I own a HP 655 notebook (Hewlett-Packard HP 655 Notebook PC/1885, BIOS F.26 05/21/2013) with an integrated Radeon HD 7340.

About two months ago, I updated the kernel and shut down the computer. For one subsequent boot with the new kernel, the computer worked fine.

However, the next boot, the screen was constantly black, even on power-on, when the BIOS/EFI was still in full control.

First I thought the computer was defective, but I noticed on HD activity that it apparently booted. Long story short, I connected an external display and it worked fine.

After some simple diagnosis, I found that the LVDS isn't detected at all as if it wouldn't exist.

dmesg gives me a single error message: "[drm:radeon_acpi_init] *ERROR* Cannot find a backlight controller"

xrandr shows HDMI-0 and VGA-0 but not LVDS-0 as it was the case before the trouble started.

I know what a display with a defective background controller looks like. This display, however, doesn't show anything at all.

Here is the output of radeontool --debug

radeontool --debug
Found card 1002:9808 (30000)
Radeon found. Base control address is 7fb730825000; base framebuffer address is 7fb720825000.
reading RADEON_DAC_CNTL (58) is 00000000
reading RADEON_LVDS_GEN_CNTL (2d0) is 00000000

I didn't move the computer at all, so I guess a hardware fault would be rather unlikely.

I suspect the kernel to have changed some registers, maybe something in EFI or some NVRAM parts
that cause the computer to shut-down the LVDS immediately on boot.

I even installed Windows on the computer using the external display, tried different Linux distributions
and different kernel versions, flashed the BIOS to the most recent version, all to no avail.

So I wonder if there is a possibility for this to be a software/EFI/NVRAM/... issue.

Is there a way to check LVDS operation with radeontool?

Is there any other way to get more detailed information about the issue, like reading special
registers with radeontool?