i2c-XX (chan_id M), whereas
>> modprobe i2c-dev would default to the current behavior: i.e. i2c-N-mux
>> (chan_id M).
>
> I don't like the need to set a module parameter to fix a flaw. I do
> consider changing the ABI to have better strings in "name". But as said,
> I need to think about it a little more...
>
> Thanks,
>
>     Wolfram
>

Re: Problem with multiple i2c multiplexers on the same bus

[Martin Belanger]

Hi all, and thanks for your response.

You are correct Guenter.  We need to address both i2c- and gpio-based
multiplexers.  You and Jean suggested the following solution:

I2C Mux:  i2c-N-mux-i2c-XX (chan_id M)
GPIO Mux: i2c-N-mux-gpio-XX (chan_id M)

Where XX is the i2c address or the first GPIO pin number.  This
ensures unique IDs for both technologies.

Adding a parameter to i2c_add_mux_adapter() (as you and Jean
suggested) would solve the problem.  Basically, we pass a unique id
string (e.g. "-i2c-XX" or "-gpio-XX") to the API, which then adds to
the "name" as described above. In cases where a unique id is not
required, we can simply pass NULL to i2c_add_mux_adapter().  Here's an
example of what the new API would look like:

struct i2c_adapter *i2c_add_mux_adapter(struct i2c_adapter *parent,
    struct device *mux_dev,
    void *mux_priv, u32 force_nr, u32 chan_id,
    unsigned int class,
    int (*select) (struct i2c_adapter *,
          void *mux_dev, u32 chan_id),
    int (*deselect) (struct i2c_adapter *,
    void *mux_dev, u32 chan_id),
    const char *explicit_id);

The parameter explicit_id gets used as follows:

if (NULL == explicit_id)
    explicit_id = "";
snprintf(priv->adap.name, sizeof(priv->adap.name),
    "i2c-%d-mux%s (chan_id %d)", i2c_adapter_id(parent), explicit_id, chan_id);

If it's OK will all parties, I can submit a patch for it (and I'll
make sure to reference Guenter and Jean as the designers).  This,
however, is a first for me.  I read all the documents about submitting
patches.  I was just wondering if I should submit against the current
3.18 development or some other "stable" releases.

Regards,
Martin
Martin Belanger
Sr. Software Engineer
1383 North McDowell Blvd.
Petaluma, CA 94954
M(707) 481-3392
Emartin.belanger-Ir6+u9MVKBtBDgjK7y7TUQ@public.gmane.org
www.cyaninc.com


On Fri, Oct 31, 2014 at 2:17 PM, Guenter Roeck <linux-0h96xk9xTtrk1uMJSBkQmQ@public.gmane.org> wrote:
> On 10/31/2014 02:03 PM, Wolfram Sang wrote:
>>
>> Hi,
>>
>> On Mon, Oct 27, 2014 at 11:46:01AM -0700, Martin Belanger wrote:
>>>
>>> This is regarding a series of emails between Guenter Roeck and Jean
>>> Delvare titled "Problem with multiple i2c multiplexers on one bus, and
>>> mux bus naming" sent in November 2013. Ref:
>>> http://thread.gmane.org/gmane.linux.drivers.i2c/16980
>>
>>
>> Please CC those people then, too. That helps getting their attention.
>> I've done this now.
>>
>>> I'm having the same problem with multiple PCA954x multiplexers on the
>>> same bus and there is no way to tell them apart just by looking at the
>>> "name" file.
>>>
>>> There was a suggestion to change the name from "i2c-N-mux (chan_id M)"
>>> to "i2c-N-mux-XX (chan_id M)" or even "i2c-N-mux-i2c-XX (chan_id M)",
>>> where XX is the multiplexer's i2c address. That would solve my
>>> problem, but unfortunately it looks like Guenter never submitted the
>>> patch (or maybe it was rejected?).
>>
>>
>> It just dropped off :( But you guys have my attention now, let's fix
>> this issue for 3.19! I am just reading through the old mails and will
>> think about it. Input is welcome.
>>
> I didn't follow up on the issue since it was not an immediate concern,
> and my proposed solution had some problems. If I remember correctly,
> one of the problems was that the multiplexer does not have to be an
> i2c chip. In that case XX would be unknown and/or have to be omitted.
>
> Guenter
>
>
>>> I would like to submit a similar change, but I was thinking of adding
>>> a module parameter so that the change is not the default behavior.
>>> The idea is to preserve backward compatibility for applications that
>>> don't require this fix. For example, modprobe i2c-dev
>>> explicit_mux_id=1 would use i2c-N-mux-i2c-XX (chan_id M), whereas
>>> modprobe i2c-dev would default to the current behavior: i.e. i2c-N-mux
>>> (chan_id M).
>>
>>
>> I don't like the need to set a module parameter to fix a flaw. I do
>> consider changing the ABI to have better strings in "name". But as said,
>> I need to think about it a little more...
>>
>> Thanks,
>>
>>     Wolfram
>>
>

Re: Problem with multiple i2c multiplexers on the same bus

[Wolfram Sang]

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> You are correct Guenter.  We need to address both i2c- and gpio-based
> multiplexers.  You and Jean suggested the following solution:
> 
> I2C Mux:  i2c-N-mux-i2c-XX (chan_id M)
> GPIO Mux: i2c-N-mux-gpio-XX (chan_id M)
> 
> Where XX is the i2c address or the first GPIO pin number.  This
> ensures unique IDs for both technologies.

I rediscovered this series:

http://www.spinics.net/lists/linux-i2c/msg11542.html
("i2c-mux: expose i2c bus topology under sysfs")

From a glimpse, I like this approach better. I wonder if we can maintain
backward compatibility somehow. I need to play with it. But not today,
this has to wait till tomorrow.


