coresight: etm4x: Add sysreg access helpers

ETM architecture defines the system instructions for accessing
via register accesses. Add basic support for accessing a given
register via system instructions.

We split the list of registers as :
 1) Accessible only from memory mapped interface
 2) Accessible from system register instructions.

All registers are accessible via the memory-mapped interface.
However, some registers are not accessible via the system
instructions. This list is then used to further filter out
the files we expose via sysfs.

Link: https://lore.kernel.org/r/20210110224850.1880240-12-suzuki.poulose@arm.com
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20210201181351.1475223-14-mathieu.poirier@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/coresight/coresight-etm4x-core.c

@@ -75,6 +75,38 @@ static inline bool etm4x_sspcicrn_present(struct etmv4_drvdata *drvdata, int n)
 	       (drvdata->config.ss_status[n] & TRCSSCSRn_PC);
 }
 
+u64 etm4x_sysreg_read(u32 offset, bool _relaxed, bool _64bit)
+{
+	u64 res = 0;
+
+	switch (offset) {
+	ETM4x_READ_SYSREG_CASES(res)
+	default :
+		pr_warn_ratelimited("etm4x: trying to read unsupported register @%x\n",
+			 offset);
+	}
+
+	if (!_relaxed)
+		__iormb(res);	/* Imitate the !relaxed I/O helpers */
+
+	return res;
+}
+
+void etm4x_sysreg_write(u64 val, u32 offset, bool _relaxed, bool _64bit)
+{
+	if (!_relaxed)
+		__iowmb();	/* Imitate the !relaxed I/O helpers */
+	if (!_64bit)
+		val &= GENMASK(31, 0);
+
+	switch (offset) {
+	ETM4x_WRITE_SYSREG_CASES(val)
+	default :
+		pr_warn_ratelimited("etm4x: trying to write to unsupported register @%x\n",
+			offset);
+	}
+}
+
 static void etm4_os_unlock_csa(struct etmv4_drvdata *drvdata, struct csdev_access *csa)
 {
 	/* Writing 0 to TRCOSLAR unlocks the trace registers */