kernel-source-code-drivers: add libgpiod for userspace driver

Simple userspace drivers can also use libgpiod (often in combination
with SPI or I2C or serial), so add that.

To make it fit on the slide, make the intro sentence a little shorter.

While we're at it, fix the emphasis of the *not* on the slide.

Signed-off-by: Arnout Vandecappelle <[email protected]>

slides/kernel-source-code-drivers/kernel-source-code-drivers.tex

@@ -245,12 +245,12 @@ \subsection{Linux kernel source code}
 \begin{frame}
   \frametitle{User space device drivers 1/2}
   \begin{itemize}
-  \item The kernel provides certain mechanisms to access hardware
-    directly from userspace:
+  \item The kernel provides some mechanisms to access hardware from userspace:
     \begin{itemize}
     \item USB devices with {\em libusb}, \url{https://libusb.info/}
     \item SPI devices with {\em spidev}, \kdochtml{spi/spidev}
     \item I2C devices with {\em i2cdev}, \kdochtml{i2c/dev-interface}
+    \item GPIOs with {\em libgpiod}, \url{https://libgpiod.readthedocs.io}
     \item Memory-mapped devices with {\em UIO}, including interrupt
       handling, \kdochtml{driver-api/uio-howto}
     \end{itemize}
@@ -264,7 +264,7 @@ \subsection{Linux kernel source code}
   \item Certain classes of devices like printers and scanners do not
     have any kernel support, they have always been handled in user space
     for historical reasons.
-  \item Otherwise this is *not* how the system should be
+  \item Otherwise this is {\bf \em not} how the system should be
     architectured. Kernel drivers should always be preferred!
   \end{itemize}
 \end{frame}