From 4a7d08def09b2b8b30cb76dee15144a9cfaeb5c7 Mon Sep 17 00:00:00 2001
From: Indan Zupancic <Indan.Zupancic@mep-info.com>
Date: Thu, 19 May 2022 15:22:18 +0200
Subject: [PATCH] MCS, ARM: Introduce TIMER_OVERHEAD_TICKS

For ARM currently TIMER_PRECISION exists, but that is in microseconds
and not fine-grained enough.

This is needed to make periodic tasks synchronous with the system clock.
If this value is zero every period will be extended with the overhead of
taking an interrupt and reading the system clock. To avoid this drift,
the configured value should be set to at least the average overhead.

See also issue #844.

Signed-off-by: Indan Zupancic <Indan.Zupancic@mep-info.com>
---
 .cmake-format.yaml                    | 1 +
 config.cmake                          | 3 +++
 configs/seL4Config.cmake              | 2 +-
 include/arch/arm/arch/machine/timer.h | 2 +-
 src/arch/arm/platform_gen.h.in        | 1 +
 src/plat/tqma8xqp1gb/config.cmake     | 1 +
 6 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/.cmake-format.yaml b/.cmake-format.yaml
index cb4813890..eb4985d57 100644
--- a/.cmake-format.yaml
+++ b/.cmake-format.yaml
@@ -52,5 +52,6 @@ additional_commands:
         CLK_MAGIC: '*'
         CLK_SHIFT: '*'
         TIMER_PRECISION: '*'
+        TIMER_OVERHEAD_TICKS: '*'
         MAX_SID: '*'
         MAX_CB: '*'
diff --git a/config.cmake b/config.cmake
index 589a93f63..4b8d0cec0 100644
--- a/config.cmake
+++ b/config.cmake
@@ -74,6 +74,9 @@ if(DEFINED CALLED_declare_default_headers)
     if(NOT DEFINED CONFIGURE_TIMER_PRECISION)
         set(CONFIGURE_TIMER_PRECISION "0")
     endif()
+    if(NOT DEFINED CONFIGURE_TIMER_OVERHEAD_TICKS)
+        set(CONFIGURE_TIMER_OVERHEAD_TICKS "0")
+    endif()
     configure_file(
         src/arch/${KernelArch}/platform_gen.h.in
         ${CMAKE_CURRENT_BINARY_DIR}/gen_headers/plat/platform_gen.h @ONLY
diff --git a/configs/seL4Config.cmake b/configs/seL4Config.cmake
index 1a4d879fd..cdbc14dcc 100644
--- a/configs/seL4Config.cmake
+++ b/configs/seL4Config.cmake
@@ -118,7 +118,7 @@ macro(declare_default_headers)
     cmake_parse_arguments(
         CONFIGURE
         ""
-        "TIMER_FREQUENCY;MAX_IRQ;NUM_PPI;PLIC_MAX_NUM_INT;INTERRUPT_CONTROLLER;TIMER;SMMU;CLK_SHIFT;CLK_MAGIC;KERNEL_WCET;TIMER_PRECISION;MAX_SID;MAX_CB"
+        "TIMER_FREQUENCY;MAX_IRQ;NUM_PPI;PLIC_MAX_NUM_INT;INTERRUPT_CONTROLLER;TIMER;SMMU;CLK_SHIFT;CLK_MAGIC;KERNEL_WCET;TIMER_PRECISION;TIMER_OVERHEAD_TICKS;MAX_SID;MAX_CB"
         ""
         ${ARGN}
     )
diff --git a/include/arch/arm/arch/machine/timer.h b/include/arch/arm/arch/machine/timer.h
index b184576c5..0df9b80df 100644
--- a/include/arch/arm/arch/machine/timer.h
+++ b/include/arch/arm/arch/machine/timer.h
@@ -50,7 +50,7 @@ static inline CONST ticks_t usToTicks(time_t us)
 
 static inline CONST ticks_t getTimerPrecision(void)
 {
-    return usToTicks(TIMER_PRECISION);
+    return usToTicks(TIMER_PRECISION) + TIMER_OVERHEAD_TICKS;
 }
 #else /* CONFIG_KERNEL_MCS */
 #include <mode/machine/timer.h>
diff --git a/src/arch/arm/platform_gen.h.in b/src/arch/arm/platform_gen.h.in
index 9c44d5475..bfb0c0bf6 100644
--- a/src/arch/arm/platform_gen.h.in
+++ b/src/arch/arm/platform_gen.h.in
@@ -12,6 +12,7 @@
 #define CLK_MAGIC @CONFIGURE_CLK_MAGIC@
 #define CLK_SHIFT @CONFIGURE_CLK_SHIFT@
 #define TIMER_PRECISION @CONFIGURE_TIMER_PRECISION@
+#define TIMER_OVERHEAD_TICKS @CONFIGURE_TIMER_OVERHEAD_TICKS@
 
 enum IRQConstants {
     maxIRQ                      = @CONFIGURE_MAX_IRQ@
diff --git a/src/plat/tqma8xqp1gb/config.cmake b/src/plat/tqma8xqp1gb/config.cmake
index c50230859..6b4b54e93 100644
--- a/src/plat/tqma8xqp1gb/config.cmake
+++ b/src/plat/tqma8xqp1gb/config.cmake
@@ -22,6 +22,7 @@ if(KernelPlatformTqma8xqp1gb)
         TIMER_FREQUENCY 8000000
         MAX_IRQ 512
         TIMER drivers/timer/arm_generic.h
+        TIMER_OVERHEAD_TICKS 1
         INTERRUPT_CONTROLLER arch/machine/gic_v3.h
         NUM_PPI 32
         CLK_MAGIC 1llu