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Date: Wed, 13 Feb 2013 14:09:08 -0700
From: Stephen Warren <swarren@wwwdotorg.org>
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To: Doug Anderson <dianders@chromium.org>
CC: Wolfram Sang <w.sang@pengutronix.de>, Simon Glass <sjg@chromium.org>,
        Naveen Krishna Chatradhi <ch.naveen@samsung.com>,
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Subject: Re: [PATCH v1 4/4] i2c-mux: i2c_add_mux_adapter() should use -1 for
 auto bus num
References: <1360778532-7480-1-git-send-email-dianders@chromium.org> <1360778532-7480-4-git-send-email-dianders@chromium.org>
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On 02/13/2013 11:02 AM, Doug Anderson wrote:
> The force_nr parameter to i2c_add_mux_adapter() uses 0 to signify that
> we don't want to force the bus number of the adapter.  This is
> non-ideal because:
> * 0 is actually a valid bus number to request
> * i2c_add_numbered_adapter() (which i2c_add_mux_adapter() calls) uses
>   -1 to mean the same thing.  That means extra logic in
>   i2c_add_mux_adapter().
> 
> Fix i2c_add_mux_adapter() to use -1 and update all mux drivers
> accordingly.
> 
> Signed-off-by: Doug Anderson <dianders@chromium.org>
> ---
> Notes:
> - If there's a good reason that force_nr uses 0 for auto then feel
>   free to drop this patch.  I've place it at the end of the series to
>   make it easy to just drop it.

IIRC (and I only vaguely do...) it's because:

> diff --git a/drivers/i2c/muxes/i2c-mux-gpio.c b/drivers/i2c/muxes/i2c-mux-gpio.c
> index 9f50ef0..301ed0b 100644
> --- a/drivers/i2c/muxes/i2c-mux-gpio.c
> +++ b/drivers/i2c/muxes/i2c-mux-gpio.c
> @@ -208,7 +208,7 @@ static int i2c_mux_gpio_probe(struct platform_device *pdev)
>  	}
>  
>  	for (i = 0; i < mux->data.n_values; i++) {
> -		u32 nr = mux->data.base_nr ? (mux->data.base_nr + i) : 0;
> +		int nr = mux->data.base_nr ? (mux->data.base_nr + i) : -1;

Here, mux->data.base_nr is platform data (or copied directly from it),
and any field in a platform data struct stored in a global variable not
explicitly initialized would be 0, hence 0 would typically mean "no
explicit bus number desired". Since a mux can't exist without a parent
I2C bus, it's unlikely anyone would want a mux to be I2C bus 0, but
rather the parent to have that number.