Re: [PATCH 1/2] i2c: core: Add mux root adapter operations

[Eddie James <eajames@linux.ibm.com>]

On 6/3/22 03:01, Joel Stanley wrote:
> On Thu, 26 May 2022 at 20:53, Eddie James <eajames@linux.ibm.com> wrote:
>> Some I2C clients need the ability to control the root I2C bus even if the
>> endpoint device is behind a mux. For example, a driver for a chip that
>> can't handle any I2C traffic on the bus while coming out of reset
>> (including an I2C-driven mux switching channels) may need to lock the root
>> bus with the mux selection fixed for the entire time the device is in
>> reset.
>> For this purpose, add a new structure containing two function pointers to
>> the adapter structure. These functions pointers should be defined for
>> every adapter. The lock_select operation, for a mux adapter, locks the
>> parent adpaters up to the root and selects the adapter's channel. The
>> unlock_deselect operation deselects the mux channel and unlocks all the
>> adapters. For a non-mux adapter, the operations lock and unlock the
>> adapters up to the root. This scheme should work with multiple levels of
>> muxes and regular adapters in between.
>>
>> Signed-off-by: Eddie James <eajames@linux.ibm.com>
> I think this looks okay. It was hard to understand at first, but makes
> more sense with the context of the existing bus locking code.
>
> There's a typo in one of your comments that the 0day bot found.
>
> Have you tested with CONFIG_DEBUG_MUTEXES?


Getting back around to this... Yes, I just tested with 
CONFIG_DEBUG_MUTEXES and didn't see any issues.

I will address the comment typo in v2 now.


