[Openchrome-users] HDMI support clarification

[Xavier Bachelot]

On 04/04/2013 07:51 PM, Amit Uttamchandani wrote:
> Hello,
> 
> I've been following the latest development of HDMI support for
> openchrome. From my understanding HDMI support requires the
> drm-openchrome kernel. However, I just tried plugging in an HDMI monitor
> and I was able to get output, albeit grainy. This is on a VX900 chipset
> with 0.3.1 openchrome driver.
> 
> So is the drm-openchrome kernel required only for accelerated or 1080p
> out? Is there a way to fix this grainy output by passing certain
> options to xorg conf?
> 
Since release 0.3.0, the openchrome X driver supports both UMS and KMS.
UMS stands for User Mode Setting, which means the X driver is
responsible for properly setting up the GPU to display on the screen(s).
KMS stands for Kernel Mode Setting, that is the kernel is responsible
for for properly setting up the GPU to display on the screen(s).
In other words, the code to drive the displays can live either in the X
driver or the kernel.

The X driver contains some support for HDMI output, but it's not very
reliable, as you have already found out. As the way forward is KMS and
UMS is on the decline, it was better to spend time properly implementing
HDMI and DVI support for KMS, that is in the via drm kernel module,
rather than fixing the X driver code.

So, in some cases, you'll be happy with just the X driver. In other
cases, UMS will not work or not properly and then you'll need the
drm-openchrome kernel to be able to use KMS.

However, the via DRM driver as found in the drm-openchrome kernel is not
yet feature complete, and lacks some features that are available with
UMS. Most notably, there's no support for 2D acceleration, Xv/XvMC, TV
out, etc... Those features will be added over time, and KMS is expected
to be on par with UMS and even better in the next few months.

I've been building a page with the status for both UMS and KMS in the
wiki. It's not yet finished, but it gives a good overview of the current
status :
http://www.freedesktop.org/wiki/Openchrome/SupportMatrix

Regards,
Xavier