10bit output via KMS

[Volker Vogelhuber]

Hi,

> On 24 July 2017 at 14:56, Volker Vogelhuber
> <v.vogelhuber at digitalendoscopy.de> wrote:
>> I wonder if it is possible to have drmModeAddFB2 to handle
>> 'X', 'R', '3', '0' at all. So is this supported in any way?
> Secondly, you're correct that you would need (theoretically) to extend
> drmModeAddFB2, however it already has DRM_FORMAT_XRGB2101010 and
> DRM_FORMAT_XBGR2101010 for i965 and above, which covers everything in
> the last 10 years. This is the 'XR30' FourCC you mention, so it should
> already be there and working in the kernel.
This question probably better fits to the drm mailing list, although the
discussion started on the mesa list.

I got it working now that my 10bit buffers are correctly displayed
on a Samsung QM49F via drmModeAddFB2 and the 'X', 'R', '3', '0'
fourcc code. Based on the Samsung's datasheet it should support 10bit
signals, although the EDID does not seem to contain a vendor
specific data block that would confirm that assumption.

I have connected an Apollo Lake module with a display port cable
to that display but I'm unsure if it's really 10bit or if there is some
conversion logic somewhere in between that may downsample my
10 bit buffer to 8bit before sending it via displayport to the display.
So far I stumbled over some patches regarding color management
in the drm part of the kernel, but couldn't figure out how one could
configure what is really send to the display. It seem to mostly be used 
to manipulate gamma values and so on. As the only indicator
what is provided to the display is the buffer format handled by
drmModeAddFB2 I doubt this is enough to configure the signal.
Otherwise one could argue that the link speed of the display signal
would have to toggle every time I send a different buffer format, which 
is certainly not the case. So what portion of the kms/drm chain do I
miss currently? The drmModeModeInfo pointer has a clock field, but I 
guess this is meant to be for the pixel clock not the clock of the 
serialized signal. Probably that's why I couldn't find modelines for 
4k at 60 especially for 10bit compared to 8bit.

Thanks
    Volker