Date:	Mon, 22 Sep 2014 17:41:44 +0300
From:	Mika Westerberg <mika.westerberg@...ux.intel.com>
To:	Lars-Peter Clausen <lars@...afoo.de>
Cc:	Jean-Michel Hautbois <jean-michel.hautbois@...alys.com>,
	wsa@...-dreams.de, linux-i2c@...r.kernel.org,
	linux-kernel@...r.kernel.org, mark.rutland@....com,
	srinivas.pandruvada@...ux.intel.com
Subject: Re: [PATCH] i2c: Add generic support passing secondary devices
 addresses

On Mon, Sep 22, 2014 at 04:11:52PM +0200, Lars-Peter Clausen wrote:
> On 09/22/2014 03:45 PM, Mika Westerberg wrote:
> >On Mon, Sep 22, 2014 at 03:27:36PM +0200, Lars-Peter Clausen wrote:
> >>On 09/22/2014 12:45 PM, Mika Westerberg wrote:
> >>>On Fri, Sep 05, 2014 at 04:02:19PM +0200, Jean-Michel Hautbois wrote:
> >>>>Some I2C devices have multiple addresses assigned, for example each address
> >>>>corresponding to a different internal register map page of the device.
> >>>>So far drivers which need support for this have handled this with a driver
> >>>>specific and non-generic implementation, e.g. passing the additional address
> >>>>via platform data.
> >>>>
> >>>>This patch provides a new helper function called i2c_new_secondary_device()
> >>>>which is intended to provide a generic way to get the secondary address
> >>>>as well as instantiate a struct i2c_client for the secondary address.
> >>>>
> >>>>The function expects a pointer to the primary i2c_client, a name
> >>>>for the secondary address and an optional default address. The name is used
> >>>>as a handle to specify which secondary address to get.
> >>>>
> >>>>The default address is used as a fallback in case no secondary address
> >>>>was explicitly specified. In case no secondary address and no default
> >>>>address were specified the function returns NULL.
> >>>>
> >>>>For now the function only supports look-up of the secondary address
> >>>>from devicetree, but it can be extended in the future
> >>>>to for example support board files and/or ACPI.
> >>>>
> >>>>Signed-off-by: Jean-Michel Hautbois <jean-michel.hautbois@...alys.com>
> >>>
> >>>Sorry, just noticed this one.
> >>>
> >>>Srinivas (CC'd) and I did similar patch series here:
> >>>
> >>>http://patchwork.ozlabs.org/patch/338342/
> >
> >Sorry I gave wrong link. That one is older version.
> >
> >Here is the current:
> >
> >http://patchwork.ozlabs.org/patch/386409/
> >http://patchwork.ozlabs.org/patch/386410/
> >
> >>>
> >>>We should probably collaborate on this one to get both DT and ACPI
> >>>supported.
> >>
> >>Yes. The idea was to keep the interface of the API generic so it can be used
> >>by ACPI or other device topology description mechanisms as well.
> >>
> >>But it looks as if the ACPI case is a bit more complex and we may need a
> >>revision of the API. How for example in the ACPI case do you know which
> >>address is which, when different parts of a chip are addressed using
> >>different addresses?
> >
> >Unfortunately there is no way in ACPI 5.0 to find out which is which. So
> >we trust the ordering of I2cSerialBus() resources. Even that has been
> >problematic because some vendors then list things like SMBus ARA
> >addresses there in random order :-(
> >
> >Our API has following signature:
> >
> >int i2c_address_by_index(struct i2c_client *client, int index,
> >			 struct i2c_board_info *info,
> >			 struct i2c_adapter **adapter)
> >
> >and we use index to find out which address to use.
> >
> >Note also that in ACPI it is possible that the I2cSerialBus() resource
> >points to another I2C host controller, so we need to have 'adapter'
> >parameter as well.
> >
> 
> Ok, looks like there are two main differences in the two implementations.
> 
> 1) The ACPI one uses a integer index and the DT one uses a string index to
> lookup the device.
> 
> The problem with the index lookup is that the order is binding specific. So
> it might be different between e.g. the devicetree binding and the ACPI
> binding. This makes it quite hard to use the API in a generic way and you'd
> end up with hacks like:
> 
> if (client->dev.of_node)
> 	index = 3;
> else if (ACPI_COMPANION(client->dev))
> 	index = 1;
> else
> 	index = 5;
> 

Indeed.

> So we might need a extra translation table which maps a name to a ACPI index
> and then we could use the name as the generic index in the driver.

Good thing is that ACPI 5.1 _DSD finally allows us to use similar naming
as the DT has been doing. Problem is that we need to support both the
new way *and* the older index lookup somehow :-/

> 2) The ACPI implementation returns the i2c_board_info and the adapter, while
> the DT implementation returns the instantiated I2C client device.
> 
> It might make sense to have both. I image that most drivers are just
> interested in creating a new client device and will simply pass the board
> info and adapter they got to i2c_new_device(). In this case it makes sense
> to have a helper function which already does this internally to avoid
> boilerplate code duplication.

I agree. How about making that helper a wrapper around the function that
returns both i2c_board_info and an adapter?

> There will probably some special cases though in which case the driver wants
> to get the adapter and the board info and then manually call
> i2c_new_device() after having done some additional steps.

Yes, if the alternative address happens to be on another bus. That
should at least be possible with this API.
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