linux-kernel - Re: [PATCH] [media] ov5645: I2C address change

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Date:   Mon, 09 Oct 2017 15:52:32 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Sakari Ailus <sakari.ailus@....fi>
Cc:     Todor Tomov <todor.tomov@...aro.org>, 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

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 ?

> I wonder what others think.

-- 
Regards,

Laurent Pinchart