powerpc/setup: Refactor/untangle panic notifiers

The panic notifiers infrastructure is a bit limited in the scope of
the callbacks - basically every kind of functionality is dropped
in a list that runs in the same point during the kernel panic path.
This is not really on par with the complexities and particularities
of architecture / hypervisors' needs, and a refactor is ongoing.

As part of this refactor, it was observed that powerpc has 2 notifiers,
with mixed goals: one is just a KASLR offset dumper, whereas the other
aims to hard-disable IRQs (necessary on panic path), warn firmware of
the panic event (fadump) and run low-level platform-specific machinery
that might stop kernel execution and never come back.

Clearly, the 2nd notifier has opposed goals: disable IRQs / fadump
should run earlier while low-level platform actions should
run late since it might not even return. Hence, this patch decouples
the notifiers splitting them in three:

- First one is responsible for hard-disable IRQs and fadump,
should run early;

- The kernel KASLR offset dumper is really an informative notifier,
harmless and may run at any moment in the panic path;

- The last notifier should run last, since it aims to perform
low-level actions for specific platforms, and might never return.
It is also only registered for 2 platforms, pseries and ps3.

The patch better documents the notifiers and clears the code too,
also removing a useless header.

Currently no functionality change should be observed, but after
the planned panic refactor we should expect more panic reliability
with this patch.

Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Reviewed-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220427224924.592546-9-gpiccoli@igalia.com

arch/powerpc/kernel/setup-common.c

@@ -23,7 +23,6 @@
 #include <linux/console.h>
 #include <linux/screen_info.h>
 #include <linux/root_dev.h>
-#include <linux/notifier.h>
 #include <linux/cpu.h>
 #include <linux/unistd.h>
 #include <linux/serial.h>
@@ -681,8 +680,25 @@ int check_legacy_ioport(unsigned long base_port)
 }
 EXPORT_SYMBOL(check_legacy_ioport);
 
-static int ppc_panic_event(struct notifier_block *this,
-                             unsigned long event, void *ptr)
+/*
+ * Panic notifiers setup
+ *
+ * We have 3 notifiers for powerpc, each one from a different "nature":
+ *
+ * - ppc_panic_fadump_handler() is a hypervisor notifier, which hard-disables
+ *   IRQs and deal with the Firmware-Assisted dump, when it is configured;
+ *   should run early in the panic path.
+ *
+ * - dump_kernel_offset() is an informative notifier, just showing the KASLR
+ *   offset if we have RANDOMIZE_BASE set.
+ *
+ * - ppc_panic_platform_handler() is a low-level handler that's registered
+ *   only if the platform wishes to perform final actions in the panic path,
+ *   hence it should run late and might not even return. Currently, only
+ *   pseries and ps3 platforms register callbacks.
+ */
+static int ppc_panic_fadump_handler(struct notifier_block *this,
+				    unsigned long event, void *ptr)
 {
 	/*
 	 * panic does a local_irq_disable, but we really
@@ -692,45 +708,63 @@ static int ppc_panic_event(struct notifier_block *this,
 
 	/*
 	 * If firmware-assisted dump has been registered then trigger
-	 * firmware-assisted dump and let firmware handle everything else.
+	 * its callback and let the firmware handles everything else.
 	 */
 	crash_fadump(NULL, ptr);
-	if (ppc_md.panic)
-		ppc_md.panic(ptr);  /* May not return */
+
 	return NOTIFY_DONE;
 }
 
-static struct notifier_block ppc_panic_block = {
-	.notifier_call = ppc_panic_event,
-	.priority = INT_MIN /* may not return; must be done last */
-};
-
-/*
- * Dump out kernel offset information on panic.
- */
 static int dump_kernel_offset(struct notifier_block *self, unsigned long v,
 			      void *p)
 {
 	pr_emerg("Kernel Offset: 0x%lx from 0x%lx\n",
 		 kaslr_offset(), KERNELBASE);
 
-	return 0;
+	return NOTIFY_DONE;
 }
 
+static int ppc_panic_platform_handler(struct notifier_block *this,
+				      unsigned long event, void *ptr)
+{
+	/*
+	 * This handler is only registered if we have a panic callback
+	 * on ppc_md, hence NULL check is not needed.
+	 * Also, it may not return, so it runs really late on panic path.
+	 */
+	ppc_md.panic(ptr);
+
+	return NOTIFY_DONE;
+}
+
+static struct notifier_block ppc_fadump_block = {
+	.notifier_call = ppc_panic_fadump_handler,
+	.priority = INT_MAX, /* run early, to notify the firmware ASAP */
+};
+
 static struct notifier_block kernel_offset_notifier = {
-	.notifier_call = dump_kernel_offset
+	.notifier_call = dump_kernel_offset,
+};
+
+static struct notifier_block ppc_panic_block = {
+	.notifier_call = ppc_panic_platform_handler,
+	.priority = INT_MIN, /* may not return; must be done last */
 };
 
 void __init setup_panic(void)
 {
+	/* Hard-disables IRQs + deal with FW-assisted dump (fadump) */
+	atomic_notifier_chain_register(&panic_notifier_list,
+				       &ppc_fadump_block);
+
 	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE) && kaslr_offset() > 0)
 		atomic_notifier_chain_register(&panic_notifier_list,
 					       &kernel_offset_notifier);
 
-	/* PPC64 always does a hard irq disable in its panic handler */
-	if (!IS_ENABLED(CONFIG_PPC64) && !ppc_md.panic)
-		return;
-	atomic_notifier_chain_register(&panic_notifier_list, &ppc_panic_block);
+	/* Low-level platform-specific routines that should run on panic */
+	if (ppc_md.panic)
+		atomic_notifier_chain_register(&panic_notifier_list,
+					       &ppc_panic_block);
 }
 
 #ifdef CONFIG_CHECK_CACHE_COHERENCY