fix(cpus): workaround for Cortex-A710 erratum 2768515

Cortex-A710 erratum 2768515 is a Cat B erratum that applies to
all revisions <=r2p1 and is still open. The workaround is to
insert a dsb before the isb in the power down sequence.

SDEN documentation:
https://developer.arm.com/documentation/SDEN1775101/latest

Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: If17fe04d3fda0dba6b8aabdd837a1c53e1830ed5

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

@@ -518,6 +518,10 @@ For Cortex-A710, the following errata build flags are defined :
    Cortex-A710 CPU. This needs to be enabled for revisions r0p0, r1p0 and r2p0
    of the CPU and is fixed in r2p1.
 
+-  ``ERRATA_A710_2768515``: This applies errata 2768515 workaround to
+   Cortex-A710 CPU. This needs to be enabled for revisions r0p0, r1p0, r2p0 and
+   r2p1 of the CPU and is still open.
+
 For Neoverse N2, the following errata build flags are defined :
 
 -  ``ERRATA_N2_2002655``: This applies errata 2002655 workaround to Neoverse-N2

lib/cpus/aarch64/cortex_a710.S

@@ -482,6 +482,30 @@ func check_errata_2371105
 	b	cpu_rev_var_ls
 endfunc check_errata_2371105
 
+/* ----------------------------------------------------
+ * Errata Workaround for Cortex-A710 Errata #2768515
+ * This applies to revisions <= r2p1 and is still open.
+ * x0: variant[4:7] and revision[0:3] of current cpu.
+ * Shall clobber: x0-x17
+ * ----------------------------------------------------
+ */
+func errata_a710_2768515_wa
+	mov	x17, x30
+	bl	check_errata_2768515
+	cbz	x0, 1f
+
+	/* dsb before isb of power down sequence */
+	dsb	sy
+1:
+	ret	x17
+endfunc errata_a710_2768515_wa
+
+func check_errata_2768515
+	/* Applies to all revisions <= r2p1 */
+	mov	x1, #0x21
+	b	cpu_rev_var_ls
+endfunc check_errata_2768515
+
 func check_errata_cve_2022_23960
 #if WORKAROUND_CVE_2022_23960
 	mov	x0, #ERRATA_APPLIES
@@ -518,6 +542,12 @@ func cortex_a710_core_pwr_dwn
 	mrs	x0, CORTEX_A710_CPUPWRCTLR_EL1
 	orr	x0, x0, #CORTEX_A710_CPUPWRCTLR_EL1_CORE_PWRDN_BIT
 	msr	CORTEX_A710_CPUPWRCTLR_EL1, x0
+#if ERRATA_A710_2768515
+	mov	x15, x30
+	bl	cpu_get_rev_var
+	bl	errata_a710_2768515_wa
+	mov	x30, x15
+#endif /* ERRATA_A710_2768515 */
 	isb
 	ret
 endfunc cortex_a710_core_pwr_dwn
@@ -550,6 +580,7 @@ func cortex_a710_errata_report
 	report_errata ERRATA_A710_2216384, cortex_a710, 2216384
 	report_errata ERRATA_A710_2291219, cortex_a710, 2291219
 	report_errata ERRATA_A710_2371105, cortex_a710, 2371105
+	report_errata ERRATA_A710_2768515, cortex_a710, 2768515
 	report_errata WORKAROUND_CVE_2022_23960, cortex_a710, cve_2022_23960
 	report_errata ERRATA_DSU_2313941, cortex_a710, dsu_2313941
 

lib/cpus/cpu-ops.mk

@@ -572,6 +572,11 @@ ERRATA_A710_2008768	?=0
 # to revision r0p0, r1p0 and r2p0 of the Cortex-A710 cpu and is fixed in r2p1.
 ERRATA_A710_2371105	?=0
 
+# Flag to apply erratum 2768515 workaround during power down. This erratum
+# applies to revision r0p0, r1p0, r2p0 and r2p1 of the Cortex-A710 cpu and is
+# still open.
+ERRATA_A710_2768515	?=0
+
 # Flag to apply erratum 2067956 workaround during reset. This erratum applies
 # to revision r0p0 of the Neoverse N2 cpu and is still open.
 ERRATA_N2_2067956	?=0
@@ -1208,6 +1213,10 @@ $(eval $(call add_define,ERRATA_A710_2008768))
 $(eval $(call assert_boolean,ERRATA_A710_2371105))
 $(eval $(call add_define,ERRATA_A710_2371105))
 
+# Process ERRATA_A710_2768515 flag
+$(eval $(call assert_boolean,ERRATA_A710_2768515))
+$(eval $(call add_define,ERRATA_A710_2768515))
+
 # Process ERRATA_N2_2067956 flag
 $(eval $(call assert_boolean,ERRATA_N2_2067956))
 $(eval $(call add_define,ERRATA_N2_2067956))