DRM_UDL and GPU under Xserver

[Alexey Brodkin]

Hi Daniel,

On Thu, 2018-04-05 at 08:18 +0200, Daniel Vetter wrote:
> On Wed, Apr 4, 2018 at 10:06 PM, Alexey Brodkin
> <Alexey.Brodkin at synopsys.com> wrote:
> > Hello,
> > 
> > We're trying to use DisplayLink USB2-to-HDMI adapter to render GPU-accelerated graphics.
> > Hardware setup is as simple as a devboard + DisplayLink adapter.
> > Devboards we use for this experiment are:
> >  * Wandboard Quad (based on IMX6 SoC with Vivante GPU) or
> >  * HSDK (based on Synopsys ARC HS38 SoC with Vivante GPU as well)
> > 
> > I'm sure any other board with DRM supported GPU will work, those we just used
> > as the very recent Linux kernels could be easily run on them both.
> > 
> > Basically the problem is UDL needs to be explicitly notified about new data
> > to be rendered on the screen compared to typical bit-streamers that infinitely
> > scan a dedicated buffer in memory.
> > 
> > In case of UDL there're just 2 ways for this notification:
> >  1) DRM_IOCTL_MODE_PAGE_FLIP that calls drm_crtc_funcs->page_flip()
> >  2) DRM_IOCTL_MODE_DIRTYFB that calls drm_framebuffer_funcs->dirty()
> > 
> > But neither of IOCTLs happen when we run Xserver with xf86-video-armada driver
> > (see https://urldefense.proofpoint.com/v2/url?u=http-3A__git.arm.linux.org.uk_cgit_xf86-2Dvideo-2Darmada.git_log_-3Fh-3Dunstable-2Ddevel&d=DwIBaQ&
> > c=DPL6_X_6JkXFx7AXWqB0tg&r=lqdeeSSEes0GFDDl656eViXO7breS55ytWkhpk5R81I&m=oEAlP64L9vkuUs_k3kGwwwlN1WJbDMJbCo0uDhwKwwk&s=3ZHj-
> > 6JXZBLSTWg_4KMnL0VNi7z8c0RxHzj2U5ywVIw&e=).
> > 
> > Is it something missing in Xserver or in UDL driver?
> 
> Use the -modesetting driverr for UDL, that one works correctly.

If you're talking about "modesetting" driver of Xserver [1] then indeed
picture is displayed on the screen. But there I guess won't be any 3D acceleration.

At least that's what was suggested to me earlier here [2] by Lucas:
---------------------------->8---------------------------
For 3D acceleration to work under X you need the etnaviv specific DDX
driver, which can be found here:

http://git.arm.linux.org.uk/cgit/xf86-video-armada.git/log/?h=unstable-devel
---------------------------->8---------------------------

[1] https://cgit.freedesktop.org/xorg/xserver/tree/hw/xfree86/drivers/modesetting
[2] http://lists.infradead.org/pipermail/linux-snps-arc/2017-November/003031.html

-Alexey