Thanks,

Eddie


>
> Reviewed-by: Joel Stanley <joel@jms.id.au>
>
>> ---
>>   drivers/i2c/i2c-core-base.c | 38 ++++++++++++++++++++++++++++
>>   drivers/i2c/i2c-mux.c       | 50 +++++++++++++++++++++++++++++++++++++
>>   include/linux/i2c.h         | 42 +++++++++++++++++++++++++++++++
>>   3 files changed, 130 insertions(+)
>>
>> diff --git a/drivers/i2c/i2c-core-base.c b/drivers/i2c/i2c-core-base.c
>> index d43db2c3876e..e2c365348e1f 100644
>> --- a/drivers/i2c/i2c-core-base.c
>> +++ b/drivers/i2c/i2c-core-base.c
>> @@ -1357,6 +1357,41 @@ static const struct i2c_lock_operations i2c_adapter_lock_ops = {
>>          .unlock_bus =  i2c_adapter_unlock_bus,
>>   };
>>
>> +/*
>> + * For a non-mux adapter, the lock_select operation locks the chain of
>> + * adapters upwards, returning the root. If there's a mux above this adapter
>> + * somehow, it should also get locked and the desired channel selected.
>> + */
> Recursive lock the set of adaptors.
>
>> +static struct i2c_adapter *i2c_adapter_lock_select(struct i2c_adapter *adapter)
>> +{
>> +       struct i2c_adapter *ret = adapter;
>> +       struct i2c_adapter *parent = i2c_parent_is_i2c_adapter(adapter);
>> +
>> +       if (parent) {
>> +               ret = parent->mux_root_ops->lock_select(parent);
>> +               if (IS_ERR(ret))
>> +                       return ret;
>> +       }
>> +
>> +       adapter->lock_ops->lock_bus(adapter, I2C_LOCK_ROOT_ADAPTER);
>> +       return ret;
>> +}
>> +
>> +static void i2c_adapter_unlock_deselect(struct i2c_adapter *adapter)
>> +{
>> +       struct i2c_adapter *parent = i2c_parent_is_i2c_adapter(adapter);
>> +
>> +       adapter->lock_ops->unlock_bus(adapter, I2C_LOCK_ROOT_ADAPTER);
>> +
>> +       if (parent)
>> +               parent->mux_root_ops->unlock_deselect(parent);
>> +}
>> +
>> +static const struct i2c_mux_root_operations i2c_adapter_mux_root_ops = {
>> +       .lock_select = i2c_adapter_lock_select,
>> +       .unlock_deselect = i2c_adapter_unlock_deselect,
>> +};
>> +
>>   static void i2c_host_notify_irq_teardown(struct i2c_adapter *adap)
>>   {
>>          struct irq_domain *domain = adap->host_notify_domain;
>> @@ -1452,6 +1487,9 @@ static int i2c_register_adapter(struct i2c_adapter *adap)
>>          if (!adap->lock_ops)
>>                  adap->lock_ops = &i2c_adapter_lock_ops;
>>
>> +       if (!adap->mux_root_ops)
>> +               adap->mux_root_ops = &i2c_adapter_mux_root_ops;
>> +
>>          adap->locked_flags = 0;
>>          rt_mutex_init(&adap->bus_lock);
>>          rt_mutex_init(&adap->mux_lock);
>> diff --git a/drivers/i2c/i2c-mux.c b/drivers/i2c/i2c-mux.c
>> index 774507b54b57..c7db770e4198 100644
>> --- a/drivers/i2c/i2c-mux.c
>> +++ b/drivers/i2c/i2c-mux.c
>> @@ -210,6 +210,49 @@ static void i2c_parent_unlock_bus(struct i2c_adapter *adapter,
>>          rt_mutex_unlock(&parent->mux_lock);
>>   }
>>
>> +/*
>> + * For a mux adapter, the lock_select operation first locks just like the
>> + * lock_bus operation. Then it selects the channel for this adapter and
>> + * returns the root adapter. If there is another mux above this one, calling
>> + * the parent lock_select should ensure that the channel is correctly
>> + * selected.
>> + */
>> +static struct i2c_adapter *i2c_mux_lock_select(struct i2c_adapter *adapter)
>> +{
>> +       int ret;
>> +       struct i2c_mux_priv *priv = adapter->algo_data;
>> +       struct i2c_mux_core *muxc = priv->muxc;
>> +       struct i2c_adapter *parent = muxc->parent;
>> +
>> +       rt_mutex_lock_nested(&parent->mux_lock, i2c_adapter_depth(adapter));
>> +
>> +       adapter = parent->mux_root_ops->lock_select(parent);
>> +       if (IS_ERR(adapter))
>> +               return adapter;
>> +
>> +       ret = muxc->select(muxc, priv->chan_id);
>> +       if (ret < 0) {
>> +               parent->mux_root_ops->unlock_deselect(parent);
>> +               rt_mutex_unlock(&parent->mux_lock);
>> +               return ERR_PTR(ret);
>> +       }
>> +
>> +       return adapter;
>> +}
>> +
>> +static void i2c_mux_unlock_deselect(struct i2c_adapter *adapter)
>> +{
