fix(cpus): workaround for Cortex-A715 erratum 2420947

Cortex-A715 erratum 2420947 is a cat B erratum that applies only
to revision r1p0 and is fixed in r1p1. The workaround is to set
bit[33] of CPUACTLR2_EL1. This will prevent store and store-release
to merge inside the write buffer, and it is not expected to have
much performance impacts.

SDEN can be found here:
https://developer.arm.com/documentation/SDEN2148827/latest

Change-Id: I01a71b878cd958e742ff8357f8cdfbfc5625de47
Signed-off-by: Bipin Ravi <biprav01@u203721.austin.arm.com>

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

@@ -881,6 +881,10 @@ For Cortex-A520, the following errata build flags are defined :
 
 For Cortex-A715, the following errata build flags are defined :
 
+-  ``ERRATA_A715_2420947``: This applies errata 2420947 workaround to
+   Cortex-A715 CPU. This needs to be enabled only for revision r1p0.
+   It is fixed in r1p1.
+
 -  ``ERRATA_A715_2429384``: This applies errata 2429384 workaround to
    Cortex-A715 CPU. This needs to be enabled for revision r1p0. There is no
    workaround for revision r0p0. It is fixed in r1p1.

lib/cpus/aarch64/cortex_a715.S

@@ -26,6 +26,12 @@
 	wa_cve_2022_23960_bhb_vector_table CORTEX_A715_BHB_LOOP_COUNT, cortex_a715
 #endif /* WORKAROUND_CVE_2022_23960 */
 
+workaround_reset_start cortex_a715, ERRATUM(2420947), ERRATA_A715_2420947
+        sysreg_bit_set CORTEX_A715_CPUACTLR2_EL1, BIT(33)
+workaround_reset_end cortex_a715, ERRATUM(2420947)
+
+check_erratum_range cortex_a715, ERRATUM(2420947), CPU_REV(1, 0), CPU_REV(1, 0)
+
 workaround_reset_start cortex_a715, ERRATUM(2429384), ERRATA_A715_2429384
         sysreg_bit_set CORTEX_A715_CPUACTLR2_EL1, BIT(27)
 workaround_reset_end cortex_a715, ERRATUM(2429384)

lib/cpus/cpu-ops.mk

@@ -911,6 +911,10 @@ CPU_FLAG_LIST += ERRATA_V2_2779510
 # This erratum applies to revisions r0p0, r0p1. Fixed in r0p2.
 CPU_FLAG_LIST += ERRATA_V2_2801372
 
+# Flag to apply erratum 2420947 workaround during reset. This erratum applies
+# only to revision r1p0. It is fixed in r1p1.
+CPU_FLAG_LIST += ERRATA_A715_2420947
+
 # Flag to apply erratum 2429384 workaround during reset. This erratum applies
 # to revision r1p0. There is no workaround for r0p0. It is fixed in r1p1.
 CPU_FLAG_LIST += ERRATA_A715_2429384