Xtensa: Add EXPERIMENTAL hooks to support lazy co-processor state restore in the future.

arch/xtensa/Kconfig

@@ -31,6 +31,30 @@ config ARCH_FAMILY_LX6
 		Cadence® Tensilica® Xtensa® LX6 data plane processing unit (DPU).
 		The LX6 is a configurable and extensible processor core.
 
+config ARCH_CHIP
+	string
+	default "esp32"		if ARCH_CHIP_ESP32
+
+config XTENSA_CP_LAZY
+	bool "Lazy co-processor state restoration"
+	default n
+	depends on EXPERIMENTAL
+	---help---
+		NuttX logic saves and restores the co-processor enabled (CPENABLE)
+		register on each context switch.  This has disadvantages in that (1)
+		co-processor context will be saved and restored even if the co-
+		processor was never used, and (2) tasks must explicitly enable and
+		disable co-processors.
+
+		An alternative, "lazy" co-processor state restore is enabled with 
+		this option.  That logic works like as follows:
+
+		a. CPENABLE is set to zero on each context switch, disabling all co-
+		   processors.
+		b. If/when the task attempts to use the disabled co-processor, an
+		   exception occurs
+		c. The co-processor exception handler re-enables the co-processor.
+
 config XTENSA_USE_OVLY
 	bool
 	default n
@@ -41,6 +65,7 @@ config XTENSA_CP_INITSET
 	hex "Default co-processor enables"
 	default 0x0001
 	range 0 0xffff
+	depends on !XTENSA_CP_LAZY
 	---help---
 		Co-processors may be enabled on a thread by calling xtensa_coproc_enable()
 		and disabled by calling xtensa_coproc_disable().  Some co-processors
@@ -48,10 +73,6 @@ config XTENSA_CP_INITSET
 		is provided by CONFIG_XTENSA_CP_INITSET.  Each bit corresponds to one
 		coprocessor with the same bit layout as for the CPENABLE register.
 
-config ARCH_CHIP
-	string
-	default "esp32"		if ARCH_CHIP_ESP32
-
 source arch/xtensa/src/lx6/Kconfig
 if ARCH_CHIP_ESP32
 source arch/xtensa/src/esp32/Kconfig

arch/xtensa/src/common/xtensa_coproc.S

@@ -306,7 +306,11 @@ _xtensa_coproc_restorestate:
 
 	mov		a15, a2							/* A15 is now the address of the save area */
 
+#ifdef CONFIG_XTENSA_CP_LAZY
+	movi	a2, 0							/* a2 = Will disable all coprocessors */
+#else
 	l16ui	a2, a15, XTENSA_CPENABLE		/* a2 = Which CPs have been enable for this thread? */
+#endif
 	wsr		a2, CPENABLE					/* Set CPENABLE correctly for this thread */
 	l16ui	a2, a15, XTENSA_CPSTORED		/* a2 = Which CPs have been saved for this thread? */
 	movi	a3, 0							/* Clear the ones being restored (all of them) */

arch/xtensa/src/common/xtensa_initialstate.c

@@ -98,10 +98,15 @@ void up_initial_state(struct tcb_s *tcb)
 
 #if XCHAL_CP_NUM > 0
   /* Set up the co-processors that will be enabled initially when the thread
-   * starts (see xtensa_coproc.h)
+   * starts (see xtensa_coproc.h).  If the lazy co-processor state restore
+   * logic is selected, that would be the empty set.
    */
 
+#ifdef CONFIG_XTENSA_CP_LAZY
+  xcp->cpstate.cpenable = 0;  /* No co-processors are enabled */
+#else
   xcp->cpstate.cpenable = (CONFIG_XTENSA_CP_INITSET & XTENSA_CP_ALLSET);
-  xcp->cpstate.cpstored = 0;  /* No coprocessors haved statee saved for this thread */
+#endif
+  xcp->cpstate.cpstored = 0;  /* No co-processors haved state saved */
 #endif
 }

arch/xtensa/src/common/xtensa_user_handler.S

@@ -63,12 +63,6 @@
 #include <arch/xtensa/core.h>
 #include <arch/xtensa/xtensa_specregs.h>
 
