arm: tegra: Set Core to 0.95V in LP1

When the device prepares for LP1, the Core voltage is set to the
highest value(1.2V for Enterprise and Kai, and 1.3V for AP37 and
Cardhu). This is to set for some of the driver suspend along the
sequence need a higher emc frequency and thus a higher Core voltage.

Since the sequence of drivers suspend depends on the sequence of
their registration in the table, which in turn is platform-dependent,
there is no right place in the LP1 entry path where the Core voltage
can be set to a lower voltage. Hence, the Core voltage remains high
in LP1 resulting in higher power.

Thus, the only safe location where the Core voltage can be lowered is
once all the drivers are suspended and the DRAM is set in self-refresh,
at the final point just before the system is suspended in the IRAM code.
This location at the assembly code ensures that no other module will be
running and thus that nothing will require a higher core voltage. The Core
is set to the lowest possible value since nothing requires it. It is then
restored to the highest voltage as soon as the LP1 resume code is started
so that all drivers are resumed safely.

At the execution point in IRAM during LP1 suspend path, even the I2C clocks
are gated. They must be reset first and then the I2C transaction is performed.
An I2C transaction involves 4 bytes of data, to send the slave address,
the Core voltage register address and 2 bytes of data which has the value
to set the voltage(the second byte is not required for this transaction).
Once these registers are set, the I2C transaction is performed by setting
the I2C transaction register to 0xA02. After sending the I2C transaction,
we wait for about 250us to check the status of the transaction and if not
updated, wait for more time to check again. If after 2ms and the transaction
fails to register, the transaction is aborted and the device is allowed
to enter at high voltage. Since the failure rate of I2C transaction is very
low at this point in execution where there will be no conflicts in the bus,
it is okay to have Core high for some of the LP1 cycles.

However, it is unacceptable for the I2C transaction to fail on the way
from LP1 resume since the device cannot come up with a lower Core
voltage. In this case, the transaction is retried again and again till
it is successful. There is no way but to keep trying as the device
would fail to resume with Core at 0.95V.

Each platform(or each PMU) has different values for the I2C transaction
ie. slave address, Core voltage register and the value to set the
voltage. For the device in IRAM, it cannot access anything in SDRAM
memory, these values needs to be pushed to IRAM memory before the device
starts execution in IRAM. This is done during initialization of suspend
code when it picks values from the board files and copies it to IRAM
part of code, before the whole memory is copied to IRAM.

This new feature is controlled by a KConfig variable TEGRA_LP1_950 which
should be enabled once the board file of the device is updated with the
right values. The device hangs when it does not have the right values for
the I2C transaction.

With this change in Core, LP1 power is reduced by 12mW in Enterprise,
20mW in AP37 and about 24mW in Kai.

Bug 1035684

Change-Id: I4318c66fd70ab227ef0786d6a13286e020e4541d
Signed-off-by: Karthik Ramakrishnan <karthikr@nvidia.com>
(cherry picked from commit ab476f287376fd0ae51a9f298659f5eba19f0296)
Reviewed-on: http://git-master/r/124779
Reviewed-by: Lokesh Pathak <lpathak@nvidia.com>
Tested-by: Lokesh Pathak <lpathak@nvidia.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/arch/arm/mach-tegra/Kconfig b/arch/arm/mach-tegra/Kconfig
index b9627372564f..422ad20bb094 100644
--- a/arch/arm/mach-tegra/Kconfig
+++ b/arch/arm/mach-tegra/Kconfig
@@ -577,6 +577,13 @@ config TEGRA_SKIN_THROTTLE
 	help
 	  Enable throttling to control the temperature of the skin/case
 	  of the device.
+
+config TEGRA_LP1_950
+	bool "LP1 low core voltage"
+	default n
+	depends on ARCH_TEGRA_3x_SOC
+	help
+		Enable support for LP1 Core voltage to set to lowest
 endif
 
 config TEGRA_DC_USE_HW_BPP