On 3/30/20 2:32 PM, Hans de Goede wrote: > Hi, > > On 3/30/20 11:25 PM, Randy Dunlap wrote: >> On 3/30/20 1:51 PM, Hans de Goede wrote: >>> Hi, >>> >>> On 3/29/20 11:36 AM, Paul Menzel wrote: >>>> Dear Linux folks, >>>> >>>> >>>> On the MSI desktop board [1] >>>> >>>> [ 0.000000] DMI: Micro-Star International Co., Ltd. MS-7A37/B350M MORTAR (MS-7A37), BIOS 1.MR 12/02/2019 >>>> >>>> with an AMD Ryzen 3 2200G with Radeon Vega Graphics, the ACPI Video Driver `video` is loaded and creates a backlight device. >>>> >>>> $ readlink -f /sys/class/backlight/acpi_video0 >>>> /sys/devices/pci0000:00/0000:00:08.1/0000:26:00.0/backlight/acpi_video0 >>>> >>>> I wonder what the driver is used for as the AMDGPU driver exists for the graphics device. >>> >>> Backlight on x86 hw is a bit of a mess, there is an ACPI standard for accessing it >>> and with older (Windows XP era) laptops that is the interface which usually works, >>> then there are a bunch of vendor specific SMBIOS or WMI backlight interfaces and >>> then there is the option of directly accessing the hardware as the amdgpu driver >>> is doing. >>> >>> We have a bunch of heuristics to avoid the acpi_video driver registering a >>> backlight interface when it should not, either because the direct hw access >>> should be used instead; or because there simply is no builtin LCD panel and thus >>> no backlight to control. >>> >>> These heuristics are failing on your board. >>> >>>> If it’s useful, is there a way to prevent the backlight interface from getting created? >>> >>> You can pass "acpi_backlight=ignore" on the kernel commandline to disable the >> >> Hi Hans, >> Should that be "acpi_backlight=none"? I don't see 'ignore' allowed here: >> >> static void acpi_video_parse_cmdline(void) >> { >> if (!strcmp("vendor", acpi_video_backlight_string)) >> acpi_backlight_cmdline = acpi_backlight_vendor; >> if (!strcmp("video", acpi_video_backlight_string)) >> acpi_backlight_cmdline = acpi_backlight_video; >> if (!strcmp("native", acpi_video_backlight_string)) >> acpi_backlight_cmdline = acpi_backlight_native; >> if (!strcmp("none", acpi_video_backlight_string)) >> acpi_backlight_cmdline = acpi_backlight_none; >> } > > Yes you are right that should be "acpi_backlight=none". > >> and Documentation/admin-guide/kernel-parameters.txt could stand to be updated >> with a few of those options. > > Ack, I've put this on my (much too long) TODO list. Feel free to > beat me to it. Sure, I'll send a patch shortly. -- ~Randy