qcs615: Configure RT CPU isolation on best-capacity core (CPU7) for iq-615-evk and qcs615-ride (#1657)

Configure RT-oriented CPU isolation defaults for QCS615 machine configs
by selecting the best-capacity (highest-frequency) core as the dedicated
RT CPU and steering system noise away from it.

CPU7 is chosen as the RT CPU because it is the best-capacity /
highest-frequency core, which helps meet RT latency/throughput needs.
IRQ handling is kept on CPUs 0-6 to reduce interrupt noise on the
isolated RT CPU.
RCU settings are applied to further reduce latency effects on the RT
CPU.
QCOM_CPUIDLE_OFF=1 avoids latency from deep idle state transitions for
RT use-cases.

Author: ricardosalveti

conf/machine/iq-615-evk.conf

@@ -25,3 +25,10 @@ MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS += " \
 QCOM_BOOT_FILES_SUBDIR = "qcs615"
 QCOM_PARTITION_FILES_SUBDIR ?= "partitions/iq-615-evk/emmc"
 QCOM_BOOT_FIRMWARE = "firmware-qcom-boot-qcs615"
+
+# Isolate rt cpu
+QCOM_RT_CPU        = "7"
+QCOM_IRQAFF        = "0-6"
+QCOM_RCU_NOCBS     = "7"
+QCOM_RCU_EXPEDITED = "1"
+QCOM_CPUIDLE_OFF   = "1"

conf/machine/qcs615-ride.conf

@@ -23,3 +23,10 @@ QCOM_BOOT_FILES_SUBDIR = "qcs615"
 QCOM_PARTITION_FILES_SUBDIR ?= "partitions/qcs615-ride/ufs"
 
 QCOM_BOOT_FIRMWARE = "firmware-qcom-boot-qcs615"
+
+# Isolate rt cpu
+QCOM_RT_CPU        = "7"
+QCOM_IRQAFF        = "0-6"
+QCOM_RCU_NOCBS     = "7"
+QCOM_RCU_EXPEDITED = "1"
+QCOM_CPUIDLE_OFF   = "1"