linux-kernel - Re: [PATCH 1/2] i2c: Add possibility for user-defined (i2c-)devices for bus-drivers.

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Date:	Wed, 14 Nov 2012 13:38:36 +0100
From:	Alexander Holler <holler@...oftware.de>
To:	Jean Delvare <khali@...ux-fr.org>
CC:	linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org,
	Till Harbaum <till@...baum.org>
Subject: Re: [PATCH 1/2] i2c: Add possibility for user-defined (i2c-)devices
 for bus-drivers.

Hello,

Am 14.11.2012 10:40, schrieb Jean Delvare:

>> It isn't possible to do such, because the only ID available for
>> i2c-busses is given them at runtime. So people have to live with that
>> imho artificially problem, if they use my patch. I don't have any other
>> solution until the numbering is predictable. But I assume you already
>> know all that, otherwise you wouldn't have mentioned it.
>
> The problem is inherent to external, hot-plug I2C adapters. You can't
> predict their bus number, and actually you can't even always predict
> their name. In the case of usb-tiny-i2c, you may be able to look-up the
> bus number from the adapter name, but you won't be able to always
> differentiate between two adapters, if they are connected to paired USB
> ports.
>
> This is a design issue with the i2c-tiny-usb hardware in the first
> place. If it is needed to differentiate between adapters, their would
> need to be a locally unique ID in every adapter, which the kernel can
> query. I suppose this was not deemed necessary at design time, as most
> people will only connect one such adapter at a time to their system.

Actually many of the available USB devices (e.g. many usb-serials) don't 
offer a unique ID by themself. That isn't just a problem of the 
i2c-tiny-usb. But in contrast to other solutions, it shouldn't be very 
hard to give those adapters a unique ID. As the whole SUB solution is 
just software (and open source), one likely just would to write some 
small piece of additional code.

> I have already experienced this and yes, it is a pain. But I would
> think that a system designed without a RTC in the first place has
> another way of getting its time correct at boot time, for example NTP.
> As I understand it the RTC chip is only there to set the system time at
> boot, right, the actual timekeeping during run-time is still done by the
> CPU?

Whatever those people which want to us it decide. If I didn't want to 
help other people by offering them some small documentation about how to 
build such theirself, I wouldn't have taken the usual and almost 
unavoidable pain trying feed some silly patches into the kernel. ;)

Anyway, maybe Till Harbaum will like that solution and won't get blocked 
by you. And maybe in some years we will see how many other bus-drivers 
have adopted the same solution. In fact the in-driver solution was my 
first one and I've thought others might be interested too, so I've moved 
the few lines from the driver itself into the i2c-core before I sent the 
patches. Unfortunately a waste of time.

Regards,

Alexander

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