x86: baytrail/cherrytrail: Rework and move P-Unit PMIC bus semaphore code

On some BYT/CHT systems the SoC's P-Unit shares the I2C bus with the
kernel. The P-Unit has a semaphore for the PMIC bus which we can take to
block it from accessing the shared bus while the kernel wants to access it.

Currently we have the I2C-controller driver acquiring and releasing the
semaphore around each I2C transfer. There are 2 problems with this:

1) PMIC accesses often come in the form of a read-modify-write on one of
the PMIC registers, we currently release the P-Unit's PMIC bus semaphore
between the read and the write. If the P-Unit modifies the register during
this window?, then we end up overwriting the P-Unit's changes.
I believe that this is mostly an academic problem, but I'm not sure.

2) To safely access the shared I2C bus, we need to do 3 things:
a) Notify the GPU driver that we are starting a window in which it may not
access the P-Unit, since the P-Unit seems to ignore the semaphore for
explicit power-level requests made by the GPU driver
b) Make a pm_qos request to force all CPU cores out of C6/C7 since entering
C6/C7 while we hold the semaphore hangs the SoC
c) Finally take the P-Unit's PMIC bus semaphore
All 3 these steps together are somewhat expensive, so ideally if we have
a bunch of i2c transfers grouped together we only do this once for the
entire group.

Taking the read-modify-write on a PMIC register as example then ideally we
would only do all 3 steps once at the beginning and undo all 3 steps once
at the end.

For this we need to be able to take the semaphore from within e.g. the PMIC
opregion driver, yet we do not want to remove the taking of the semaphore
from the I2C-controller driver, as that is still necessary to protect many
other code-paths leading to accessing the shared I2C bus.

This means that we first have the PMIC driver acquire the semaphore and
then have the I2C controller driver trying to acquire it again.

To make this possible this commit does the following:

1) Move the semaphore code from being private to the I2C controller driver
into the generic iosf_mbi code, which already has other code to deal with
the shared bus so that it can be accessed outside of the I2C bus driver.

2) Rework the code so that it can be called multiple times nested, while
still blocking I2C accesses while e.g. the GPU driver has indicated the
P-Unit needs the bus through a iosf_mbi_punit_acquire() call.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Tested-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Acked-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 29 insertions, 10 deletions

diff --git a/arch/x86/include/asm/iosf_mbi.h b/arch/x86/include/asm/iosf_mbi.h
index 3de0489deadef..5270ff39b9afe 100644
--- a/arch/x86/include/asm/iosf_mbi.h
+++ b/arch/x86/include/asm/iosf_mbi.h
@@ -105,8 +105,10 @@ int iosf_mbi_modify(u8 port, u8 opcode, u32 offset, u32 mdr, u32 mask);
  * the PMIC bus while another driver is also accessing the PMIC bus various bad
  * things happen.
  *
- * To avoid these problems this function must be called before accessing the
- * P-Unit or the PMIC, be it through iosf_mbi* functions or through other means.
+ * Call this function before sending requests to the P-Unit which may make it
+ * access the PMIC, be it through iosf_mbi* functions or through other means.
+ * This function will block all kernel access to the PMIC I2C bus, so that the
+ * P-Unit can safely access the PMIC over the shared I2C bus.
  *
  * Note on these systems the i2c-bus driver will request a sempahore from the
  * P-Unit for exclusive access to the PMIC bus when i2c drivers are accessing
@@ -123,6 +125,31 @@ void iosf_mbi_punit_acquire(void);
 void iosf_mbi_punit_release(void);
 
 /**
+ * iosf_mbi_block_punit_i2c_access() - Block P-Unit accesses to the PMIC bus
+ *
+ * Call this function to block P-Unit access to the PMIC I2C bus, so that the
+ * kernel can safely access the PMIC over the shared I2C bus.
+ *
+ * This function acquires the P-Unit bus semaphore and notifies
+ * pmic_bus_access_notifier listeners that they may no longer access the
+ * P-Unit in a way which may cause it to access the shared I2C bus.
+ *
+ * Note this function may be called multiple times and the bus will not
+ * be released until iosf_mbi_unblock_punit_i2c_access() has been called the
+ * same amount of times.
+ *
+ * Return: Nonzero on error
+ */
+int iosf_mbi_block_punit_i2c_access(void);
+
+/*
+ * iosf_mbi_unblock_punit_i2c_access() - Release PMIC I2C bus block
+ *
+ * Release i2c access block gotten through iosf_mbi_block_punit_i2c_access().
+ */
+void iosf_mbi_unblock_punit_i2c_access(void);
+
+/**
  * iosf_mbi_register_pmic_bus_access_notifier - Register PMIC bus notifier
  *
  * This function can be used by drivers which may need to acquire P-Unit
@@ -159,14 +186,6 @@ int iosf_mbi_unregister_pmic_bus_access_notifier_unlocked(
 	struct notifier_block *nb);
 
 /**
- * iosf_mbi_call_pmic_bus_access_notifier_chain - Call PMIC bus notifier chain
- *
- * @val: action to pass into listener's notifier_call function
- * @v: data pointer to pass into listener's notifier_call function
- */
-int iosf_mbi_call_pmic_bus_access_notifier_chain(unsigned long val, void *v);
-
-/**
  * iosf_mbi_assert_punit_acquired - Assert that the P-Unit has been acquired.
  */
 void iosf_mbi_assert_punit_acquired(void);