coresight: etm4x: Add commentary on the registers

As we are about define a switch..case table for individual register
access by offset for implementing the system instruction support,
document the possible set of registers for each group to make
it easier to correlate.

Link: https://lore.kernel.org/r/20210110224850.1880240-11-suzuki.poulose@arm.com
Cc: Mike Leach <mike.leach@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/20210201181351.1475223-13-mathieu.poirier@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/coresight/coresight-etm4x.h

@@ -45,13 +45,13 @@
 #define TRCVDSACCTLR			0x0A4
 #define TRCVDARCCTLR			0x0A8
 /* Derived resources registers */
-#define TRCSEQEVRn(n)			(0x100 + (n * 4))
+#define TRCSEQEVRn(n)			(0x100 + (n * 4)) /* n = 0-2 */
 #define TRCSEQRSTEVR			0x118
 #define TRCSEQSTR			0x11C
 #define TRCEXTINSELR			0x120
-#define TRCCNTRLDVRn(n)			(0x140 + (n * 4))
-#define TRCCNTCTLRn(n)			(0x150 + (n * 4))
-#define TRCCNTVRn(n)			(0x160 + (n * 4))
+#define TRCCNTRLDVRn(n)			(0x140 + (n * 4)) /* n = 0-3 */
+#define TRCCNTCTLRn(n)			(0x150 + (n * 4)) /* n = 0-3 */
+#define TRCCNTVRn(n)			(0x160 + (n * 4)) /* n = 0-3 */
 /* ID registers */
 #define TRCIDR8				0x180
 #define TRCIDR9				0x184
@@ -60,7 +60,7 @@
 #define TRCIDR12			0x190
 #define TRCIDR13			0x194
 #define TRCIMSPEC0			0x1C0
-#define TRCIMSPECn(n)			(0x1C0 + (n * 4))
+#define TRCIMSPECn(n)			(0x1C0 + (n * 4)) /* n = 1-7 */
 #define TRCIDR0				0x1E0
 #define TRCIDR1				0x1E4
 #define TRCIDR2				0x1E8
@@ -69,9 +69,12 @@
 #define TRCIDR5				0x1F4
 #define TRCIDR6				0x1F8
 #define TRCIDR7				0x1FC
-/* Resource selection registers */
+/*
+ * Resource selection registers, n = 2-31.
+ * First pair (regs 0, 1) is always present and is reserved.
+ */
 #define TRCRSCTLRn(n)			(0x200 + (n * 4))
-/* Single-shot comparator registers */
+/* Single-shot comparator registers, n = 0-7 */
 #define TRCSSCCRn(n)			(0x280 + (n * 4))
 #define TRCSSCSRn(n)			(0x2A0 + (n * 4))
 #define TRCSSPCICRn(n)			(0x2C0 + (n * 4))
@@ -81,11 +84,13 @@
 #define TRCPDCR				0x310
 #define TRCPDSR				0x314
 /* Trace registers (0x318-0xEFC) */
-/* Comparator registers */
+/* Address Comparator registers n = 0-15 */
 #define TRCACVRn(n)			(0x400 + (n * 8))
 #define TRCACATRn(n)			(0x480 + (n * 8))
+/* Data Value Comparator Value registers, n = 0-7 */
 #define TRCDVCVRn(n)			(0x500 + (n * 16))
 #define TRCDVCMRn(n)			(0x580 + (n * 16))
+/* ContextID/Virtual ContextID comparators, n = 0-7 */
 #define TRCCIDCVRn(n)			(0x600 + (n * 8))
 #define TRCVMIDCVRn(n)			(0x640 + (n * 8))
 #define TRCCIDCCTLR0			0x680