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

Joel Stanley
2022-06-03T08:02:03Z

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?

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
>