linux-kernel - Re: [RFC/RFT 01/10] i2c: add 'is_suspended' flag for i2c adapters

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Date:   Mon, 10 Dec 2018 22:03:09 +0000
From:   Peter Rosin <peda@...ntia.se>
To:     Wolfram Sang <wsa+renesas@...g-engineering.com>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>
CC:     "linux-renesas-soc@...r.kernel.org" 
        <linux-renesas-soc@...r.kernel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Wolfram Sang <wsa@...-dreams.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC/RFT 01/10] i2c: add 'is_suspended' flag for i2c adapters

On 2018-12-10 22:02, Wolfram Sang wrote:
> Some drivers open code handling of suspended adapters. It should be
> handled by the core, though, to ensure generic handling. This patch adds
> the flag and an accessor function.
> 
> Signed-off-by: Wolfram Sang <wsa+renesas@...g-engineering.com>
> ---
>  drivers/i2c/i2c-core-base.c | 1 +
>  include/linux/i2c.h         | 9 +++++++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/drivers/i2c/i2c-core-base.c b/drivers/i2c/i2c-core-base.c
> index 28460f6a60cc..9f89e258c8ff 100644
> --- a/drivers/i2c/i2c-core-base.c
> +++ b/drivers/i2c/i2c-core-base.c
> @@ -1232,6 +1232,7 @@ static int i2c_register_adapter(struct i2c_adapter *adap)
>  	if (!adap->lock_ops)
>  		adap->lock_ops = &i2c_adapter_lock_ops;
>  
> +	adap->is_suspended = false;
>  	rt_mutex_init(&adap->bus_lock);
>  	rt_mutex_init(&adap->mux_lock);
>  	mutex_init(&adap->userspace_clients_lock);
> diff --git a/include/linux/i2c.h b/include/linux/i2c.h
> index 65b4eaed1d96..9852038ee3a7 100644
> --- a/include/linux/i2c.h
> +++ b/include/linux/i2c.h
> @@ -680,6 +680,7 @@ struct i2c_adapter {
>  	int timeout;			/* in jiffies */
>  	int retries;
>  	struct device dev;		/* the adapter device */
> +	unsigned int is_suspended:1;	/* owned by the I2C core */

When more stuff is added to this bit field (which always happens at
some point) updates to all members of the bit field will have to use
the same root-adapter-locking, or we will suffer from RMW-races. So
this feels like an invitation for future disaster. Maybe a comment
about that to remind our future selves? Or perhaps the bit field
should be avoided altogether?

Cheers,
Peter

>  
>  	int nr;
>  	char name[48];
> @@ -762,6 +763,14 @@ i2c_unlock_bus(struct i2c_adapter *adapter, unsigned int flags)
>  	adapter->lock_ops->unlock_bus(adapter, flags);
>  }
>  
> +static inline void i2c_mark_adapter_suspended(struct i2c_adapter *adap,
> +					      bool suspended)
> +{
> +	i2c_lock_bus(adap, I2C_LOCK_ROOT_ADAPTER);
> +	adap->is_suspended = suspended;
> +	i2c_unlock_bus(adap, I2C_LOCK_ROOT_ADAPTER);
> +}
> +
>  /*flags for the client struct: */
>  #define I2C_CLIENT_PEC		0x04	/* Use Packet Error Checking */
>  #define I2C_CLIENT_TEN		0x10	/* we have a ten bit chip address */
>