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commit cc9febb4533e350aeac95329bda5239b956da73c
Author: Kerogit <kr....@kerogit.eu>
AuthorDate: Mon Mar 24 22:43:33 2025 +0100
nuttx/compiler: replace AVR __flash qualifier with __memx for some chips
The __flash qualifier only allows access to program memory below 64kB
mark (uses 16 bit pointer.) Since many chips currently supported
have more than that, this qualifier can prove insufficient.
When flagged __memx the compiler will use 24 bit pointer to access
the variable.
---
include/nuttx/compiler.h | 32 +++++++++++++++++++++++++++++++-
1 file changed, 31 insertions(+), 1 deletion(-)
diff --git a/include/nuttx/compiler.h b/include/nuttx/compiler.h
index 8ff05a0781..38f4c815c1 100644
--- a/include/nuttx/compiler.h
+++ b/include/nuttx/compiler.h
@@ -437,7 +437,37 @@
# if defined(CONFIG_AVR_HAS_MEMX_PTR)
/* I-space access qualifiers needed by Harvard architecture */
-# define IOBJ __flash
+# if defined(CONFIG_AVR_HAS_RAMPZ)
+# define IOBJ __memx
+/* Compiler will not copy const variables declared with this qualifier
+ * to RAM, they will be kept in .progmemx.data section and read from there.
+ * This is used for chips with more than 64kB program memory, the compiler
+ * will use 24 bit pointers when accessing such variables. 16 bit pointer
+ * register "Z" is extended using additional "RAMPZ" register. All of this
+ * happens automatically.
+ *
+ * When working with these variables - passing pointers to them as function
+ * parameters for example, the same qualifier needs to be used (see IPTR
+ * below.) IPTR (__memx) linearizes flash and RAM and allows using generic
+ * pointer to both. Underlying memory for such access is then determined
+ * in runtime, with some impact on performance.
+ *
+ * To alleviate this, avr-gcc introduces __flashx qualifier which still
+ * provides 24 bit access to the variable but only allows program memory
+ * access. The check for pointer's underlying memory can then be skipped.
+ *
+ * This is currently not in use, it was introduced in avr-gcc 15 which
+ * is still fairly fresh. Can be considered at later time.
+ */
+# else
+# define IOBJ __flash
+/* Compiler will not copy variables declared with this qualifier to RAM.
+ * Instead, it will read them using LPM (Load from Program Memory.)
+ * Only first 64kB of program memory can be accessed this way so for
+ * chips with more memory than that, __memx is preferred
+ */
+# endif
+
# define IPTR __memx
# else
