add information about cirrusdrm driver and Linux Cirrus driver

%opening

\title{OpenBSD DRM/KMS driver implementation guide}

\author{Léo Grange}

\author{Léo Grange \\ \href{mailto:grangeleo@gmail.com}{grangeleo@gmail.com}}

\bibliographystyle{plain}

holes I encountered myself.

As explaining abstract things with a concrete example often helps to understand,

we will use the OpenBSD Cirrus DRM driver for illustration purpose.

The old Cirrus chipsets are significantly easier to program than their today's

equivalents (Intel or AMD/ATI cards).

we will use the OpenBSD Cirrus DRM driver\footnotemark for illustration purpose.

This driver was originally written as part of a Google Summer of Code 2015 project,

to provide an understandable basis for future DRM drivers.

Indeed, the old Cirrus chipsets are significantly easier to program than their

today's equivalents (Intel or AMD/ATI cards).

In addition, the Cirrus CL-GD5446 card is well emulated in QEMU, which makes it

easier to test the driver and to experiment with it.

% talk about Cirrus driver

\footnotetext{

Until \texttt{cirrusdrm} driver integration to upstream OpenBSD,

a version of the driver is available through a git repository, at

\url{http://lab.knightsofnii.com/kristaba/openbsd.git}, on the branch

\texttt{cirrusdrm-current}.

A web interface is also provided at

\url{http://lab.knightsofnii.com/kristaba/openbsd/tree/cirrusdrm-current/sys/dev/pci/drm/cirrus}.

}

\end{abstract}

% TODO explain the need of userland X.Org drivers

\subsection{Userland API}

%\subsection{Userland API}

% TODO

\subsection{Kernel driver model}

APIs, it's difficult to create a new driver without some examples.

The example used here is \texttt{cirrusdrm}, a pretty simple driver which supports

mainly the Cirrus CL-GD5446 chipset.

This driver was written from scratch, based on \texttt{radeon} and \texttt{i915}

DRM driver sources as well as the X.org DDX driver \texttt{xf86-video-cirrus}.

A similar driver, designed to work only with the QEMU and Xen implementations of

this chipset, exists in upstream Linux kernel for a while.

Unfortunately, the GPL-only license of the driver prevents it to be ported to

OpenBSD's kernel, and its authors didn't seem to agree for changing the license.

As explained above, this chipset has the huge advantage to be emulated in some

virtual machines and hypervisors (QEMU or Xen for example), resulting in an ease

virtual machines and hypervisors (QEMU or Xen for instance), resulting in an ease

of testing and experimenting with it.

As it's a graphics card from the 90's, it has the other advantage to expose a

pretty simple hardware, with only one CRTC, a single VGA connector and little

acceleration capability.

Some modes are invalidated later, both in X.org and KMS drivers, for instance

when memory is insufficient for a mode.

\pagebreak

\bibliography{openbsd-biblio}