>> +       struct i2c_mux_priv *priv = adapter->algo_data;
>> +       struct i2c_mux_core *muxc = priv->muxc;
>> +       struct i2c_adapter *parent = muxc->parent;
>> +
>> +       if (muxc->deselect)
>> +               muxc->deselect(muxc, priv->chan_id);
>> +
>> +       parent->mux_root_ops->unlock_deselect(parent);
>> +       rt_mutex_unlock(&parent->mux_lock);
>> +}
>> +
>>   struct i2c_adapter *i2c_root_adapter(struct device *dev)
>>   {
>>          struct device *i2c;
>> @@ -279,6 +322,11 @@ static const struct i2c_lock_operations i2c_parent_lock_ops = {
>>          .unlock_bus =  i2c_parent_unlock_bus,
>>   };
>>
>> +static const struct i2c_mux_root_operations i2c_mux_root_ops = {
>> +       .lock_select = i2c_mux_lock_select,
>> +       .unlock_deselect = i2c_mux_unlock_deselect,
>> +};
>> +
>>   int i2c_mux_add_adapter(struct i2c_mux_core *muxc,
>>                          u32 force_nr, u32 chan_id,
>>                          unsigned int class)
>> @@ -339,6 +387,8 @@ int i2c_mux_add_adapter(struct i2c_mux_core *muxc,
>>          else
>>                  priv->adap.lock_ops = &i2c_parent_lock_ops;
>>
>> +       priv->adap.mux_root_ops = &i2c_mux_root_ops;
>> +
>>          /* Sanity check on class */
>>          if (i2c_mux_parent_classes(parent) & class)
>>                  dev_err(&parent->dev,
>> diff --git a/include/linux/i2c.h b/include/linux/i2c.h
>> index fbda5ada2afc..a3596f61b417 100644
>> --- a/include/linux/i2c.h
>> +++ b/include/linux/i2c.h
>> @@ -583,6 +583,26 @@ struct i2c_lock_operations {
>>          void (*unlock_bus)(struct i2c_adapter *adapter, unsigned int flags);
>>   };
>>
>> +/**
>> + * struct i2c_mux_root_operations - represent operations to lock and select
>> + * the adapter's mux channel (if a mux is present)
>> + * @lock_select: Get exclusive access to the root I2C bus adapter with the
>> + *   correct mux channel selected for the adapter
>> + * @unlock_deslect: Release exclusive access to the root I2C bus adapter and
>> + *   deselect the mux channel for the adapter
>> + *
>> + * Some I2C clients need the ability to control the root I2C bus even if the
>> + * endpoint device is behind a mux. For example, a driver for a chip that
>> + * can't handle any I2C traffic on the bus while coming out of reset (including
>> + * an I2C-driven mux switching channels) may need to lock the root bus with
>> + * the mux selection fixed for the entire time the device is in reset.
>> + * These operations are for such a purpose.
>> + */
>> +struct i2c_mux_root_operations {
>> +       struct i2c_adapter *(*lock_select)(struct i2c_adapter *adapter);
>> +       void (*unlock_deselect)(struct i2c_adapter *adapter);
>> +};
>> +
>>   /**
>>    * struct i2c_timings - I2C timing information
>>    * @bus_freq_hz: the bus frequency in Hz
>> @@ -725,6 +745,7 @@ struct i2c_adapter {
>>
>>          /* data fields that are valid for all devices   */
>>          const struct i2c_lock_operations *lock_ops;
>> +       const struct i2c_mux_root_operations *mux_root_ops;
>>          struct rt_mutex bus_lock;
>>          struct rt_mutex mux_lock;
>>
>> @@ -817,6 +838,27 @@ i2c_unlock_bus(struct i2c_adapter *adapter, unsigned int flags)
>>          adapter->lock_ops->unlock_bus(adapter, flags);
>>   }
>>
>> +/**
>> + * i2c_lock_select_bus - Get exclusive access to the root I2C bus with the
>> + *   target's mux channel (if a mux is present) selected.
>> + * @adapter: Target I2C bus
>> + *
>> + * Return the root I2C bus if mux selection succeeds, an ERR_PTR otherwise
>> + */
>> +static inline struct i2c_adapter *i2c_lock_select_bus(struct i2c_adapter *adapter)
>> +{
>> +       return adapter->mux_root_ops->lock_select(adapter);
>> +}
>> +
>> +/**
>> + * i2c_unlock_deslect_bus - Release exclusive access to the root I2C bus
>> + * @adapter: Target I2C bus
>> + */
>> +static inline void i2c_unlock_deselect_bus(struct i2c_adapter *adapter)
>> +{
>> +       adapter->mux_root_ops->unlock_deselect(adapter);
>> +}
>> +
>>   /**
>>    * i2c_mark_adapter_suspended - Report suspended state of the adapter to the core
>>    * @adap: Adapter to mark as suspended
>> --
>> 2.27.0
>>