Take 2: [RFC] Debugging with a HW probe.

[Jimi Xenidis]

A rework of the original patch, some on and off list comments had
suggested that this be a generic service, and to make it very hard
to unintentionally turn it on.  I fact we should make sure that the
feature is indeed turned off.

Any suggestions on how to operate on the HID bits of all CPUs based on
the value of the config _and_ 'hw_probe_enabled' would be welcom.

I'd also like to wait for Olof's:
  http://ozlabs.org/pipermail/linuxppc-dev/2006-August/025024.html
patch to make it to the tree (or not).

Signed-off-by: Jimi Xenidis <jimix at watson.ibm.com>

---
diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
index e29ef77..bc4cdf9 100644
--- a/arch/powerpc/Kconfig.debug
+++ b/arch/powerpc/Kconfig.debug
@@ -61,6 +61,17 @@ config KGDB_CONSOLE
 	  over the gdb stub.
 	  If unsure, say N.
 
+config ENABLE_HW_PROBE
+       bool "Allow instructions that contact a hardware probe (dangerous)"
+       depends on PPC64
+       help
+         If you enable this AND you add "hwprobe" to the cmdline, the
+         processor will enable instructions that contact the hardware
+         probe.  These instructions ca be used in all processor modes
+         _including_ user mode and are only useful for kernel
+         development and debugging.  DO NOT enable this unless you
+         plan to use it.
+
 config XMON
 	bool "Include xmon kernel debugger"
 	depends on DEBUGGER && !PPC_ISERIES
diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
index bf2005b..02c5b83 100644
--- a/arch/powerpc/kernel/prom.c
+++ b/arch/powerpc/kernel/prom.c
@@ -68,6 +68,9 @@ int __initdata iommu_is_off;
 int __initdata iommu_force_on;
 unsigned long tce_alloc_start, tce_alloc_end;
 #endif
+#ifdef CONFIG_ENABLE_HW_PROBE
+int hw_probe_enabled;
+#endif
 
 typedef u32 cell_t;
 
@@ -693,6 +696,11 @@ #ifdef CONFIG_PPC64
 	if (of_get_flat_dt_prop(node, "linux,iommu-force-on", NULL) != NULL)
 		iommu_force_on = 1;
 #endif
+#ifdef CONFIG_HW_PROBE_ENABLE
+	if (of_get_flat_dt_prop(node, "linux,hw-probe-enable", NULL) != NULL) {
+		hw_probe_enabled = 1;
+		DBG("HW Probe will be enabled\n");
+#endif
 
 	/* mem=x on the command line is the preferred mechanism */
  	lprop = of_get_flat_dt_prop(node, "linux,memory-limit", NULL);
diff --git a/arch/powerpc/kernel/prom_init.c b/arch/powerpc/kernel/prom_init.c
index 90972ef..26428de 100644
--- a/arch/powerpc/kernel/prom_init.c
+++ b/arch/powerpc/kernel/prom_init.c
@@ -587,6 +587,14 @@ #ifdef CONFIG_PPC64
 			RELOC(iommu_force_on) = 1;
 	}
 #endif
+#ifdef CONFIG_HW_PROBE_ENABLE
+	opt = strstr(RELOC(prom_cmd_line), RELOC("hwprobe"));
+	if (opt) {
+		prom_printf("WARNING! HW Probe will be activated!\n");
+		prom_setprop(_prom->chosen, "/chosen",
+			     "linux,hw-probe-enable", NULL, 0);
+	}
+#endif
 }
 
 #ifdef CONFIG_PPC_PSERIES
diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index 179b10c..51a1e4e 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c
@@ -189,7 +189,12 @@ #endif
   dd	dump double values\n\
   dr	dump stream of raw bytes\n\
   e	print exception information\n\
-  f	flush cache\n\
+  f	flush cache\n"
+#ifdef CONFIG_ENABLE_HW_PROBE
+  "\
+  H     Contact hardware probe, if available\n"
+#endif
+  "\
   la	lookup symbol+offset of specified address\n\
   ls	lookup address of specified symbol\n\
   m	examine/change memory\n\
@@ -217,6 +222,18 @@ #endif
   zh	halt\n"
 ;
 
+#ifdef CONFIG_ENABLE_HW_PROBE
+/* try to keep this funtion from being inlined so its easier to move
+ * around the ATTN instruction */
+extern int hw_probe_enabled;
+static noinline void xmon_hw_probe(void)
+{
+	if (!hw_probe_enabled)
+		return;
+	ATTN();
+}
+#endif
+
 static struct pt_regs *xmon_regs;
 
 static inline void sync(void)
@@ -819,6 +836,11 @@ #endif /* CONFIG_SMP */
 			if (cpu_cmd())
 				return 0;
 			break;
+#ifdef CONFIG_ENABLE_HW_PROBE
+		case 'H':
+			xmon_hw_probe();
+			break;
+#endif
 		case 'z':
 			bootcmds();
 			break;
diff --git a/include/asm-powerpc/reg.h b/include/asm-powerpc/reg.h
index cf73475..478097e 100644
--- a/include/asm-powerpc/reg.h
+++ b/include/asm-powerpc/reg.h
@@ -207,6 +207,13 @@ #define SPRN_EAR	0x11A		/* External Addr
 #define SPRN_HASH1	0x3D2		/* Primary Hash Address Register */
 #define SPRN_HASH2	0x3D3		/* Secondary Hash Address Resgister */
 #define SPRN_HID0	0x3F0		/* Hardware Implementation Register 0 */
+#ifdef __ASSEMBLY__
+#define HID0_QATTN	(1<<35)		/* Sup. Proc. attn insn all threads */
+#define HID0_ATTN	(1<<32)		/* Sup. Proc. attn insn */
+#else
+#define HID0_QATTN	(1UL<<35)	/* Sup. Proc. attn insn all threads */
+#define HID0_ATTN	(1UL<<32)	/* Sup. Proc. attn insn */
+#endif
 #define HID0_EMCP	(1<<31)		/* Enable Machine Check pin */
 #define HID0_EBA	(1<<29)		/* Enable Bus Address Parity */
 #define HID0_EBD	(1<<28)		/* Enable Bus Data Parity */
@@ -641,6 +648,13 @@ extern void ppc64_runlatch_off(void);
 extern unsigned long scom970_read(unsigned int address);
 extern void scom970_write(unsigned int address, unsigned long value);
 
+/*
+ * Support Processor Attention Instruction instroduced in POWER
+ * architecture processors as of RS64, tho may not be supported by
+ * POWER 3.
+ */
+#define ATTN() asm volatile("attn; nop")
+
 #else
 #define ppc64_runlatch_on()
 #define ppc64_runlatch_off()