linux-kernel - Re: [PATCH v2] drm/display: Select DP helper for DRM_DP_AUX_CHARDEV and DRM_DP

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Date:   Thu, 28 Apr 2022 09:02:32 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Javier Martinez Canillas <javierm@...hat.com>,
        linux-kernel@...r.kernel.org
Cc:     Lyude Paul <lyude@...hat.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2] drm/display: Select DP helper for DRM_DP_AUX_CHARDEV
 and DRM_DP_CEC

Hi

Am 27.04.22 um 23:55 schrieb Javier Martinez Canillas:
> The DRM_DP_AUX_CHARDEV and DRM_DP_CEC Kconfig symbols enable code that use
> DP helper functions, that are only present if CONFIG_DRM_DISPLAY_DP_HELPER
> is also enabled.
> 
> But these don't select the DRM_DISPLAY_DP_HELPER symbol, meaning that it
> is possible to enable any of them without CONFIG_DRM_DISPLAY_DP_HELPER.
> 
> That will lead to the following linking errors with the mentioned config:
> 
>    LD      vmlinux.o
>    MODPOST vmlinux.symvers
>    MODINFO modules.builtin.modinfo
>    GEN     modules.builtin
>    LD      .tmp_vmlinux.kallsyms1
>    KSYMS   .tmp_vmlinux.kallsyms1.S
>    AS      .tmp_vmlinux.kallsyms1.S
>    LD      .tmp_vmlinux.kallsyms2
>    KSYMS   .tmp_vmlinux.kallsyms2.S
>    AS      .tmp_vmlinux.kallsyms2.S
>    LD      vmlinux
>    SYSMAP  System.map
>    SORTTAB vmlinux
>    OBJCOPY arch/arm64/boot/Image
>    MODPOST modules-only.symvers
> ERROR: modpost: "drm_dp_dpcd_write" [drivers/gpu/drm/display/drm_display_helper.ko] undefined!
> ERROR: modpost: "drm_dp_read_desc" [drivers/gpu/drm/display/drm_display_helper.ko] undefined!
> ERROR: modpost: "drm_dp_dpcd_read" [drivers/gpu/drm/display/drm_display_helper.ko] undefined!
> make[1]: *** [scripts/Makefile.modpost:134: modules-only.symvers] Error 1
> make[1]: *** Deleting file 'modules-only.symvers'
> make: *** [Makefile:1749: modules] Error 2
> 
> Note: It seems this has been an issue for a long time but was made easier
> to reproduce after the commit 1e0f66420b13 ("drm/display: Introduce a DRM
> display-helper module"). Adding a Fixes: tag just to make sure that this
> fix will be picked for stable once the mentioned change also lands there.
> 
> Fixes: 1e0f66420b13 ("drm/display: Introduce a DRM display-helper module")
> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
> ---
> 
> Changes in v2:
> - Explain better the issue in the change description.
> - Only select DRM_DISPLAY_DP_HELPER and not DRM_DISPLAY_HELPER.
> 
>   drivers/gpu/drm/display/Kconfig | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/gpu/drm/display/Kconfig b/drivers/gpu/drm/display/Kconfig
> index f84f1b0cd23f..9fe80c4e555c 100644
> --- a/drivers/gpu/drm/display/Kconfig
> +++ b/drivers/gpu/drm/display/Kconfig
> @@ -32,6 +32,7 @@ config DRM_DISPLAY_HDMI_HELPER
>   config DRM_DP_AUX_CHARDEV
>   	bool "DRM DP AUX Interface"
>   	depends on DRM
> +	select DRM_DISPLAY_DP_HELPER

You cannot select DISPLAY_DP_HELPER without DISPLAY_HELPER.

Can't you simply make it depend on DISPLAY_DP_HELPER.  The menu entry 
will show up as soon as there's a driver that selcets DISPLAY_DP_HELPER.

Best regards
Thomas

>   	help
>   	  Choose this option to enable a /dev/drm_dp_auxN node that allows to
>   	  read and write values to arbitrary DPCD registers on the DP aux
> @@ -40,6 +41,7 @@ config DRM_DP_AUX_CHARDEV
>   config DRM_DP_CEC
>   	bool "Enable DisplayPort CEC-Tunneling-over-AUX HDMI support"
>   	depends on DRM
> +	select DRM_DISPLAY_DP_HELPER
>   	select CEC_CORE
>   	help
>   	  Choose this option if you want to enable HDMI CEC support for

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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