[PATCH v2] coresight: etm4x: Do not access TRCIDR1 for identification

[Suzuki K Poulose]

CoreSight ETM4x architecture clearly provides ways to identify a device
via registers in the "Management" class, TRCDEVARCH and TRCDEVTYPE. These
registers can be accessed without the Trace domain being powered on.
We additionally added TRCIDR1 as fallback in order to cover for any
ETMs that may not have implemented TRCDEVARCH. So far, nobody has
reported hitting a WARNING we placed to catch such systems.

Also, more importantly it is problematic to access TRCIDR1, which is a
"Trace" register via MMIO access, without clearing the OSLK. But we cannot
mess with the OSLK until we know for sure that this is an ETMv4 device.
Thus, this kind of creates a chicken and egg problem unnecessarily for
systems "which are compliant" to the ETMv4 architecture.

Let us remove the TRCIDR1 fall back check and rely only on TRCDEVARCH.

Reported-by: Steve Clevenger <scclevenger at os.amperecomputing.com>
Link: https://lore.kernel.org/all/143540e5623d4c7393d24833f2b80600d8d745d2.1677881753.git.scclevenger@os.amperecomputing.com/
Cc: Mike Leach <mike.leach at linaro.org>
Cc: James Clark <james.clark at arm.com>
Fixes: 8b94db1edaee ("coresight: etm4x: Use TRCDEVARCH for component discovery")
Signed-off-by: Suzuki K Poulose <suzuki.poulose at arm.com>
Reviewed-by: Mike Leach <mike.leach at linaro.org>
---

Changes since v2:
 - Remove unused etm_tridr_to_arch() helper
 - Add comment to explain why TRCIDR1 cannot be used.

---
 .../coresight/coresight-etm4x-core.c          | 22 ++++++++-----------
 drivers/hwtracing/coresight/coresight-etm4x.h | 20 +++++------------
 2 files changed, 15 insertions(+), 27 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
index 104333c2c8a3..4c15fae534f3 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
+++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
@@ -1070,25 +1070,21 @@ static bool etm4_init_iomem_access(struct etmv4_drvdata *drvdata,
 				   struct csdev_access *csa)
 {
 	u32 devarch = readl_relaxed(drvdata->base + TRCDEVARCH);
-	u32 idr1 = readl_relaxed(drvdata->base + TRCIDR1);
 
 	/*
 	 * All ETMs must implement TRCDEVARCH to indicate that
-	 * the component is an ETMv4. To support any broken
-	 * implementations we fall back to TRCIDR1 check, which
-	 * is not really reliable.
+	 * the component is an ETMv4. Even though TRCIDR1 also
+	 * contains the information, it is part of the "Trace"
+	 * register and must be accessed with the OSLK cleared,
+	 * with MMIO. But we cannot touch the OSLK until we are
+	 * sure this is an ETM. So rely only on the TRCDEVARCH.
 	 */
-	if ((devarch & ETM_DEVARCH_ID_MASK) == ETM_DEVARCH_ETMv4x_ARCH) {
-		drvdata->arch = etm_devarch_to_arch(devarch);
-	} else {
-		pr_warn("CPU%d: ETM4x incompatible TRCDEVARCH: %x, falling back to TRCIDR1\n",
-			smp_processor_id(), devarch);
-
-		if (ETM_TRCIDR1_ARCH_MAJOR(idr1) != ETM_TRCIDR1_ARCH_ETMv4)
-			return false;
-		drvdata->arch = etm_trcidr_to_arch(idr1);
+	if ((devarch & ETM_DEVARCH_ID_MASK) != ETM_DEVARCH_ETMv4x_ARCH) {
+		pr_warn_once("TRCDEVARCH doesn't match ETMv4 architecture\n");
+		return false;
 	}
 
+	drvdata->arch = etm_devarch_to_arch(devarch);
 	*csa = CSDEV_ACCESS_IOMEM(drvdata->base);
 	return true;
 }
diff --git a/drivers/hwtracing/coresight/coresight-etm4x.h b/drivers/hwtracing/coresight/coresight-etm4x.h
index 434f4e95ee17..27c8a9901868 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x.h
+++ b/drivers/hwtracing/coresight/coresight-etm4x.h
@@ -753,14 +753,12 @@
  * TRCDEVARCH	- CoreSight architected register
  *                - Bits[15:12] - Major version
  *                - Bits[19:16] - Minor version
- * TRCIDR1	- ETM architected register
- *                - Bits[11:8] - Major version
- *                - Bits[7:4]  - Minor version
- * We must rely on TRCDEVARCH for the version information,
- * however we don't want to break the support for potential
- * old implementations which might not implement it. Thus
- * we fall back to TRCIDR1 if TRCDEVARCH is not implemented
- * for memory mapped components.
+ *
+ * We must rely only on TRCDEVARCH for the version information. Even though,
+ * TRCIDR1 also provides the architecture version, it is a "Trace" register
+ * and as such must be accessed only with Trace power domain ON. This may
+ * not be available at probe time.
+ *
  * Now to make certain decisions easier based on the version
  * we use an internal representation of the version in the
  * driver, as follows :
@@ -786,12 +784,6 @@ static inline u8 etm_devarch_to_arch(u32 devarch)
 				ETM_DEVARCH_REVISION(devarch));
 }
 
-static inline u8 etm_trcidr_to_arch(u32 trcidr1)
-{
-	return ETM_ARCH_VERSION(ETM_TRCIDR1_ARCH_MAJOR(trcidr1),
-				ETM_TRCIDR1_ARCH_MINOR(trcidr1));
-}
-
 enum etm_impdef_type {
 	ETM4_IMPDEF_HISI_CORE_COMMIT,
 	ETM4_IMPDEF_FEATURE_MAX,
-- 
2.34.1