sysview: per-family RTT buffer defaults, 65536 on RAM-rich parts

The dual-role dogfood measured the 4096 fallback losing 96.7% of ISR exit
contexts on mimxrt1064_evk and 99.1% on metro_m4_express; a local-board run
measured stm32u575nucleo losing ~19% of the whole stream under a 12 s
cdc_msc workload. The default was too small for any real capture on fast
parts and every user had to rediscover that per board.

Families now declare SYSVIEW_BUFFER_SIZE_DEFAULT in family.cmake (the
SYSVIEW_RAM_BASE_DEFAULT pattern): imxrt, nrf, rp2040, samd5x_e5x, stm32f4,
stm32f7, stm32h7 and stm32u5 set 65536, the measured-safe value.
-DSYSVIEW_BUFFER_SIZE still beats everything; small-RAM parts keep the 4096
fallback and their per-board overrides (stm32f072disco's 2048 verified
still honoured). lpc55 measured 34 lost events on the same route, an order
of magnitude lighter, so it keeps the fallback rather than inheriting a
default on thin evidence.

9 files changed, 42 insertions, 1 deletions

diff --git a/hw/bsp/family_support.cmake b/hw/bsp/family_support.cmake
index 2aac00eb5..adb4cde13 100644
--- a/hw/bsp/family_support.cmake
+++ b/hw/bsp/family_support.cmake
@@ -505,7 +505,16 @@ function(family_configure_common TARGET RTOS)
         "(it's a mandatory dependency, fetched regardless of family)")
     endif ()
     if (NOT DEFINED SYSVIEW_BUFFER_SIZE)
-      set(SYSVIEW_BUFFER_SIZE 4096)   # overflows under bulk USB load -- raise per board
+      # -DSYSVIEW_BUFFER_SIZE always wins. Otherwise a family may declare its own default
+      # (SYSVIEW_BUFFER_SIZE_DEFAULT in family.cmake, the SYSVIEW_RAM_BASE_DEFAULT pattern):
+      # RAM-rich families set 65536, the dual-role dogfood's measured-safe value -- at the
+      # global 4096 fallback mimxrt1064_evk lost 96.7% of ISR exit contexts at only ~109 Hz.
+      # Small-RAM parts keep the 4096 fallback and override per board (see boards.md).
+      if (DEFINED SYSVIEW_BUFFER_SIZE_DEFAULT)
+        set(SYSVIEW_BUFFER_SIZE ${SYSVIEW_BUFFER_SIZE_DEFAULT})
+      else ()
+        set(SYSVIEW_BUFFER_SIZE 4096)
+      endif ()
     endif ()
     if (NOT DEFINED SYSVIEW_RAM_BASE)
       # A family whose SRAM does not start at the Cortex-M-canonical 0x20000000 (SEGGER's
diff --git a/hw/bsp/imxrt/family.cmake b/hw/bsp/imxrt/family.cmake
index 009b15da3..132e3bd5a 100644
--- a/hw/bsp/imxrt/family.cmake
+++ b/hw/bsp/imxrt/family.cmake
@@ -1,3 +1,7 @@
+# RAM-rich family: SystemView RTT buffer defaults to the dogfood's measured-safe size
+# (-DSYSVIEW_BUFFER_SIZE overrides; see hw/bsp/family_support.cmake and boards.md)
+set(SYSVIEW_BUFFER_SIZE_DEFAULT 65536)
+
 include_guard()
 
 set(MCUX_CORE ${TOP}/hw/mcu/nxp/mcuxsdk-core)
diff --git a/hw/bsp/nrf/family.cmake b/hw/bsp/nrf/family.cmake
index fba5e6e69..579716378 100644
--- a/hw/bsp/nrf/family.cmake
+++ b/hw/bsp/nrf/family.cmake
@@ -1,3 +1,7 @@
+# RAM-rich family: SystemView RTT buffer defaults to the dogfood's measured-safe size
+# (-DSYSVIEW_BUFFER_SIZE overrides; see hw/bsp/family_support.cmake and boards.md)
+set(SYSVIEW_BUFFER_SIZE_DEFAULT 65536)
+
 include_guard()
 
