drmModeSetCrtc and how to use x,y

[Rian Quinn]

In case someone else reads this, I think I sort of answered my questions.
We tested on the AMD w600 with 6 monitors and were able to do the following:

- The X,Y in drmModeSetCrtc does in fact specify the "source" offset into
your framebuffer (not the destination on the CRTC) which is what I was
looking for.
- We were able to allocate both a Dumb buffer, and a GBM buffer the size of
6 1920x1200 monitors in a 1x6 configuration, so basically we didn't have
any memory issues with a single buffer that big.
- drmModeSetCrtc worked with both Dumb and GBM buffers, and we didn't have
to do anything special on our end, so it clearly is handling the tiling
issues behind the scenes (woot).

I didn't try it with drmModePageFlip, but I don't think that should be an
issue since you have to run drmModeSetCrtc first to set your mode anyways.
If you need a good example of page flipping, David Herrmann has a good blog
post on it:
http://dvdhrm.wordpress.com/2012/12/21/advanced-drm-mode-setting-api/

Thanks,
- Rian


On Wed, Feb 12, 2014 at 2:05 PM, Rian Quinn <rianquinn at gmail.com> wrote:

> With respect to Windows XDDM, in the past I was able to map more than one
> display to a single framebuffer by leveraging width, height, stride, and
> then adding an offset to the framebuffer's char*. I can see how this would
> work with dumb buffers as they are basically a simple linear ARGB buffer,
> but how does this work with respect to GBM buffers that are tiled?
>
> My initial thought was to use the x,y in drmModeSetCrtc. Basically, I
> could create a framebuffer in GBM that spanned multiple displays. From
> there, I could use drmModeSetCrtc to tell the CRTC where, within the
> framebuffer, to get it's data. Is that the proper use of x,y in
> drmModeSetCrtc, and if so, does it handle the titling issues behind the
> scenes?
>
> Also, are there any limitations with a design like that. For example, is
> it possible to create GBM buffers that span 6 monitors wide?
>
> Thanks,
> - Rian
>
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