On 10/14/2018 04:17 PM, Wolfram Sang wrote:
On Thu, Oct 11, 2018 at 04:29:09PM +0200, Hans de Goede wrote:
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@xxxxxxxxxx>
For the record: once the designware maintainers are okay with this
change, I am also okay with it going via the x86 platform tree.
Acked-by: Jarkko Nikula <jarkko.nikula@xxxxxxxxxxxxxxx>
Tested-by: Jarkko Nikula <jarkko.nikula@xxxxxxxxxxxxxxx>