 set(NRFX_PATH ${TOP}/hw/mcu/nordic/nrfx)
diff --git a/hw/bsp/rp2040/family.cmake b/hw/bsp/rp2040/family.cmake
index 57be416a2..281fa64b6 100644
--- a/hw/bsp/rp2040/family.cmake
+++ b/hw/bsp/rp2040/family.cmake
@@ -1,3 +1,7 @@
+# RAM-rich family: SystemView RTT buffer defaults to the dogfood's measured-safe size
+# (-DSYSVIEW_BUFFER_SIZE overrides; see hw/bsp/family_support.cmake and boards.md)
+set(SYSVIEW_BUFFER_SIZE_DEFAULT 65536)
+
 cmake_minimum_required(VERSION 3.13)
 include_guard(GLOBAL)
 
diff --git a/hw/bsp/samd5x_e5x/family.cmake b/hw/bsp/samd5x_e5x/family.cmake
index bb022ab76..4e259ae0a 100644
--- a/hw/bsp/samd5x_e5x/family.cmake
+++ b/hw/bsp/samd5x_e5x/family.cmake
@@ -1,3 +1,7 @@
+# RAM-rich family: SystemView RTT buffer defaults to the dogfood's measured-safe size
+# (-DSYSVIEW_BUFFER_SIZE overrides; see hw/bsp/family_support.cmake and boards.md)
+set(SYSVIEW_BUFFER_SIZE_DEFAULT 65536)
+
 include_guard()
 
 include(${CMAKE_CURRENT_LIST_DIR}/boards/${BOARD}/board.cmake)
diff --git a/hw/bsp/stm32f4/family.cmake b/hw/bsp/stm32f4/family.cmake
index 0d3d9ec93..b2aa60973 100644
--- a/hw/bsp/stm32f4/family.cmake
+++ b/hw/bsp/stm32f4/family.cmake
@@ -1,3 +1,7 @@
+# RAM-rich family: SystemView RTT buffer defaults to the dogfood's measured-safe size
+# (-DSYSVIEW_BUFFER_SIZE overrides; see hw/bsp/family_support.cmake and boards.md)
+set(SYSVIEW_BUFFER_SIZE_DEFAULT 65536)
+
 include_guard()
 
 set(ST_FAMILY f4)
diff --git a/hw/bsp/stm32f7/family.cmake b/hw/bsp/stm32f7/family.cmake
index d405753d4..342a59374 100644
--- a/hw/bsp/stm32f7/family.cmake
+++ b/hw/bsp/stm32f7/family.cmake
@@ -1,3 +1,7 @@
+# RAM-rich family: SystemView RTT buffer defaults to the dogfood's measured-safe size
+# (-DSYSVIEW_BUFFER_SIZE overrides; see hw/bsp/family_support.cmake and boards.md)
+set(SYSVIEW_BUFFER_SIZE_DEFAULT 65536)
+
 include_guard()
 
 set(ST_FAMILY f7)
diff --git a/hw/bsp/stm32h7/family.cmake b/hw/bsp/stm32h7/family.cmake
index 8b6086356..e2a7376e0 100644
--- a/hw/bsp/stm32h7/family.cmake
+++ b/hw/bsp/stm32h7/family.cmake
@@ -1,3 +1,7 @@
+# RAM-rich family: SystemView RTT buffer defaults to the dogfood's measured-safe size
+# (-DSYSVIEW_BUFFER_SIZE overrides; see hw/bsp/family_support.cmake and boards.md)
+set(SYSVIEW_BUFFER_SIZE_DEFAULT 65536)
+
 include_guard()
 
 set(ST_FAMILY h7)
diff --git a/hw/bsp/stm32u5/family.cmake b/hw/bsp/stm32u5/family.cmake
index c580892b8..2a3a583da 100644
--- a/hw/bsp/stm32u5/family.cmake
+++ b/hw/bsp/stm32u5/family.cmake
@@ -1,3 +1,7 @@
+# RAM-rich family: SystemView RTT buffer defaults to the dogfood's measured-safe size
+# (-DSYSVIEW_BUFFER_SIZE overrides; see hw/bsp/family_support.cmake and boards.md)
+set(SYSVIEW_BUFFER_SIZE_DEFAULT 65536)
+
 include_guard()
 
 set(ST_FAMILY u5)