[PATCH 3/8] drm/display: Introduce a DRM display-helper module

[Javier Martinez Canillas]

On 3/22/22 20:27, Thomas Zimmermann wrote:
> Replace the DP-helper module with a display-helper module. Update
> all related Kconfig and Makefile rules.
> 
> Besides the existing code for DisplayPort, the new module will
> contain helpers for other video-output standards, such as HDMI.
> Drivers will still be able to select the required video-output
> helpers. Linking all such code into a single module avoids the
> proliferation of small kernel modules.
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
> ---

[snip]

> +config DRM_DISPLAY_HELPER
> +	tristate
> +	depends on DRM
> +	help
> +	  DRM helpers for display adapters.
> +
>  config DRM_DP_HELPER
>  	tristate
>  	depends on DRM
> +	select DRM_DISPLAY_HELPER
>  	help
>  	  DRM helpers for DisplayPort.
> 

I was about to ask why this would still be needed but then re-read the
commit message that says drivers will still be able to select required
video-output helpers.

That makes sense since the fact that all helpers will be in the same module
would be transparent to drivers.

> diff --git a/drivers/gpu/drm/display/Makefile b/drivers/gpu/drm/display/Makefile
> index 75faffc706b1..90f12e9b4735 100644
> --- a/drivers/gpu/drm/display/Makefile
> +++ b/drivers/gpu/drm/display/Makefile
> @@ -2,8 +2,9 @@
>  
>  obj-$(CONFIG_DRM_DP_AUX_BUS) += drm_dp_aux_bus.o
>  
> -drm_dp_helper-y := drm_dp.o drm_dp_dual_mode_helper.o drm_dp_helper_mod.o drm_dp_mst_topology.o
> -drm_dp_helper-$(CONFIG_DRM_DP_AUX_CHARDEV) += drm_dp_aux_dev.o
> -drm_dp_helper-$(CONFIG_DRM_DP_CEC) += drm_dp_cec.o
> +drm_display_helper-y := drm_display_helper_mod.o
> +drm_display_helper-$(CONFIG_DRM_DP_HELPER) := drm_dp_helper.o drm_dp_dual_mode_helper.o drm_dp_mst_topology.o
> +drm_display_helper-$(CONFIG_DRM_DP_AUX_CHARDEV) += drm_dp_aux_dev.o
> +drm_display_helper-$(CONFIG_DRM_DP_CEC) += drm_dp_cec.o
>  
> -obj-$(CONFIG_DRM_DP_HELPER) += drm_dp_helper.o
> +obj-$(CONFIG_DRM_DISPLAY_HELPER) += drm_display_helper.o

The drm_dp_helper.ko module has some parameters and this change will break
existing kernel cmdline that are using it:

$ modinfo drivers/gpu/drm/dp/drm_dp_helper.ko | grep parm | cut -d : -f2
           drm_dp_cec_unregister_delay
           dp_aux_i2c_speed_khz
           dp_aux_i2c_transfer_size

I don't know whether those are considered a kernel ABI or not though, and
some already changed when the DP helpers were moved from drm_kms_helper.ko

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat