kernel 6.6.x compilation crash when CONFIG_BACKLIGHT_CLASS_DEVICE is not y

[jlsalvador]

Describe the bug

When CONFIG_BACKLIGHT_CLASS_DEVICE is not y; backlight_set_display_name will not be defined (

linux/include/linux/backlight.h

Lines 486 to 488 in a2fb37f

	#if IS_ENABLED(CONFIG_BACKLIGHT_CLASS_DEVICE)
	struct backlight_device *devm_of_find_backlight(struct device *dev);
	int backlight_set_display_name(struct backlight_device *bd, const char *name);

), so compiling drivers/gpu/drm/bridge/panel.o will crash with an backlight_set_display_name undefined

This issue was added by commit 573f8fd

Tested on the 6.6.x branch.

Steps to reproduce the behaviour

• Generate a kernel config with CONFIG_BACKLIGHT_CLASS_DEVICE not set.
• Generate a kernel config with CONFIG_BACKLIGHT_CLASS_DEVICE=m
• Try to compile the kernel.

Device (s)

Raspberry Pi 4 Mod. B, Raspberry Pi 5, Other

System

Cross compiling kernel by Buildroot.

Logs

https://gist.github.com/jlsalvador/37316ff616a0428adbac7cc2cecdddab

Additional context

No response

[6by9]

It should be defined as an empty function via the #else at https://github.com/raspberrypi/linux/blob/rpi-6.6.y/include/linux/backlight.h#L496-L500

I'll test on Monday.

[pelwell]

Please provide an example configuration showing the build failure.

[jlsalvador]

Please provide an example configuration showing the build failure.

https://gist.github.com/jlsalvador/37316ff616a0428adbac7cc2cecdddab

Please, take a look at the bottom, the output.txt file.

In this example CONFIG_BACKLIGHT_CLASS_DEVICE=m the error is:

/usr/src/simplek8s/per-package/linux/host/bin/aarch64-none-linux-gnu-ld: drivers/gpu/drm/bridge/panel.o: in function `panel_bridge_attach':
panel.c:(.text+0x200): undefined reference to `backlight_set_display_name'
make[3]: *** [scripts/Makefile.vmlinux:37: vmlinux] Error 1
make[2]: *** [Makefile:1165: vmlinux] Error 2
make[1]: *** [package/pkg-generic.mk:283: /usr/src/simplek8s/build/linux-custom/.stamp_built] Error 2
make: *** [Makefile:21: _all] Error 2

[6by9]

Your original description said CONFIG_BACKLIGHT_CLASS_DEVICE not set. I believe that should be fine.

The awkwardness is actually CONFIG_DRM_PANEL=y at line 4404 with CONFIG_BACKLIGHT_CLASS_DEVICE=m. Having that built in when backlight is a module will lead to a link error. There are magic Kconfig runes around forcing modules to be built in to match other built ins, but it becomes ugly - there's no obligation for selecting BACKLIGHT from DRM_PANEL, and having BACKLIGHT declare all the modules that could use it and change structure accordingly is equally ugly.

I suspect I need to resort to #ifdef in the code. It looks like drm_panel already has
#if IS_REACHABLE(CONFIG_BACKLIGHT_CLASS_DEVICE) (https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_panel.c#L515)
for accessing devm_of_find_backlight which is similarly stubbed.

[adminy]

I got:

