errata: workaround for Neoverse N2 erratum 2138956

Neoverse N2 erratum 2138956 is a Cat B erratum that applies to
revision r0p0 and is still open. This erratum can be avoided by
inserting a sequence of 16 DMB ST instructions prior to WFI or WFE.

SDEN can be found here:
https://developer.arm.com/documentation/SDEN1982442/latest

Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: I1aac87b3075992f875451e4767b21857f596d0b2

docs/design/cpu-specific-build-macros.rst

@@ -395,8 +395,10 @@ For Neoverse N2, the following errata build flags are defined :
    CPU. This needs to be enabled for revision r0p0 of the CPU and is still open.
 
 -  ``ERRATA_N2_2189731``: This applies errata 2189731 workaround to Neoverse-N2
-   CPU. This needs to be enabled only for revision r0p0 of the CPU and is still
-   open.
+   CPU. This needs to be enabled for revision r0p0 of the CPU and is still open.
+
+-  ``ERRATA_N2_2138956``: This applies errata 2138956 workaround to Neoverse-N2
+   CPU. This needs to be enabled for revision r0p0 of the CPU and is still open.
 
 DSU Errata Workarounds
 ----------------------

lib/cpus/aarch64/neoverse_n2.S

@@ -141,6 +141,48 @@ func check_errata_2189731
 	b       cpu_rev_var_ls
 endfunc check_errata_2189731
 
+/* --------------------------------------------------
+ * Errata Workaround for Neoverse N2 Erratum 2138956.
+ * This applies to revision r0p0 of Neoverse N2. it is still open.
+ * Inputs:
+ * x0: variant[4:7] and revision[0:3] of current cpu.
+ * Shall clobber: x0-x17
+ * --------------------------------------------------
+ */
+func errata_n2_2138956_wa
+	/* Check revision. */
+	mov	x17, x30
+	bl	check_errata_2138956
+	cbz	x0, 1f
+
+	/* Apply instruction patching sequence */
+	ldr	x0,=0x3
+	msr	S3_6_c15_c8_0,x0
+	ldr	x0,=0xF3A08002
+	msr	S3_6_c15_c8_2,x0
+	ldr	x0,=0xFFF0F7FE
+	msr	S3_6_c15_c8_3,x0
+	ldr	x0,=0x10002001003FF
+	msr	S3_6_c15_c8_1,x0
+	ldr	x0,=0x4
+	msr	S3_6_c15_c8_0,x0
+	ldr	x0,=0xBF200000
+	msr	S3_6_c15_c8_2,x0
+	ldr	x0,=0xFFEF0000
+	msr	S3_6_c15_c8_3,x0
+	ldr	x0,=0x10002001003F3
+	msr	S3_6_c15_c8_1,x0
+	isb
+1:
+	ret	x17
+endfunc errata_n2_2138956_wa
+
+func check_errata_2138956
+	/* Applies to r0p0 */
+	mov	x1, #0x00
+	b	cpu_rev_var_ls
+endfunc check_errata_2138956
+
 	/* -------------------------------------------
 	 * The CPU Ops reset function for Neoverse N2.
 	 * -------------------------------------------
@@ -176,6 +218,12 @@ func neoverse_n2_reset_func
 	bl      errata_n2_2189731_wa
 #endif
 
+
+#if ERRATA_N2_2138956
+	mov	x0, x18
+	bl	errata_n2_2138956_wa
+#endif
+
 #if ENABLE_AMU
 	/* Make sure accesses from EL0/EL1 and EL2 are not trapped to EL3 */
 	mrs	x0, cptr_el3
@@ -240,6 +288,7 @@ func neoverse_n2_errata_report
 	report_errata ERRATA_N2_2067956, neoverse_n2, 2067956
 	report_errata ERRATA_N2_2025414, neoverse_n2, 2025414
         report_errata ERRATA_N2_2189731, neoverse_n2, 2189731
+	report_errata ERRATA_N2_2138956, neoverse_n2, 2138956
 
 	ldp	x8, x30, [sp], #16
 	ret

lib/cpus/cpu-ops.mk

@@ -433,6 +433,10 @@ ERRATA_N2_2025414	?=0
 # to revision r0p0 of the Neoverse N2 cpu and is still open.
 ERRATA_N2_2189731	?=0
 
+# Flag to apply erratum 2138956 workaround during reset. This erratum applies
+# to revision r0p0 of the Neoverse N2 cpu and is still open.
+ERRATA_N2_2138956	?=0
+
 # Flag to apply erratum 2055002 workaround during reset. This erratum applies
 # to revision r1p0, r2p0 of the Cortex-A710 cpu and is still open.
 ERRATA_A710_2055002	?=0
@@ -806,6 +810,10 @@ $(eval $(call add_define,ERRATA_N2_2025414))
 $(eval $(call assert_boolean,ERRATA_N2_2189731))
 $(eval $(call add_define,ERRATA_N2_2189731))
 
+# Process ERRATA_N2_2138956 flag
+$(eval $(call assert_boolean,ERRATA_N2_2138956))
+$(eval $(call add_define,ERRATA_N2_2138956))
+
 # Process ERRATA_A710_2055002 flag
 $(eval $(call assert_boolean,ERRATA_A710_2055002))
 $(eval $(call add_define,ERRATA_A710_2055002))