Re: [RFC PATCH 4/5] i2c: core: add simple caching to the 'alias' scanning

[Peter Rosin <peda@axentia.se>]

On 2019-12-31 17:13, Wolfram Sang wrote:
> Add some simple caching of the last used alias to skip some unneeded
> scanning of the I2C bus. When freeing the device, the cache will be set
> back.
> 
> Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
> ---
>  drivers/i2c/i2c-core-base.c | 15 ++++++++++++++-
>  include/linux/i2c.h         |  2 ++
>  2 files changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/i2c/i2c-core-base.c b/drivers/i2c/i2c-core-base.c
> index 5a010e7e698f..0cc4a5c49a15 100644
> --- a/drivers/i2c/i2c-core-base.c
> +++ b/drivers/i2c/i2c-core-base.c
> @@ -830,6 +830,8 @@ EXPORT_SYMBOL_GPL(i2c_new_device);
>   */
>  void i2c_unregister_device(struct i2c_client *client)
>  {
> +	struct i2c_adapter *adap = client->adapter;
> +
>  	if (IS_ERR_OR_NULL(client))
>  		return;
>  
> @@ -840,6 +842,14 @@ void i2c_unregister_device(struct i2c_client *client)
>  
>  	if (ACPI_COMPANION(&client->dev))
>  		acpi_device_clear_enumerated(ACPI_COMPANION(&client->dev));
> +
> +	i2c_lock_bus(adap, I2C_LOCK_SEGMENT);
> +
> +	if (client->flags & I2C_CLIENT_ALIAS && client->addr < adap->alias_idx)
> +		adap->alias_idx = client->addr;
> +
> +	i2c_unlock_bus(adap, I2C_LOCK_SEGMENT);
> +
>  	device_unregister(&client->dev);
>  }
>  EXPORT_SYMBOL_GPL(i2c_unregister_device);
> @@ -1297,6 +1307,7 @@ static int i2c_register_adapter(struct i2c_adapter *adap)
>  		adap->lock_ops = &i2c_adapter_lock_ops;
>  
>  	adap->locked_flags = 0;
> +	adap->alias_idx = 0x08;	/* first valid I2C address */
>  	rt_mutex_init(&adap->bus_lock);
>  	rt_mutex_init(&adap->mux_lock);
>  	mutex_init(&adap->userspace_clients_lock);
> @@ -2249,10 +2260,12 @@ struct i2c_client *i2c_new_alias_device(struct i2c_adapter *adap)
>  
>  	i2c_lock_bus(adap, I2C_LOCK_SEGMENT);
>  
> -	for (addr = 0x08; addr < 0x78; addr++) {
> +	for (addr = adap->alias_idx; addr < 0x78; addr++) {
>  		ret = i2c_scan_for_client(adap, addr, i2c_unlocked_read_byte_probe);
>  		if (ret == -ENODEV) {
>  			alias = i2c_new_dummy_device(adap, addr);
> +			alias->flags |= I2C_CLIENT_ALIAS;
> +			adap->alias_idx = addr + 1;
>  			dev_dbg(&adap->dev, "Found alias: 0x%x\n", addr);
>  			break;
>  		}
> diff --git a/include/linux/i2c.h b/include/linux/i2c.h
> index 583ca2aec022..6427c2db5ee0 100644
> --- a/include/linux/i2c.h
> +++ b/include/linux/i2c.h
> @@ -309,6 +309,7 @@ struct i2c_driver {
>  struct i2c_client {
>  	unsigned short flags;		/* div., see below		*/
>  #define I2C_CLIENT_PEC		0x04	/* Use Packet Error Checking */
> +#define I2C_CLIENT_ALIAS	0x08	/* client is an alias */
>  #define I2C_CLIENT_TEN		0x10	/* we have a ten bit chip address */
>  					/* Must equal I2C_M_TEN below */
>  #define I2C_CLIENT_SLAVE	0x20	/* we are the slave */
> @@ -715,6 +716,7 @@ struct i2c_adapter {
>  	const struct i2c_adapter_quirks *quirks;
>  
>  	struct irq_domain *host_notify_domain;
> +	u16 alias_idx;
>  };
>  #define to_i2c_adapter(d) container_of(d, struct i2c_adapter, dev)
>  
> 

If you have a situation where a driver for a device with reprogrammable
I2C address is, say, built as a module and is reloaded repeatedly for some
reason, and also have the situation that the device keeps responding to
the last used address even after unloading the driver. Then I think you
have a problem, because as I read the code the alias address will increase
with every reload, and eventually hit the 0x78 roof.

No?

Cheers,
Peter