[PATCH 1/7] i2c: mux: add the ability to share mux-address with child nodes

[Farouk Bouabid]

Hi Peter,

On 29.04.24 17:46, Peter Rosin wrote:
> Hi!
>
> 2024-04-26 at 18:49, Farouk Bouabid wrote:
>> Allow the mux to have the same address as a child device. This is useful
>> when the mux can only use an i2c-address that is used by a child device
>> because no other addresses are free to use. eg. the mux can only use
>> address 0x18 which is used by amc6821 connected to the mux.
>>
>> Signed-off-by: Farouk Bouabid <farouk.bouabid at theobroma-systems.com>
>> ---
>>   drivers/i2c/i2c-mux.c   | 10 +++++++++-
>>   include/linux/i2c-mux.h |  1 +
>>   2 files changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/i2c/i2c-mux.c b/drivers/i2c/i2c-mux.c
>> index 57ff09f18c37..f5357dff8cc5 100644
>> --- a/drivers/i2c/i2c-mux.c
>> +++ b/drivers/i2c/i2c-mux.c
>> @@ -331,7 +331,6 @@ int i2c_mux_add_adapter(struct i2c_mux_core *muxc,
>>   	priv->adap.owner = THIS_MODULE;
>>   	priv->adap.algo = &priv->algo;
>>   	priv->adap.algo_data = priv;
>> -	priv->adap.dev.parent = &parent->dev;
>>   	priv->adap.retries = parent->retries;
>>   	priv->adap.timeout = parent->timeout;
>>   	priv->adap.quirks = parent->quirks;
>> @@ -348,6 +347,15 @@ int i2c_mux_add_adapter(struct i2c_mux_core *muxc,
>>   	else
>>   		priv->adap.class = class;
>>   
>> +	/*
>> +	 * When creating the adapter, the node devices are checked for i2c address
>> +	 * match with other devices on the parent adapter, among which is the mux itself.
>> +	 * If a match is found the node device is not probed successfully.
>> +	 * Allow the mux to have the same address as a child device by skipping this check.
>> +	 */
>> +	if (!(muxc->share_addr_with_children))
>> +		priv->adap.dev.parent = &parent->dev;
> This is a dirty hack that will not generally do the right thing.
>
> The adapter device parent is not there solely for the purpose of
> detecting address clashes, so the above has other implications
> that are not desirable.
>
> Therefore, NACK on this approach. It simply needs to be more involved.
> Sorry.
>
> Cheers,
> Peter
>

Another way to approach this is by implementing this flag as a quirk for 
the added adapter:

(tested but not cleaned up)

"""

diff --git a/drivers/i2c/i2c-core-base.c b/drivers/i2c/i2c-core-base.c
index ff5c486a1dbb..6a0237f750db 100644
--- a/drivers/i2c/i2c-core-base.c
+++ b/drivers/i2c/i2c-core-base.c
@@ -821,9 +821,21 @@ static int i2c_check_mux_children(struct device 
*dev, void *addrp)
  static int i2c_check_addr_busy(struct i2c_adapter *adapter, int addr)
  {
         struct i2c_adapter *parent = i2c_parent_is_i2c_adapter(adapter);
+       bool skip_check = false;
         int result = 0;

-       if (parent)
+       if (adapter->quirks) {
+                if (adapter->quirks->flags & I2C_AQ_SHARE_ADDR) {
+                       struct i2c_client *client = 
of_find_i2c_device_by_node(adapter->dev.of_node->parent);
+
+                       if (client) {
+                               skip_check = client->addr == addr;
+                               put_device(&client->dev);
+                       }
+                }
+       }
+
+       if (parent && !skip_check)
                 result = i2c_check_mux_parents(parent, addr);

         if (!result)
diff --git a/drivers/i2c/i2c-mux.c b/drivers/i2c/i2c-mux.c
index 57ff09f18c37..e87cb0e43725 100644
--- a/drivers/i2c/i2c-mux.c
+++ b/drivers/i2c/i2c-mux.c
@@ -334,7 +334,26 @@ int i2c_mux_add_adapter(struct i2c_mux_core *muxc,
         priv->adap.dev.parent = &parent->dev;
         priv->adap.retries = parent->retries;
         priv->adap.timeout = parent->timeout;
-       priv->adap.quirks = parent->quirks;
+       /*
+        * When creating the adapter, the node devices are checked for 
i2c address
+        * match with other devices on the parent adapter, among which 
is the mux itself.
+        * If a match is found the node device is not probed successfully.
+        * Allow the mux to have the same address as a child device by 
skipping this check.
+        */
+       if (!muxc->share_addr_with_children)
+               priv->adap.quirks = parent->quirks;
+       else {
+               struct i2c_adapter_quirks *quirks = 
kzalloc(sizeof(*quirks), GFP_KERNEL);
+               if (!quirks)
+                       return -ENOMEM;
+
+               if (parent->quirks)
+                       *quirks = *(parent->quirks); // @fixme memcpy
+
+               quirks->flags |= I2C_AQ_SHARE_ADDR;
+               priv->adap.quirks = quirks;
+       }
+
         if (muxc->mux_locked)
                 priv->adap.lock_ops = &i2c_mux_lock_ops;
         else
diff --git a/include/linux/i2c-mux.h b/include/linux/i2c-mux.h
index 98ef73b7c8fd..17ac68bf1703 100644
--- a/include/linux/i2c-mux.h
+++ b/include/linux/i2c-mux.h
@@ -21,6 +21,7 @@ struct i2c_mux_core {
         unsigned int mux_locked:1;
         unsigned int arbitrator:1;
         unsigned int gate:1;
+       unsigned int share_addr_with_children:1;

         void *priv;

diff --git a/include/linux/i2c.h b/include/linux/i2c.h
index 5e6cd43a6dbd..2ebac9e672ef 100644
--- a/include/linux/i2c.h
+++ b/include/linux/i2c.h
@@ -711,6 +711,8 @@ struct i2c_adapter_quirks {
  #define I2C_AQ_NO_ZERO_LEN             (I2C_AQ_NO_ZERO_LEN_READ | 
I2C_AQ_NO_ZERO_LEN_WRITE)
  /* adapter cannot do repeated START */
  #define I2C_AQ_NO_REP_START            BIT(7)
+/* @fixme document and find proper name */
+#define I2C_AQ_SHARE_ADDR              BIT(8)

  /*
   * i2c_adapter is the structure used to identify a physical i2c bus along

"""

This works, however this only supports device-tree because of 
of_find_i2c_device_by_node. If we want to support acpi then we can either:


1. Get the Mule i2c device address from fwnode_get_next_parent_dev but 
this is static since v6.9-rcx.

2. Pass the Mule i2c device address as a new member of  struct 
i2c_adapter_quirks.


I would go for 2. Do you suggest something else?

Best regards

Farouk