libdrm: drmSetMaster yields Invalid argument
 

I'm using libdrm in effort to change gamma on my Chromebook. I find that when I call drmModeCrtcSetGamma, I get a "Permission denied" error, regardless of whether I run my program with sudo. (See line 91 of my source code.)

So I insert a call to drmSetMaster(dat->fd) above drmModeCrtcSetGamma, and that results in an "Invalid argument" error. What is the correct approach, please? Documentation seems rather sparse, with the best I've seen being this.