arm64/mte: Add a little bit of documentation for mte_update_sctlr_user()

The code isn't that obscure but it probably won't hurt to have a little
bit more documentation for anyone trying to find out where everything
actually takes effect.
Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Vincenzo Frascino <Vincenzo.Frascino@arm.com>
Tested-by: Branislav Rankov <branislav.rankov@arm.com>
Link: https://lore.kernel.org/r/20220216173224.2342152-3-broonie@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/mte.c

@@ -186,6 +186,11 @@ void mte_check_tfsr_el1(void)
 }
 #endif
 
+/*
+ * This is where we actually resolve the system and process MTE mode
+ * configuration into an actual value in SCTLR_EL1 that affects
+ * userspace.
+ */
 static void mte_update_sctlr_user(struct task_struct *task)
 {
 	/*
@@ -199,8 +204,17 @@ static void mte_update_sctlr_user(struct task_struct *task)
 	unsigned long pref, resolved_mte_tcf;
 
 	pref = __this_cpu_read(mte_tcf_preferred);
+	/*
+	 * If there is no overlap between the system preferred and
+	 * program requested values go with what was requested.
+	 */
 	resolved_mte_tcf = (mte_ctrl & pref) ? pref : mte_ctrl;
 	sctlr &= ~SCTLR_EL1_TCF0_MASK;
+	/*
+	 * Pick an actual setting. The order in which we check for
+	 * set bits and map into register values determines our
+	 * default order.
+	 */
 	if (resolved_mte_tcf & MTE_CTRL_TCF_ASYNC)
 		sctlr |= SCTLR_EL1_TCF0_ASYNC;
 	else if (resolved_mte_tcf & MTE_CTRL_TCF_SYNC)