From bug #503577, on a Dell mini 10, 'apci_backlight=vendor' kernel parameter seems to make it work.
"acpi_backlight= [HW,ACPI]
acpi_backlight=vendor
acpi_backlight=video
If set to vendor, prefer vendor specific driver
(e.g. thinkpad_acpi, sony_acpi, etc.) instead
of the ACPI video.ko driver."
From bug #503577, on a Dell mini 10, 'apci_backlight =vendor' kernel parameter seems to make it work. vendor video
"acpi_backlight= [HW,ACPI]
acpi_backlight=
acpi_backlight=
If set to vendor, prefer vendor specific driver
(e.g. thinkpad_acpi, sony_acpi, etc.) instead
of the ACPI video.ko driver."