powerpc/xive: Add a kernel parameter for StoreEOI

StoreEOI is activated by default on platforms supporting the feature
(POWER10) and will be used as soon as firmware advertises its
availability. The kernel parameter provides a way to deactivate its
use. It can be still be reactivated through debugfs.
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211105102636.1016378-10-clg@kaod.org

Documentation/admin-guide/kernel-parameters.txt

@@ -6446,6 +6446,12 @@
 				  controller on both pseries and powernv
 				  platforms. Only useful on POWER9 and above.
 
+	xive.store-eoi=off	[PPC]
+			By default on POWER10 and above, the kernel will use
+			stores for EOI handling when the XIVE interrupt mode
+			is active. This option allows the XIVE driver to use
+			loads instead, as on POWER9.
+
 	xhci-hcd.quirks		[USB,KNL]
 			A hex value specifying bitmask with supplemental xhci
 			host controller quirks. Meaning of each bit can be

arch/powerpc/sysdev/xive/common.c

@@ -1712,6 +1712,19 @@ static int __init xive_off(char *arg)
 }
 __setup("xive=off", xive_off);
 
+static int __init xive_store_eoi_cmdline(char *arg)
+{
+	if (!arg)
+		return -EINVAL;
+
+	if (strncmp(arg, "off", 3) == 0) {
+		pr_info("StoreEOI disabled on kernel command line\n");
+		xive_store_eoi = false;
+	}
+	return 0;
+}
+__setup("xive.store-eoi=", xive_store_eoi_cmdline);
+
 #ifdef CONFIG_DEBUG_FS
 static void xive_debug_show_ipi(struct seq_file *m, int cpu)
 {