[PATCH v2 6/9] drm/tinydrm/Kconfig: drivers: Select BACKLIGHT_CLASS_DEVICE

[David Lechner]

On 7/22/19 5:43 AM, Noralf Trønnes wrote:
> The mipi_dbi helper is missing a dependency on DRM_KMS_HELPER and putting
> that in revealed this problem:
> 
> drivers/video/fbdev/Kconfig:12:error: recursive dependency detected!
> drivers/video/fbdev/Kconfig:12: symbol FB is selected by DRM_KMS_FB_HELPER
> drivers/gpu/drm/Kconfig:75:     symbol DRM_KMS_FB_HELPER depends on DRM_KMS_HELPER
> drivers/gpu/drm/Kconfig:69:     symbol DRM_KMS_HELPER is selected by TINYDRM_MIPI_DBI
> drivers/gpu/drm/tinydrm/Kconfig:11:     symbol TINYDRM_MIPI_DBI is selected by TINYDRM_HX8357D
> drivers/gpu/drm/tinydrm/Kconfig:15:     symbol TINYDRM_HX8357D depends on BACKLIGHT_CLASS_DEVICE
> drivers/video/backlight/Kconfig:144:    symbol BACKLIGHT_CLASS_DEVICE is selected by FB_BACKLIGHT
> drivers/video/fbdev/Kconfig:187:        symbol FB_BACKLIGHT depends on FB
> 
> A symbol that selects DRM_KMS_HELPER can not depend on
> BACKLIGHT_CLASS_DEVICE. The reason for this is that DRM_KMS_FB_HELPER
> selects FB instead of depending on it.
> 
> The tinydrm drivers have somehow gotten away with depending on
> BACKLIGHT_CLASS_DEVICE because DRM_TINYDRM selects DRM_KMS_HELPER and the
> drivers depend on that symbol.
> 
> An audit shows that all DRM drivers that select DRM_KMS_HELPER and use
> BACKLIGHT_CLASS_DEVICE, selects it:
>    DRM_TILCDC, DRM_GMA500, DRM_SHMOBILE, DRM_NOUVEAU, DRM_FSL_DCU,
>    DRM_I915, DRM_RADEON, DRM_AMDGPU, DRM_PARADE_PS8622
> 
> Documentation/kbuild/kconfig-language.txt has a note regarding select:
> 1. 'select should be used with care since it doesn't visit dependencies.'
>     This is not a problem since BACKLIGHT_CLASS_DEVICE doesn't have any
>     dependencies.
> 2. 'In general use select only for non-visible symbols'
>     BACKLIGHT_CLASS_DEVICE is user visible.
> 
> The real solution to this would be to have DRM_KMS_FB_HELPER depend on the
> user visible symbol FB. That is a can of worms I'm not willing to tackle.
> I fear that such a change will result in me handling difficult fallouts
> for the next weeks. So I'm following DRM suite here.
> 
> Signed-off-by: Noralf Trønnes <noralf at tronnes.org>
> ---

Reviewed-by: David Lechner <david at lechnology.com>