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Re: Problem with multiple i2c multiplexers on the same bus

[Wolfram Sang]

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> I rediscovered this series:
> 
> http://www.spinics.net/lists/linux-i2c/msg11542.html
> ("i2c-mux: expose i2c bus topology under sysfs")
> 
> From a glimpse, I like this approach better. I wonder if we can maintain
> backward compatibility somehow. I need to play with it. But not today,
> this has to wait till tomorrow.

Sending a patch using this aproach in a minute...


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Re: Problem with multiple i2c multiplexers on the same bus

[Guenter Roeck]

On 10/31/2014 02:45 PM, Martin Belanger wrote:
> Hi all, and thanks for your response.
>
> You are correct Guenter.  We need to address both i2c- and gpio-based
> multiplexers.  You and Jean suggested the following solution:
>
> I2C Mux:  i2c-N-mux-i2c-XX (chan_id M)
> GPIO Mux: i2c-N-mux-gpio-XX (chan_id M)
>
Too long ago to remember.

> Where XX is the i2c address or the first GPIO pin number.  This
> ensures unique IDs for both technologies.
>
> Adding a parameter to i2c_add_mux_adapter() (as you and Jean
> suggested) would solve the problem.  Basically, we pass a unique id
> string (e.g. "-i2c-XX" or "-gpio-XX") to the API, which then adds to
> the "name" as described above. In cases where a unique id is not
> required, we can simply pass NULL to i2c_add_mux_adapter().  Here's an
> example of what the new API would look like:
>
> struct i2c_adapter *i2c_add_mux_adapter(struct i2c_adapter *parent,
>      struct device *mux_dev,
>      void *mux_priv, u32 force_nr, u32 chan_id,
>      unsigned int class,
>      int (*select) (struct i2c_adapter *,
>            void *mux_dev, u32 chan_id),
>      int (*deselect) (struct i2c_adapter *,
>      void *mux_dev, u32 chan_id),
>      const char *explicit_id);
>
> The parameter explicit_id gets used as follows:
>
> if (NULL == explicit_id)

Personally, I would never accept code like that; it just confuses me
to see the constant first in an expression.

API-wise I am fine with the proposed change, except I would probably
move the new argument ahead of chan_id and name it mux_id.

Guenter

>      explicit_id = "";
> snprintf(priv->adap.name, sizeof(priv->adap.name),
>      "i2c-%d-mux%s (chan_id %d)", i2c_adapter_id(parent), explicit_id, chan_id);
>
> If it's OK will all parties, I can submit a patch for it (and I'll
> make sure to reference Guenter and Jean as the designers).  This,
> however, is a first for me.  I read all the documents about submitting
> patches.  I was just wondering if I should submit against the current
> 3.18 development or some other "stable" releases.
>
> Regards,
> Martin
> Martin Belanger
> Sr. Software Engineer
> 1383 North McDowell Blvd.
> Petaluma, CA 94954
> M(707) 481-3392
> Emartin.belanger-Ir6+u9MVKBtBDgjK7y7TUQ@public.gmane.org
> www.cyaninc.com
>
>
> On Fri, Oct 31, 2014 at 2:17 PM, Guenter Roeck <linux-0h96xk9xTtrk1uMJSBkQmQ@public.gmane.org> wrote:
>> On 10/31/2014 02:03 PM, Wolfram Sang wrote:
>>>
>>> Hi,
>>>
>>> On Mon, Oct 27, 2014 at 11:46:01AM -0700, Martin Belanger wrote:
>>>>
>>>> This is regarding a series of emails between Guenter Roeck and Jean
>>>> Delvare titled "Problem with multiple i2c multiplexers on one bus, and
>>>> mux bus naming" sent in November 2013. Ref:
>>>> http://thread.gmane.org/gmane.linux.drivers.i2c/16980
>>>
>>>
>>> Please CC those people then, too. That helps getting their attention.
>>> I've done this now.
>>>
>>>> I'm having the same problem with multiple PCA954x multiplexers on the
>>>> same bus and there is no way to tell them apart just by looking at the
>>>> "name" file.
>>>>
>>>> There was a suggestion to change the name from "i2c-N-mux (chan_id M)"
>>>> to "i2c-N-mux-XX (chan_id M)" or even "i2c-N-mux-i2c-XX (chan_id M)",
>>>> where XX is the multiplexer's i2c address. That would solve my
>>>> problem, but unfortunately it looks like Guenter never submitted the
>>>> patch (or maybe it was rejected?).
>>>
>>>
>>> It just dropped off :( But you guys have my attention now, let's fix
>>> this issue for 3.19! I am just reading through the old mails and will
>>> think about it. Input is welcome.
>>>
>> I didn't follow up on the issue since it was not an immediate concern,
>> and my proposed solution had some problems. If I remember correctly,
>> one of the problems was that the multiplexer does not have to be an
>> i2c chip. In that case XX would be unknown and/or have to be omitted.
>>
>> Guenter
>>
>>
>>>> I would like to submit a similar change, but I was thinking of adding
>>>> a module parameter so that the change is not the default behavior.
>>>> The idea is to preserve backward compatibility for applications that
>>>> don't require this fix. For example, modprobe i2c-dev
>>>> explicit_mux_id=1 would use i2c-N-mux-i2c-XX (chan_id M), whereas
>>>> modprobe i2c-dev would default to the current behavior: i.e. i2c-N-mux
>>>> (chan_id M).
>>>
>>>
>>> I don't like the need to set a module parameter to fix a flaw. I do
>>> consider changing the ABI to have better strings in "name". But as said,
>>> I need to think about it a little more...
>>>
>>> Thanks,
>>>
>>>      Wolfram
>>>
>>
>