Re: [PATCH] drm/panel-edp: modify Kconfig to prevent build error

[Arnd Bergmann <arnd@arndb.de>]

On Wed, Nov 17, 2021 at 7:27 AM Randy Dunlap <rdunlap@infradead.org> wrote:
>
> When CONFIG_DRM_KMS_HELPER=m and CONFIG_DRM_PANEL_EDP=y,
> there is a build error in gpu/drm/panel/panel-edp.o:
>
> arm-linux-gnueabi-ld: drivers/gpu/drm/panel/panel-edp.o: in function `panel_edp_probe':
> panel-edp.c:(.text+0xf38): undefined reference to `drm_panel_dp_aux_backlight'
>
> Fix this by limiting DRM_PANEL_DEP by the value of the DRM_KMS_HELPER
> symbol.

I think the analysis is correct, but this is not the correct fix since
DRM_KMS_HELPER
is not user-selectable. (Almost) all other drivers that rely on DRM_KMS_HELPER
use 'select' for this, and mixing the two risks running into circular
dependencies.

I see that there are already some 'depends on DRM_KMS_HELPER' in bridge
and panel drivers, so it's possible that we have to fix them all at the same to
do this right. I ran into another problem like this the other day and
I'm currently
testing with the patch below, but I have not posted that yet since I am not
fully convinced that this is the correct fix either.

        Arnd
---
commit a836092fedaac66af03ea9ed7cb13214fd2ab8a2
Author: Arnd Bergmann <arnd@arndb.de>
Date:   Mon Nov 15 16:54:04 2021 +0100

    drm/mipi-dbi: select CONFIG_DRM_KMS_HELPER

    The driver fails to build when the KMS helpers are disabled:

    ld.lld: error: undefined symbol: drm_gem_fb_get_obj
    >>> referenced by drm_mipi_dbi.c
    >>>               gpu/drm/drm_mipi_dbi.o:(mipi_dbi_buf_copy) in
archive drivers/built-in.a
    >>> referenced by drm_mipi_dbi.c
    >>>               gpu/drm/drm_mipi_dbi.o:(mipi_dbi_fb_dirty) in
archive drivers/built-in.a

    ld.lld: error: undefined symbol: drm_gem_fb_begin_cpu_access
    >>> referenced by drm_mipi_dbi.c
    >>>               gpu/drm/drm_mipi_dbi.o:(mipi_dbi_buf_copy) in
archive drivers/built-in.a

    ld.lld: error: undefined symbol: drm_fb_swab
    >>> referenced by drm_mipi_dbi.c
    >>>               gpu/drm/drm_mipi_dbi.o:(mipi_dbi_buf_copy) in
archive drivers/built-in.a

    ld.lld: error: undefined symbol: drm_fb_xrgb8888_to_rgb565
    >>> referenced by drm_mipi_dbi.c
    >>>               gpu/drm/drm_mipi_dbi.o:(mipi_dbi_buf_copy) in
archive drivers/built-in.a

    ld.lld: error: undefined symbol: drm_fb_memcpy
    >>> referenced by drm_mipi_dbi.c
    >>>               gpu/drm/drm_mipi_dbi.o:(mipi_dbi_buf_copy) in
archive drivers/built-in.a

    This is fairly hard to hit in randconfig drivers, but it eventually
    did trigger for me in a configuration where all other DRM drivers
    are loadable modules, but DRM_PANEL_WIDECHIPS_WS2401 was built-in.

    Signed-off-by: Arnd Bergmann <arnd@arndb.de>

diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
index 0039df26854b..a03c2761c5f9 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -29,6 +29,7 @@ menuconfig DRM

 config DRM_MIPI_DBI
        tristate
+       select DRM_KMS_HELPER
        depends on DRM

 config DRM_MIPI_DSI
diff --git a/drivers/gpu/drm/bridge/Kconfig b/drivers/gpu/drm/bridge/Kconfig
index 431b6e12a81f..17a8d603e7d8 100644
--- a/drivers/gpu/drm/bridge/Kconfig
+++ b/drivers/gpu/drm/bridge/Kconfig
@@ -8,7 +8,6 @@ config DRM_BRIDGE
 config DRM_PANEL_BRIDGE
        def_bool y
        depends on DRM_BRIDGE
-       depends on DRM_KMS_HELPER
        select DRM_PANEL
        help
          DRM bridge wrapper of DRM panels
diff --git a/drivers/gpu/drm/panel/Kconfig b/drivers/gpu/drm/panel/Kconfig
index cfc8d644cedf..40ec20f3552d 100644
--- a/drivers/gpu/drm/panel/Kconfig
+++ b/drivers/gpu/drm/panel/Kconfig
@@ -140,9 +140,8 @@ config DRM_PANEL_ILITEK_IL9322
 config DRM_PANEL_ILITEK_ILI9341
        tristate "Ilitek ILI9341 240x320 QVGA panels"
        depends on OF && SPI
-       depends on DRM_KMS_HELPER
-       depends on DRM_KMS_CMA_HELPER
        depends on BACKLIGHT_CLASS_DEVICE
+       select DRM_KMS_CMA_HELPER
        select DRM_MIPI_DBI
        help
          Say Y here if you want to enable support for Ilitek IL9341