-/****************************************************************************
- * Pre-processor Definitions
- ****************************************************************************/
-
-#undef HAVE_LAZY_COPROC
-
 /****************************************************************************
  * Assembly Language Macros
  ****************************************************************************/
@@ -144,13 +138,13 @@ _xtensa_to_alloca_handler:
 _xtensa_to_syscall_handler:
 	call0	_xtensa_syscall_handler			/* Jump to syscall exception handler */
 
-#ifdef HAVE_LAZY_COPROC
+#ifdef CONFIG_XTENSA_CP_LAZY
 #if XCHAL_CP_NUM > 0
 	.align	4
 _xtensa_to_coproc_handler:
 	call0	_xtensa_coproc_handler			/* Jump to copressor exception handler */
 #endif
-#endif /* HAVE_LAZY_COPROC */
+#endif /* CONFIG_XTENSA_CP_LAZY */
 
 /****************************************************************************
  * Name: _xtensa_user_handler
@@ -173,7 +167,7 @@ _xtensa_user_handler:
 	rsr		a0, EXCCAUSE
 	beqi	a0, EXCCAUSE_LEVEL1INTERRUPT, _xtensa_to_level1_handler
 
-#ifdef HAVE_LAZY_COPROC
+#ifdef CONFIG_XTENSA_CP_LAZY
 #if XCHAL_CP_NUM > 0
 	/* Handle any coprocessor exceptions. Rely on the fact that exception
 	 * numbers above EXCCAUSE_CP0_DISABLED all relate to the coprocessors.
@@ -181,7 +175,7 @@ _xtensa_user_handler:
 
 	bgeui	a0, EXCCAUSE_CP0_DISABLED, _xtensa_to_coproc_handler
 #endif
-#endif /* HAVE_LAZY_COPROC */
+#endif /* CONFIG_XTENSA_CP_LAZY */
 
 	/* Handle alloca and syscall exceptions */
 
@@ -399,11 +393,14 @@ _xtensa_syscall_handler:
  *   NuttX does not currently implement this lazy co-process enable.  Rather,
  *   NuttX follows the model:
  *
- *   1. A set of co-processors may be enable when each thread starts as determined by CONFIG_XTENSA_CP_INITSET.
- *   2. Additional co-processors may be enabled for the thread by explicitly setting the CPENABLE register when the thread starts.
- *   3. Co-processor state, including CPENABLE, is saved an restored on each context switch.
+ *   1. A set of co-processors may be enable when each thread starts as
+ *      determined by CONFIG_XTENSA_CP_INITSET.
+ *   2. Additional co-processors may be enabled for the thread by explicitly
+ *      setting the CPENABLE register when the thread starts.
+ *   3. Co-processor state, including CPENABLE, is saved an restored on each
+ *      context switch.
  *   4. Any Coprocessor[n]Disabled exceptions result in a system PANIC.
-
+ *
  *   These exceptions are generated by co-processor instructions, which are
  *   only allowed in thread code (not in interrupts or kernel code).  This
  *   restriction is deliberately imposed to reduce the burden of state-save/
@@ -414,11 +411,14 @@ _xtensa_syscall_handler:
  *
  ****************************************************************************/
 
-/* Disabled for now:  The following logic is redundant.  It simply duplicates the
- * the logic in _xtensa_user_handler
+#ifdef CONFIG_XTENSA_CP_LAZY
+/* Lazy co-processor restoration is not implemented.  Below, the logic simply
+ * calls xtensa_user() which will crash the system with an unhandled error
+ * Duplicates logic above.
  */
 
-#ifdef HAVE_LAZY_COPROC
+#error Lazy co-processor restoration is not implemented
+
 #if XCHAL_CP_NUM > 0
 	.type	_xtensa_coproc_handler, @function
 	.align	4
@@ -477,4 +477,4 @@ _xtensa_coproc_handler:
 1:	j		1b
 
 #endif /* XCHAL_CP_NUM */
-#endif /* HAVE_LAZY_COPROC */
+#endif /* CONFIG_XTENSA_CP_LAZY */