linux-kernel - [PATCH] coresight: etm4x: Do not access TRCIDR1 for identification

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Date:   Fri, 17 Mar 2023 11:57:28 +0000
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     coresight@...ts.linaro.org
Cc:     mike.leach@...aro.org, james.clark@....com,
        anshuman.khandual@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Steve Clevenger <scclevenger@...amperecomputing.com>
Subject: [PATCH] coresight: etm4x: Do not access TRCIDR1 for identification

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@...amperecomputing.com>
Link: https://lore.kernel.org/all/143540e5623d4c7393d24833f2b80600d8d745d2.1677881753.git.scclevenger@os.amperecomputing.com/
Cc: Mike Leach <mike.leach@...aro.org>
Cc: James Clark <james.clark@....com>
Fixes: 8b94db1edaee ("coresight: etm4x: Use TRCDEVARCH for component discovery")
Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
---
 .../hwtracing/coresight/coresight-etm4x-core.c | 18 +++++-------------
 1 file changed, 5 insertions(+), 13 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
index 104333c2c8a3..c1b72d892d7d 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
+++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
@@ -1070,25 +1070,17 @@ 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
 	 */
-	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;
 }
-- 
2.34.1