kernel-driver-development-debugging: Add dedicated slies about early …

…printl WIP

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>

slides/kernel-driver-development-debugging/kernel-driver-development-debugging.tex

@@ -100,6 +100,22 @@ \section{Kernel debugging}
   \end{itemize}
 \end{frame}
 
+\begin{frame}
+  \frametitle{Printing message early during the boot}
+  \begin{itemize}
+  \item Messages are visible only once the console driver has been initialized.
+  \item The console driver is simpler than a complete serial driver
+    and is initialize earlier than serial driver
+  \item But core subsystem are still initialized before, such as
+    memory, timer or interrupt, if a crash occurred there, then no
+    message are visible
+  \item on arm (32 bits) architecture most of the SoC implement the
+    \code{early_printk} support
+  \item \Activate it with kconfig{CONFIG_DEBUG_LL} and \kconfig{CONFIG_EARLYPRINTK} and add
+    \code{earlyprintk} to the kernel command line.
+  \end{itemize}
+\end{frame}
+
 \begin{frame}
   \frametitle{DebugFS}
   A virtual filesystem to export debugging information to user space.
@@ -276,9 +292,5 @@ \section{Kernel debugging}
     \item Is turned on by default
     \item To get oops messages with symbol names instead of raw addresses
     \end{itemize}
-  \item On ARM, if your kernel doesn't boot or hangs without any
-    message, you can activate early debugging options
-    (\kconfig{CONFIG_DEBUG_LL} and \kconfig{CONFIG_EARLYPRINTK}), and add
-    \code{earlyprintk} to the kernel command line.
   \end{itemize}
 \end{frame}