Sharing a KMS device

[Troll Berserker]

On 19/09/18 01:02, Keith Packard wrote:
> Troll Berserker <berserker.troll at yandex.com> writes:
> 
>> Cool. In your talk you even mentions multiseating.
>> AFAIU with this technique one doesn't get multiple /dev/dri/card* for
>> a GPU out of the box, so it is not possible to use unmodified
>> modesetting driver to run multiple X servers.
> 
> Correct. I have posted X server patches to pass the DRM file descriptor
> in from an external program, and a sample program which creates a lease
> for that purpose.
> 

Hmm. I see that drmDropMaster became conditional:

     if (!ms->fd_passed)
         drmDropMaster(ms->fd);

(https://github.com/mirror/xserver/commit/38ff29ec8ead0a293f50ac52146199babe4205ca#diff-4db01e9b0bfda6e79f6b85426b0cdb0aR1780)

I was thinking that may be it is possible to implement a CUSE driver to expose leased descriptors as /dev/dri/card* devices so that it would be possible to run X server unmodified.
But now I see that these leased FDs are somehow special so you can't perform drmDropMaster on them.
Why?