linux-kernel - Re: [PATCH 2/2] i2c-dev: Don't block the adapter from unregistering

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Date:	Wed, 6 Jul 2016 15:55:24 +0900
From:	Wolfram Sang <wsa@...-dreams.de>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Jean Delvare <jdelvare@...e.com>, linux-i2c@...r.kernel.org,
	linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
	Johan Hovold <johan@...nel.org>, Alex Elder <elder@...aro.org>
Subject: Re: [PATCH 2/2] i2c-dev: Don't block the adapter from unregistering

On Tue, Jul 05, 2016 at 07:57:07PM -0700, Viresh Kumar wrote:
> The i2c-dev calls i2c_get_adapter() from the .open() callback, which
> doesn't let the adapter device unregister unless the .close() callback
> is called.
> 
> On some platforms (like Google ARA), this doesn't let the modules
> (hardware attached to the phone) eject from the phone as the cleanup
> path for the module hasn't finished yet (i2c adapter not removed).
> 
> We can't let the userspace block the kernel forever in such cases.
> 
> Fix this by calling i2c_get_adapter() from all other file operations,
> i.e.  read/write/ioctl, to make sure the adapter doesn't get away while
> we are in the middle of a operation, but not otherwise. In .open() we
> will release the adapter device before returning and so if there is no
> data transfer in progress, then the i2c-dev doesn't block the adapter
> from unregistering.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>

I'd think Jean has more experience with I2C hotplugging approaches and
difficulties, so I'd be interested in his high level review.

However:

> @@ -234,6 +234,7 @@ struct i2c_client {
>  	struct i2c_adapter *adapter;	/* the adapter we sit on	*/
>  	struct device dev;		/* the device structure		*/
>  	int irq;			/* irq issued by device		*/
> +	int adapter_nr;
>  	struct list_head detected;

Adding something to *every* i2c_client for this corner case sounds
pretty expensive to me.

Regards,

   Wolfram


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