I2C Power collapse issue in android build

Problem: The modem side processor keeps spinning for the spinlock acquired by the apps side processor for an I2C transaction when the apps side processor is power collapsed.



Solution:

As a solution to this, wakelocks can be used. "Wakelock" is a mechanism which can prevent the system from going into a low-power state.



To prevent the above mentioned problem, the I2C driver at the apps side grabs a wakelock at the beginning of the I2C transaction (inside the msm_i2c_xfer() and then releases it at the end of the transaction. With this, the power collapse is delayed for the time the wakelock is held by the i2c driver i.e. till the spinlock held by the apps side processor is released, and the released spinlock can, then, be used by the modem side processor.



To setup a wakelock in the kernel space:

#include <linux/wakelock.h>

wake_lock_init(struct wake_lock *lock, int type, const char *name)

where,

name: name of the lock

type: kind of wakelock



The two types are : WAKE_LOCK_SUSPEND : prevents the system from suspending

WAKE_LOCK_IDLE: prevents going into a low_power idle state



Following are the APIs to handle this lock:



void wake_lock(struct wake_lock *lock);

void wake_lock_timeout(struct wake_lock *lock, long timeout);

void wake_unlock(struct wake_lock *lock);



Note: pm_qos() is not intended to influence the suspend power collapse. It influences the idle power collapse. Thus, pm_qos() should not be used in the above mentioned scenario.