libcamera: base: log: Take LogCategory by reference

When no log category is specified, `nullptr` is passed, and
then the `_log()` function implementations replace that with
`LogCategory::defaultCategory()`. But since the call site always
knows the log category, this condition can be removed and the
`_LOG1()` macro can use `LogCategory::defaultCategory()`.

So remove the condition from the `_log()` implementations and
use references to refer to log categories.

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/base/log.h

@@ -96,12 +96,12 @@ public:
 protected:
 	virtual std::string logPrefix() const = 0;
 
-	LogMessage _log(const LogCategory *category, LogSeverity severity,
+	LogMessage _log(const LogCategory &category, LogSeverity severity,
 			const char *fileName = __builtin_FILE(),
 			unsigned int line = __builtin_LINE()) const;
 };
 
-LogMessage _log(const LogCategory *category, LogSeverity severity,
+LogMessage _log(const LogCategory &category, LogSeverity severity,
 		const char *fileName = __builtin_FILE(),
 		unsigned int line = __builtin_LINE());
 
@@ -109,9 +109,9 @@ LogMessage _log(const LogCategory *category, LogSeverity severity,
 #define _LOG_CATEGORY(name) logCategory##name
 
 #define _LOG1(severity) \
-	_log(nullptr, Log##severity).stream()
+	_log(LogCategory::defaultCategory(), Log##severity).stream()
 #define _LOG2(category, severity) \
-	_log(&_LOG_CATEGORY(category)(), Log##severity).stream()
+	_log(_LOG_CATEGORY(category)(), Log##severity).stream()
 
 /*
  * Expand the LOG() macro to _LOG1() or _LOG2() based on the number of

src/libcamera/base/log.cpp

@@ -952,12 +952,10 @@ Loggable::~Loggable()
  *
  * \return A log message
  */
-LogMessage Loggable::_log(const LogCategory *category, LogSeverity severity,
+LogMessage Loggable::_log(const LogCategory &category, LogSeverity severity,
 			  const char *fileName, unsigned int line) const
 {
-	return LogMessage(fileName, line,
-			  category ? *category : LogCategory::defaultCategory(),
-			  severity, logPrefix());
+	return LogMessage(fileName, line, category, severity, logPrefix());
 }
 
 /**
@@ -972,12 +970,10 @@ LogMessage Loggable::_log(const LogCategory *category, LogSeverity severity,
  *
  * \return A log message
  */
-LogMessage _log(const LogCategory *category, LogSeverity severity,
+LogMessage _log(const LogCategory &category, LogSeverity severity,
 		const char *fileName, unsigned int line)
 {
-	return LogMessage(fileName, line,
-			  category ? *category : LogCategory::defaultCategory(),
-			  severity);
+	return LogMessage(fileName, line, category, severity);
 }
 
 /**