Re: [PATCH] [media] ov5645: I2C address change

[Laurent Pinchart]

Hi Todor,

On Monday, 9 October 2017 19:18:17 EEST Todor Tomov wrote:
> On  9.10.2017 15:52, Laurent Pinchart wrote:
> > 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@xxxxxxxxxx>
> >>>>> 
> >>>>> 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.

Yes, that's correct. It can be even worse than that, sometimes only one of the 
two devices with the same address can be reconfigured, which means that 
powering that device requires powering up the other device and changing its 
address first, otherwise the second device can't be used as long as the first 
one is power on (this happened for real in a Nokia platform).

> 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 :)

The biggest issue I see is that there's no C code that has knowledge of the 
whole platform. It's hard to describe this in DT in a generic way, board files 
were clearly useful for this kind of situations.

> >> I wonder what others think.

-- 
Regards,

Laurent Pinchart