Comment 13 for bug 1485246

Right, so I have re-tested having installed the 4.2.0-040200rc7-generic kernel. The behaviour seems to be identical to the stock Ubuntu 15.04 kernel.

A few things I have noticed having carried out several reboots and suspends in the process of testing this. These apply to both the stock kernel and the 4.2 kernel:

With kernel parameter acpi_backlight=vendor
1. On first boot the brightness of the screen is about 60% brightness and attempts to change the brightness setting via the hotkeys causes the brightness level notification to flicker and the brightness level cannot be altered
2. After a suspend the brightness returns to about 100% and using the hotkeys may then work (i.e brightness controls have been properly restored). This occurs on about 2 in 3 wakes from suspend, the other 1 in 3 the brighness is 100% but control over brightness still non-functional

with no acpi_backlight kernel param
1. The initial boot brightness is approximately the same as above. Attempts to change this successfully move the brightness level in the indicator, but have no impact on the actual screen brighness.
2. wake from suspend results in the 100% brightness as above, but no control of screen brightness still

Not sure if any of that actually helps any...

Let me know if I can extract any further useful information to help with debugging.