EDAC, skx, i10nm: Make skx_common.c a pure library

The following Kconfig constellations fail randconfig builds:

  CONFIG_ACPI_NFIT=y
  CONFIG_EDAC_DEBUG=y
  CONFIG_EDAC_SKX=m
  CONFIG_EDAC_I10NM=y

or

  CONFIG_ACPI_NFIT=y
  CONFIG_EDAC_DEBUG=y
  CONFIG_EDAC_SKX=y
  CONFIG_EDAC_I10NM=m

with:
  ...
  CC [M]  drivers/edac/skx_common.o
  ...
  .../skx_common.o:.../skx_common.c:672: undefined reference to `__this_module'

That is because if one of the two drivers - skx_edac or i10nm_edac - is
built-in and the other one is a module, the shared file skx_common.c
gets linked into a module object by kbuild. Therefore, when linking that
same file into vmlinux, the '__this_module' symbol used in debugfs isn't
defined, leading to the above error.

Fix it by moving all debugfs code from skx_common.c to both skx_base.c
and i10nm_base.c respectively. Thus, skx_common.c doesn't refer to the
'__this_module' symbol anymore.

Clarify skx_common.c's purpose at the top of the file for future
reference, while at it.

 [ bp: Make text more readable. ]

Fixes: d4dc89d069 ("EDAC, i10nm: Add a driver for Intel 10nm server processors")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: James Morse <james.morse@arm.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: linux-edac <linux-edac@vger.kernel.org>
Link: https://lkml.kernel.org/r/20190321221339.GA32323@agluck-desk

drivers/edac/skx_base.c

@@ -540,6 +540,54 @@ static struct notifier_block skx_mce_dec = {
 	.priority	= MCE_PRIO_EDAC,
 };
 
+#ifdef CONFIG_EDAC_DEBUG
+/*
+ * Debug feature.
+ * Exercise the address decode logic by writing an address to
+ * /sys/kernel/debug/edac/skx_test/addr.
+ */
+static struct dentry *skx_test;
+
+static int debugfs_u64_set(void *data, u64 val)
+{
+	struct mce m;
+
+	pr_warn_once("Fake error to 0x%llx injected via debugfs\n", val);
+
+	memset(&m, 0, sizeof(m));
+	/* ADDRV + MemRd + Unknown channel */
+	m.status = MCI_STATUS_ADDRV + 0x90;
+	/* One corrected error */
+	m.status |= BIT_ULL(MCI_STATUS_CEC_SHIFT);
+	m.addr = val;
+	skx_mce_check_error(NULL, 0, &m);
+
+	return 0;
+}
+DEFINE_SIMPLE_ATTRIBUTE(fops_u64_wo, NULL, debugfs_u64_set, "%llu\n");
+
+static void setup_skx_debug(void)
+{
+	skx_test = edac_debugfs_create_dir("skx_test");
+	if (!skx_test)
+		return;
+
+	if (!edac_debugfs_create_file("addr", 0200, skx_test,
+				      NULL, &fops_u64_wo)) {
+		debugfs_remove(skx_test);
+		skx_test = NULL;
+	}
+}
+
+static void teardown_skx_debug(void)
+{
+	debugfs_remove_recursive(skx_test);
+}
+#else
+static inline void setup_skx_debug(void) {}
+static inline void teardown_skx_debug(void) {}
+#endif /*CONFIG_EDAC_DEBUG*/
+
 /*
  * skx_init:
  *	make sure we are running on the correct cpu model
@@ -619,7 +667,7 @@ static int __init skx_init(void)
 	/* Ensure that the OPSTATE is set correctly for POLL or NMI */
 	opstate_init();
 
-	setup_skx_debug("skx_test");
+	setup_skx_debug();
 
 	mce_register_decode_chain(&skx_mce_dec);