[mythtv-users] Xorg nouveau driver vs. nvidia-driver

Stephen Worthington stephen_agent at jsw.gen.nz
Thu Nov 5 05:04:17 UTC 2020

On Wed, 4 Nov 2020 13:13:02 -0600, you wrote:

>   I am running mythtv 31-fixes on Debian Buster (10) through 
>deb-multimedia backports. I started with Nouveau driver for Xorg as that 
>is the default in Debian buster.  After testing a while and making sure 
>that it works, I added nvidia-driver.  While mythfrontend seem to look 
>tiny bit better, I see that gui is not fitting due to overscan on TV.  
>It was perfect with nouveau driver and overscanned on nvidia-driver. 
>Another thing I noticed is nouveau drive gives me 1920x1080 at 120hz 
>whereas nvidia does 1920x1080 at 119.88hz. Not sure if nouveau is rouding 
>or giving me a true 120hz. I do see that my TV takes a while when 
>switching from another source to mythtv host when in 119.88hz rating. 
>Given this,
>Is nvidia-driver known to be superior that I should try to keep it and 
>adjust frontend gui resolution to make things fit or should I simply 
>switch back to nouveau driver?
>Also, overscan is a general issue even with Xorg, and fixing gui 
>resolution will not make Xorg better. I will have to fix Xorg overscan 
>somehow. So, I prefer nouveau over nvidia in this aspect.

I think you are confusing things here.  Nouveau uses X in the form of
Xorg just as Nvidia does.  And the overscan problem is usually in the
TV, not the video drivers.  And to complicate things further, the
Nouveau drivers do now have some VDPAU abilities so they can offload
some things onto the GPU for video playback.

>Any help suggestion in improving what I have is much appreciated.

I used Nouveau last year on my first GT1030 for a while.  That PC has
an ancient motherboard (Intel Core2 Duo E6550, from 2008) and only 4
Gibytes of RAM.  So it needs the graphics processing to be done by the
GPU.  It was fine for desktop use, but mythfrontend could not play any
TV successfully.  Even SD recordings (576p) seemed to judder a bit as
though there were dropped frames.  And HD (720p, 1080i) was impossible
to watch.  Installing the Nvidia drivers made everything work
properly.  If you have a fast modern CPU, it may be able to do the
graphics processing fast enough in the CPU with the Nouveau drivers,
but where you normally see problems is in deinterlacing.  So try
playing 1080i recordings, and watch scenes where there is fast

And it is unlikely that any 4k video will work properly.  I found that
on my main MythTV box with a GT220 (no 4k support), the CPU was unable
to rescale 4k downloaded video to 1080p.  When I changed it to a
GT1030 also, 4k rescaled on the GPU without any problems.

So, which Nvidia card are you using?  And just what is your TV?  If it
is 4k, then you definitely will want to be using the Nvidia drivers. I
would recommend using Nvidia anyway - it uses much less electricity to
use the GPU instead of the CPU for graphics processing.

You should check if your TV can be set to have its overscan disabled.
Some have options for this, and others automatically do it on inputs
that are identified as a PC rather than a video player device, so if
the TV manual says to use one specific input for PCs, that will likely
not have overscan.  Others TVs can do it only via a hidden maintenance
menu, and then there are some that have no way of disabling it.  And
in others, it is off in some modes and on in others, so try using
xrandr to change modes and see if that is what is happening.  It would
also be good to set full debug output (if that is possible) in the
Nouveau drivers and keep a copy of the log.  It may well show in there
exactly what the Nouveau drivers do that removes the overscan.  I have
also heard of a TV that drops the overscan when you use the menu
options for naming the inputs and select the name option that has a PC
like name.  A web search for your exact TV model may be able to help.
Or is there a manufacturer's help service that you can contact?

If you need to select from only specific modes to get no overscan,
then you will probably need to get a copy of all the modes from the
log file and convert them into modelines and create an xorg.conf file
from them.  That is very tedious to do, but not horribly complicated.
I am in the process of doing this at the moment, and have it working
for all the standard modes I need so far except the 24 Hz ones.  To
get the log information you need for this, you need to add this:

Option  "ModeDebug" "true"

in the Screen section of xorg.conf:

Section "Screen"
   Option  "ModeDebug" "true"

If you are unable to disable the overscan, then you can always use the
ViewPortOut setting in the Nvidia drivers.  That does reduce the
resolution of the picture a little bit though, so it is not the
preferred fix.

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