QUESTION: Lowlevel Backlight controls, NAME: BACKLIGHT_CLASS_DEVICE, ALTS: M/y/?, ANSWER: n
GOT: n
GOT: 
GOT: CONFIG_BACKLIGHT_CLASS_DEVICE:
GOT: 
GOT: This framework adds support for low-level control of the LCD
GOT: backlight. This includes support for brightness and power.
GOT: 
GOT: To have support for your specific LCD panel you will have to
GOT: select the proper drivers which depend on this option.
GOT: 
GOT: Symbol: BACKLIGHT_CLASS_DEVICE [=m]
GOT: Type  : tristate
GOT: Defined at drivers/video/backlight/Kconfig:136
GOT:   Prompt: Lowlevel Backlight controls
GOT:   Depends on: HAS_IOMEM [=y]
GOT:   Location:
GOT:     -> Device Drivers
GOT:       -> Graphics support
GOT:         -> Backlight & LCD device support
GOT:           -> Lowlevel Backlight controls (BACKLIGHT_CLASS_DEVICE [=m])
GOT: Selected by [m]:
GOT:   - DRM_RADEON [=m] && HAS_IOMEM [=y] && DRM [=y] && PCI [=y] && MMU [=y] && (AGP [=n] || !AGP [=n])
GOT:   - DRM_AMDGPU [=m] && HAS_IOMEM [=y] && DRM [=y] && PCI [=y] && MMU [=y] && !UML
GOT:   - DRM_NOUVEAU [=m] && HAS_IOMEM [=y] && DRM [=y] && PCI [=y] && MMU [=y] && DRM_NOUVEAU_BACKLIGHT [=y]
GOT:   - DRM_PANEL_MIPI_DBI [=m] && HAS_IOMEM [=y] && DRM [=y] && SPI [=y]
GOT:   - TINYDRM_HX8357D [=m] && HAS_IOMEM [=y] && DRM [=y] && SPI [=y]
GOT:   - TINYDRM_ILI9163 [=m] && HAS_IOMEM [=y] && DRM [=y] && SPI [=y]
GOT:   - TINYDRM_ILI9341 [=m] && HAS_IOMEM [=y] && DRM [=y] && SPI [=y]
GOT:   - TINYDRM_ILI9486 [=m] && HAS_IOMEM [=y] && DRM [=y] && SPI [=y]
GOT:   - TINYDRM_MI0283QT [=m] && HAS_IOMEM [=y] && DRM [=y] && SPI [=y]
GOT:   - TINYDRM_ST7735R [=m] && HAS_IOMEM [=y] && DRM [=y] && SPI [=y]
GOT:   - DRM_GUD [=m] && HAS_IOMEM [=y] && DRM [=y] && USB [=y] && MMU [=y]
GOT:   - DRM_SSD130X [=m] && HAS_IOMEM [=y] && DRM [=y] && MMU [=y]
GOT:   - DRM_PARADE_PS8622 [=m] && HAS_IOMEM [=y] && DRM [=y] && DRM_BRIDGE [=y] && OF [=y]
GOT:   - FB_ARMCLCD [=m] && (ARM || ARM64 [=y] || COMPILE_TEST [=n]) && FB [=y] && ARM_AMBA [=y] && HAS_IOMEM [=y] && OF [=y]
GOT:   - FB_BACKLIGHT [=m] && HAS_IOMEM [=y] && FB [=y]
GOT:   - USB_APPLEDISPLAY [=m] && USB_SUPPORT [=y] && USB [=y]
GOT: Selected by [n]:
GOT:   - DRM_NOUVEAU [=m] && HAS_IOMEM [=y] && DRM [=y] && PCI [=y] && MMU [=y] && ACPI [=n] && X86
GOT:   - DRM_I915 [=n] && HAS_IOMEM [=y] && DRM [=y] && X86 && PCI [=y] && !PREEMPT_RT [=n] && ACPI [=n]
GOT:   - DRM_GMA500 [=n] && HAS_IOMEM [=y] && DRM [=y] && PCI [=y] && X86 && MMU [=y] && ACPI [=n]
GOT:   - DRM_SHMOBILE [=n] && HAS_IOMEM [=y] && DRM [=y] && (ARCH_RENESAS [=n] || ARCH_SHMOBILE || COMPILE_TEST [=n])
GOT:   - DRM_TILCDC [=n] && HAS_IOMEM [=y] && DRM [=y] && OF [=y] && ARM
GOT:   - DRM_FSL_DCU [=n] && HAS_IOMEM [=y] && DRM [=y] && OF [=y] && ARM && COMMON_CLK [=y]
GOT:   - FB_OLPC_DCON [=n] && STAGING [=y] && OLPC && FB [=y] && BROKEN [=n] && I2C [=y] && GPIO_CS5535 [=n] && ACPI [=n]
GOT:   - SAMSUNG_Q10 [=n] && X86_PLATFORM_DEVICES [=n] && ACPI [=n]
GOT:   - ACPI_CMPC [=n] && X86_PLATFORM_DEVICES [=n] && ACPI [=n] && INPUT [=y] && (RFKILL [=m] || RFKILL [=m]=n [=n])
GOT: 
GOT: 
GOT: 
QUESTION: Lowlevel Backlight controls, NAME: BACKLIGHT_CLASS_DEVICE, ALTS: M/y/?, ANSWER: n
repeated question: Lowlevel Backlight controls at /nix/store/k5nz4dqvifjqgr2m3ya4n1012jnn9zjb-generate-config.pl line 88.

x999 of:
Error in reading or end of file.

And even set CONFIG_DRM_PANEL=n

[6by9]

I'm struggling to get a configuration that fails, but #6230 should work around it anyway.

[pelwell]

That's now merged. Please test again.

[adminy]

okay, thank you

[jlsalvador]

Fixed by da87f91
Thank you all!