Date:   Mon, 9 Oct 2017 19:18:17 +0300
From:   Todor Tomov <todor.tomov@...aro.org>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Sakari Ailus <sakari.ailus@....fi>
Cc:     mchehab@...nel.org, hansverk@...co.com,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-i2c@...r.kernel.org, Wolfram Sang <wsa@...-dreams.de>
Subject: Re: [PATCH] [media] ov5645: I2C address change

Hi Laurent :)

On  9.10.2017 15:52, Laurent Pinchart wrote:
> Hello,
> 
> On Monday, 9 October 2017 12:34:26 EEST Sakari Ailus wrote:
>> On Mon, Oct 09, 2017 at 11:36:01AM +0300, Todor Tomov wrote:
>>> On  4.10.2017 13:47, Laurent Pinchart wrote:
>>>> CC'ing the I2C mainling list and the I2C maintainer.
>>>>
>>>> On Wednesday, 4 October 2017 13:30:08 EEST Sakari Ailus wrote:
>>>>> On Mon, Oct 02, 2017 at 04:28:45PM +0300, Todor Tomov wrote:
>>>>>> As soon as the sensor is powered on, change the I2C address to the one
>>>>>> specified in DT. This allows to use multiple physical sensors
>>>>>> connected to the same I2C bus.
>>>>>>
>>>>>> Signed-off-by: Todor Tomov <todor.tomov@...aro.org>
>>>>>
>>>>> The smiapp driver does something similar and I understand Laurent might
>>>>> be interested in such functionality as well.
>>>>>
>>>>> It'd be nice to handle this through the I²C framework instead and to
>>>>> define how the information is specified through DT. That way it could
>>>>> be made generic, to work with more devices than just this one.
>>>>>
>>>>> What do you think?
>>>
>>> Thank you for this suggestion.
>>>
>>> The way I have done it is to put the new I2C address in the DT and the
>>> driver programs the change using the original I2C address. The original
>>> I2C address is hardcoded in the driver. So maybe we can extend the DT
>>> binding and the I2C framework so that both addresses come from the DT and
>>> avoid hiding the original I2C address in the driver. This sounds good to
>>> me.
>>
>> Agreed.
>>
>> In this case the address is known but in general that's not the case it's
>> not that simple. There are register compatible devices that have different
>> addresses even if they're the same devices.
>>
>> It might be a good idea to make this explicit.
> 
> Yes, in the general case we need to specify the original address in DT, as the 
> chip could have a non-fixed boot-up I2C address.
> 
> In many cases the value of the new I2C address doesn't matter much, as long as 
> it's unique on the bus. I was thinking about implementing a dynamic allocator 
> for I2C addresses, but after discussing it with Wolfram we concluded that it 
> would probably not be a good idea. There could be other I2C devices on the bus 
> that Linux isn't aware of, in which case the dynamic allocator could create 
> address conflicts. Specifying the new address in DT is likely a better idea, 
> even if it could feel a bit more of system configuration information than a 
> pure hardware description.
> 
>>> Then changing the address could be device specific and also this must be
>>> done right after power on so that there are no address conflicts. So I
>>> don't think that we can support this through the I2C framework only, the
>>> drivers that we want to do that will have to be expanded with this
>>> functionality. Or do you have any other idea?
>>
>> Yes, how the address is changed is always hardware specific. This would be
>> most conveniently done in driver's probe or PM runtime_resume functions.
> 
> This patch modifies client->addr directly, which I don't think is a good idea. 
> I'd prefer making the I2C core aware of the address change through an explicit 
> API call. This would allow catching I2C adress conflicts for instance.
> 
>> It could be as simple as providing an adapter specific mutex to serialise
>> address changes on the bus so that no two address changes are taking place
>> at the same time. Which is essentially the impliementation you had, only
>> the mutex would be for the I²C adapter, not the driver. An helper functions
>> for acquiring and releasing the mutex.
> 
> Why do you need to serialize address changes ?

Correct me if I'm wrong, but if you power on more than one device with the
same I2C address and issue a command to change it, then all devices will
recognize this command as addressed to them. The only solution (which I know
about) to avoid this is to serialize the power on and address change (as a whole!)
for these devices.

I think it would be better to move the mutex out of the driver - to avoid all
client drivers which will change I2C address to add a global variable mutex 
for this. We just have to find a better place for it :)

> 
>> I wonder what others think.
> 

-- 
Best regards,
Todor Tomov

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