linux-kernel - Re: [PATCH v3 1/1] platform/chrome: Probe multiple i2c adapters of the same name

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Date:	Wed, 11 Jun 2014 23:23:02 +0100 (BST)
From:	Scot Doyle <lkml14@...tdoyle.com>
To:	Benson Leung <bleung@...omium.org>
cc:	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Olof Johansson <olof@...om.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/1] platform/chrome: Probe multiple i2c adapters of
 the same name


On Wed, 11 Jun 2014, Benson Leung wrote:
> On Wed, Jun 11, 2014 at 10:51 AM, Scot Doyle <lkml14@...tdoyle.com> wrote:
>> The chromeos_laptop module probes the first i2c adapter with a specific name
>> for expected hardware. However, the Acer C720 Chromebook has two i2c
>> adapters with the same name. This patch probes each i2c adapter with a
>> specific name until locating the expected hardware.
>>
>> Thanks to Mika Westerberg for formatting tips and identifying the need for
>> unique bus addresses within each set of identically-named adapters.
>
> Scot,
>
> Is your intention with this patch to replace the one that Mika wrote
> that specifically special cased the designware i2c busses? He already
> solved this problem in this patch.
> https://patchwork.kernel.org/patch/4288591/
>
> I'm not sure about your approach here. It looks like if there's some
> ambiguity in the names of the busses, you try to probe the device on
> every bus with that name.
>
> We know that bus 0 is going to have the touchpad, and we know that bus
> 1 is going to have the touchscreen and light sensor. I'd prefer not to
> send extra probes for all of the combination of devices that don't
> exist.

In the case of the Acer C720, will the touchpad always be the first device 
iterated by the bus_find_device function?

My quite possibly incorrect understanding is that:
- the i2c bus numbers assigned are dynamic
- dynamic bus numbers depend on the order of device registration
- there is no guarantee of the ordering of device registration
- bus_find_device iterates by insertion order (as opposed to ascending pci 
device id or ascending pci hardware address)
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