Add Thread Local Storage (TLS) support using Picolibc functions

Thread Local Storage (TLS) offers a platform-native mechanism for
managing per-task variables that doesn't rely on custom support in
FreeRTOS for things like errno.

Picolibc uses TLS internally for all variables that are intended to be
task-local. Because of the TLS architecture, only variables which are
actually used end up getting allocate space in the TLS block. Contrast
this with the newlib 'struct _reent' block which holds all possible
task-local values, even for APIs not used by the application.

Picolibc also has TLS helper functions that provide the necessary
platform-specific code which makes the FreeRTOS changes platform
independent. This patch uses those hooks instead of providing the
platform specific code in FreeRTOS itself.

The picolibc helper functions rely on elements within the linker
script to arrange the TLS data in memory and define some symbols.
Applications wanting to use this mechanism will need changes in their
linker script when migrating to picolibc.

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard

.github/lexicon.txt

@@ -1061,6 +1061,7 @@ mclk
 mconfigintcoresw
 mcr
 mcu
+md
 mddr
 mder
 mdh
@@ -1342,6 +1343,7 @@ phy
 phya
 pic
 picnt
+picolibc
 pien
 piir
 pimr

include/FreeRTOS.h

@@ -102,6 +102,43 @@
     #endif
 #endif /* if ( configUSE_NEWLIB_REENTRANT == 1 ) */
 
+#if ( configUSE_PICOLIBC_TLS == 1)
+
+    /* Use picolibc TLS support to allocate space for __thread variables,
+     * initialize them at thread creation and set the TLS context at
+     * thread switch time.
+     *
+     * See the picolibc TLS docs:
+     * https://github.com/picolibc/picolibc/blob/main/doc/tls.md
+     * for additional information. */
+    #include <picotls.h>
+
+    #define configUSE_C_RUNTIME_TLS_SUPPORT 1
+
+    #define configTLS_BLOCK_TYPE void *
+
+    /* Allocate thread local storage block off the end of the
+     * stack. The _tls_size() function returns the size (in
+     * bytes) of the total TLS area used by the application */
+    #if ( portSTACK_GROWTH < 0 )
+        #define configINIT_TLS_BLOCK( xTLSBlock ) do { \
+            pxTopOfStack = ( StackType_t *) ( ( ( portPOINTER_SIZE_TYPE) pxTopOfStack) - _tls_size() ); \
+            xTLSBlock = pxTopOfStack; \
+            _init_tls( xTLSBlock ); \
+        } while(0)
+    #else /* portSTACK_GROWTH */
+        #define configINIT_TLS_BLOCK( xTLSBlock ) do {  \
+            xTLSBlock = pxTopOfStack; \
+            pxTopOfStack = ( StackType_t *) ( ( ( portPOINTER_SIZE_TYPE) pxTopOfStack) + _tls_size() ); \
+            _init_tls(xTLSBlock); \
+        } while(0)
+    #endif /* portSTACK_GROWTH */
+
+    #define configSET_TLS_BLOCK( xTLSBlock ) _set_tls( xTLSBlock )
+
+    #define configDEINIT_TLS_BLOCK( xTLSBlock )
+#endif
+
 #ifndef configUSE_C_RUNTIME_TLS_SUPPORT
     #define configUSE_C_RUNTIME_TLS_SUPPORT    0
 #endif