[posix] check the max number of RCP supportted sleepy children

This commit adds a diag command to rcp capbility diag module to check
the max number of RCP supportted sleepy children.

src/posix/platform/README_RCP_CAPS_DIAG.md

@@ -7,6 +7,7 @@ This module provides diag commands for checking RCP capabilities.
 ## Command List
 
 - [spinel](#spinel)
+- [perf](#perf)
 
 ## Command Details
 
@@ -30,3 +31,13 @@ Optional :
 PROP_VALUE_GET PHY_CCA_THRESHOLD -------------------------- OK
 Done
 ```
+
+### perf
+
+Check the RCP performance.
+
+```bash
+> diag rcpcaps perf
+MaxNumSleepyChildren -------------------------------------- 128
+Done
+```

src/posix/platform/rcp_caps_diag.cpp

@@ -99,6 +99,10 @@ otError RcpCapsDiag::DiagProcess(char *aArgs[], uint8_t aArgsLength, char *aOutp
     {
         ProcessSpinel();
     }
+    else if (strcmp(aArgs[1], "perf") == 0)
+    {
+        ProcessPerf();
+    }
     else
     {
         error = OT_ERROR_INVALID_COMMAND;
@@ -137,22 +141,61 @@ void RcpCapsDiag::TestSpinelCommands(Category aCategory)
     }
 }
 
+void RcpCapsDiag::ProcessPerf(void) { TestNumSupportedSleepyChildren(); }
+
+void RcpCapsDiag::TestNumSupportedSleepyChildren(void)
+{
+    static constexpr uint16_t kMaxNumChildren = 512;
+    static constexpr uint16_t kValueLength    = 6;
+    otExtAddress              extAddress      = {0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x00};
+    char                      value[kValueLength];
+    uint16_t                  num = 0;
+
+    SuccessOrExit(mRadioSpinel.Set(SPINEL_PROP_MAC_SRC_MATCH_EXTENDED_ADDRESSES, nullptr));
+
+    for (num = 0; num < kMaxNumChildren; num++)
+    {
+        extAddress.m8[0] = num;
+        SuccessOrExit(
+            mRadioSpinel.Insert(SPINEL_PROP_MAC_SRC_MATCH_EXTENDED_ADDRESSES, SPINEL_DATATYPE_EUI64_S, extAddress.m8));
+    }
+
+exit:
+    if (num != 0)
+    {
+        mRadioSpinel.Set(SPINEL_PROP_MAC_SRC_MATCH_EXTENDED_ADDRESSES, nullptr);
+    }
+
+    snprintf(value, sizeof(value), "%u", num);
+    OutputFormat("MaxNumSleepyChildren", value);
+}
+
+void RcpCapsDiag::OutputFormat(const char *aName, const char *aValue)
+{
+    static constexpr uint8_t kMaxLength    = 56;
+    static const char        kPadding[]    = "----------------------------------------------------------";
+    uint16_t                 actualLength  = static_cast<uint16_t>(strlen(aName));
+    uint16_t                 paddingOffset = (actualLength > kMaxLength) ? kMaxLength : actualLength;
+
+    static_assert(kMaxLength < sizeof(kPadding), "Padding bytes are too short");
+
+    Output("%.*s %s %s\r\n", kMaxLength, aName, &kPadding[paddingOffset], aValue);
+}
+
 void RcpCapsDiag::OutputResult(const SpinelEntry &aEntry, otError error)
 {
     static constexpr uint8_t  kSpaceLength            = 1;
     static constexpr uint8_t  kMaxCommandStringLength = 20;
     static constexpr uint8_t  kMaxKeyStringLength     = 35;
-    static constexpr uint16_t kMaxLength              = kMaxCommandStringLength + kMaxKeyStringLength + kSpaceLength;
-    static const char         kPadding[]              = "----------------------------------------------------------";
-    const char               *commandString           = spinel_command_to_cstr(aEntry.mCommand);
-    const char               *keyString               = spinel_prop_key_to_cstr(aEntry.mKey);
-    uint16_t actualLength  = static_cast<uint16_t>(strlen(commandString) + strlen(keyString) + kSpaceLength);
-    uint16_t paddingOffset = (actualLength > kMaxLength) ? kMaxLength : actualLength;
-
-    static_assert(kMaxLength < sizeof(kPadding), "Padding bytes are too short");
-
-    Output("%.*s %.*s %s %s\r\n", kMaxCommandStringLength, commandString, kMaxKeyStringLength, keyString,
-           &kPadding[paddingOffset], otThreadErrorToString(error));
+    static constexpr uint16_t kMaxLength =
+        kMaxCommandStringLength + kMaxKeyStringLength + kSpaceLength + 1 /* size of '\0' */;
+    char        buffer[kMaxLength] = {0};
+    const char *commandString      = spinel_command_to_cstr(aEntry.mCommand);
+    const char *keyString          = spinel_prop_key_to_cstr(aEntry.mKey);
+
+    snprintf(buffer, sizeof(buffer), "%.*s %.*s", kMaxCommandStringLength, commandString, kMaxKeyStringLength,
+             keyString);
+    OutputFormat(buffer, otThreadErrorToString(error));
 }
 
 void RcpCapsDiag::Output(const char *aFormat, ...)

src/posix/platform/rcp_caps_diag.hpp

@@ -100,7 +100,10 @@ class RcpCapsDiag
     };
 
     void ProcessSpinel(void);
+    void ProcessPerf(void);
     void TestSpinelCommands(Category aCategory);
+    void TestNumSupportedSleepyChildren(void);
+    void OutputFormat(const char *aName, const char *aValue);
     void OutputResult(const SpinelEntry &aEntry, otError error);
     void Output(const char *aFormat, ...);