From: Alexander Stein <alexander.stein@systec-electronic.com>
To: Feng Tang <feng.tang@intel.com>
Cc: linux-i2c@vger.kernel.org, linux-kernel@vger.kernel.org,
        "Jean Delvare (PC drivers, core)" <khali@linux-fr.org>,
        "Ben Dooks (embedded platforms)" <ben-linux@fluff.org>,
        Tomoya MORINAGA <tomoya.rohm@gmail.com>
Subject: Re: i2c-eg20t: regression since i2c_add_numbered_adapter change
Date: Wed, 22 Aug 2012 11:17:51 +0200
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Hello,

Am Mittwoch, 22. August 2012, 16:04:39 schrieb Feng Tang:
> > Why use a fixed one? Give the driver (and maybe every i2c bus driver) a parameter which sets the base bus number it should use.
> > E.g. i2c-eg20t.base-bus-num=2 so it will register the bus numbers starting from 2. If this parameter is unset. It would use the first free one, thus simply using i2c_add_adapter.
> 
> The reason we need a fixed number is it is easier for platform code
> which needs to register dozens of i2c devices to different controllers
> with i2c_register_board_info, and they need provide a bus number for
> each i2c device, this _binding_ info is not detectable but have to
> be fixed.

Yes, I'm aware of that. With "Why use a fixed one?" I meant why hard-code it into the driver. I should be changeable.
Because this is/was not possible in general to use i2c_register_board_info, so we used an echo to /sys/bus/i2c/devices/i2c-0/new_device or /sys/bus/i2c/devices/i2c-1/new_device.

> Yes, your module parameter "base-bus-num" sounds like a plan too,
> at the cost of some extra setting in the kernel cmdline. And I'm fine
> with all solutions as long as I can get a _fixed_ bus number so that
> I can easily write the platform config code (I guess this is same
> for device tree, and even ACPI 5.0)

i2c_add_numbered_adapter only works reliably if only using a single driver. If you are using several, especially if one uses dynamic bus numbers, you're kinda screwed.
So, not very driver can add busses starting at bus number 0.
I guess using an Intel Atom (i2c-isch) with an eg20t is not that uncommon, so you'll end up using 2 drivers where each one should have a fixed number.
Some mechanism like udev for renaming ethernet or block devices is needed here.

Regards,
Alexander

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