The experiment is to boot Linux in a Gen2 Hyper-V guest with both the Hyper-V FB and Hyper-V DRM modules removed. In this case, the generic EFI framebuffer driver (efifb) should get used. With this driver, a program can open /dev/fb0, and while it is open, unbind the efifb driver (which is in /sys/bus/platform/drivers/efi-framebuffer). Interestingly, there's no WARN generated. But when the hyperv_fb driver is loaded and used, the WARN *is* generated, as you observed. So I looked at the code for efifb. It does the framebuffer_release() call in a function that hyperv_fb doesn't have. Based on the comments in efifb.c, we need a similar function to handle the call to framebuffer_release(). And the efifb driver also does the iounmap() in that same function, which makes we wonder if the hyperv_fb driver should do similarly. It will need a little more analysis to figure that out. You found the bug. Do you want to work on fixing the hyperv_fb driver? And maybe the Hyper-V DRM driver needs the same fix. I haven't looked. Alternatively, if you are busy, I can work on the fix. Let me know your preference. Michael
