Allow user to choose where to map the common libc heap

[tenllado]

When CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE is defined with a positive value, the common libc implementation defines an array to hold the heap for the malloc/calloc/realloc functions of the libc.

This array is placed by default in the noinit section, through the definition of the POOL_SECTION macro in line 85 of the file zephyr/lib/libc/common/source/stdlib/malloc.c:

static POOL_SECTION unsigned char __aligned(HEAP_ALIGN) malloc_arena[HEAP_SIZE];

I would like to enable the user to choose an alternative location for the malloc_arena array, giving the possiblity to choose, for instance, a section mapped on an external RAM in the target board.

A possible implementation is given in this patch:

diff --git a/lib/libc/common/Kconfig b/lib/libc/common/Kconfig
index dc22d0c21..b69422038 100644
--- a/lib/libc/common/Kconfig
+++ b/lib/libc/common/Kconfig
@@ -45,6 +45,14 @@ config COMMON_LIBC_MALLOC_ARENA_SIZE
 	  16kB and all other systems will default to using all remaining
 	  ram for the malloc heap.
 
+config COMMON_LIBC_MALLOC_ARENA_SECTION
+	string "Section name for the malloc arena"
+	default "noinit"
+	depends on COMMON_LIBC_MALLOC
+	help
+	  Select the section name (without initial ".") where to place
+	  the malloc arena. Must be defined in the linker script.
+
 config COMMON_LIBC_CALLOC
 	bool "Common C library calloc"
 	depends on COMMON_LIBC_MALLOC
diff --git a/lib/libc/common/source/stdlib/malloc.c b/lib/libc/common/source/stdlib/malloc.c
index e3a5db6f7..ef569844f 100644
--- a/lib/libc/common/source/stdlib/malloc.c
+++ b/lib/libc/common/source/stdlib/malloc.c
@@ -34,7 +34,9 @@ LOG_MODULE_DECLARE(os, CONFIG_KERNEL_LOG_LEVEL);
 K_APPMEM_PARTITION_DEFINE(z_malloc_partition);
 #  define POOL_SECTION Z_GENERIC_SECTION(K_APP_DMEM_SECTION(z_malloc_partition))
 # else
-#  define POOL_SECTION __noinit
+#  define POOL_SECTION __attribute__((section("." CONFIG_COMMON_LIBC_MALLOC_ARENA_SECTION \
+					      "." STRINGIFY(__FILE__) \
+					      "." STRINGIFY(__LINE__))))
 # endif /* CONFIG_USERSPACE */
 
 # if defined(CONFIG_MMU) && CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE < 0

An alternative would be to change the definition of the HEAP_BASE and the HEAP_SIZE to build the default heap on a different region. However this requires more careful computations that involve the size of the region and its begining address. It seems more reasonable to me to make it target specific in that case. For instance, esp32 platforms, the following patch maps the heap on the external RAM if the user chooses to do it:

diff --git a/lib/libc/common/source/stdlib/malloc.c b/lib/libc/common/source/stdlib/malloc.c
index e3a5db6f7..50c868f8b 100644
--- a/lib/libc/common/source/stdlib/malloc.c
+++ b/lib/libc/common/source/stdlib/malloc.c
@@ -92,7 +92,13 @@ static POOL_SECTION unsigned char __aligned(HEAP_ALIGN) malloc_arena[HEAP_SIZE];
  * the end of SRAM
  */
 
+
+#   if defined(CONFIG_XTENSA) && defined(CONFIG_ESP_SPIRAM_COMMON_HEAP)
+extern char _ext_ram_data_end[];
+#   define USED_RAM_END_ADDR   POINTER_TO_UINT(&_ext_ram_data_end)
+#   else
 #   define USED_RAM_END_ADDR   POINTER_TO_UINT(&_end)
+#   endif
 
 /*
  * No partition, heap can just start wherever _end is, with
@@ -101,7 +107,11 @@ static POOL_SECTION unsigned char __aligned(HEAP_ALIGN) malloc_arena[HEAP_SIZE];
 
 #   define HEAP_BASE	ROUND_UP(USED_RAM_END_ADDR, HEAP_ALIGN)
 
-#   if defined(CONFIG_XTENSA) && (defined(CONFIG_SOC_FAMILY_INTEL_ADSP) \
+#   if defined(CONFIG_XTENSA) && defined(CONFIG_ESP_SPIRAM_COMMON_HEAP)
+extern char _ext_ram_data_start[];
+#    define HEAP_SIZE	ROUND_DOWN(((size_t) CONFIG_ESP_SPIRAM_SIZE - \
+		((size_t) HEAP_BASE - (size_t)_ext_ram_data_start)), HEAP_ALIGN)
+#   elif defined(CONFIG_XTENSA) && (defined(CONFIG_SOC_FAMILY_INTEL_ADSP) \
 	|| defined(CONFIG_HAS_ESPRESSIF_HAL))
 extern char _heap_sentry[];
 #    define HEAP_SIZE  ROUND_DOWN((POINTER_TO_UINT(_heap_sentry) - HEAP_BASE), HEAP_ALIGN)
diff --git a/soc/xtensa/espressif_esp32/common/Kconfig.soc b/soc/xtensa/espressif_esp32/common/Kconfig.soc
index c8eaad62d..1a3fc3dba 100644
--- a/soc/xtensa/espressif_esp32/common/Kconfig.soc
+++ b/soc/xtensa/espressif_esp32/common/Kconfig.soc
@@ -45,6 +45,12 @@ config ESP_SPIRAM_HEAP_SIZE
 	help
 	  Specify size of SPIRAM heap.
 
+config ESP_SPIRAM_COMMON_HEAP
+	bool "Map the common libc heap on SPIRAM"
+	depends on ESP_SPIRAM
+	help
+	  Map the common libc heap on the SPIRAM
+
 menu "SPI RAM config"
 	depends on ESP_SPIRAM

Both options were tested on a esp_wrover_kit to map the heap on the external RAM, and worked well. Notice that these are only two possibilities, the former is platform independent but the latter is plaform specific. Any alternatives/suggestions are wellcomed. I just want to request your comments on this topic, and if the community finds something simmilar to the above solutions reasonable, I would work on a pull request for the agreed-upon solution. Please leave your comments.

Thank you all.

[3Nigma]

I love the first approach. This feature would also benefit other external, PSRAM, supporting SOCs such as the SiliconLabs siw91x offerings.