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    SubjectRe: [PATCH 5.18 0309/1095] drm/panel: Fix build error when CONFIG_DRM_PANEL_SAMSUNG_ATNA33XC20=y && CONFIG_DRM_DISPLAY_HELPER=m
    Hi,

    On Mon, Aug 15, 2022 at 12:09 PM Greg Kroah-Hartman
    <gregkh@linuxfoundation.org> wrote:
    >
    > From: Gao Chao <gaochao49@huawei.com>
    >
    > [ Upstream commit a67664860f7833015a683ea295f7c79ac2901332 ]
    >
    > If CONFIG_DRM_PANEL_SAMSUNG_ATNA33XC20=y && CONFIG_DRM_DISPLAY_HELPER=m,
    > bulding fails:
    >
    > drivers/gpu/drm/panel/panel-samsung-atna33xc20.o: In function `atana33xc20_probe':
    > panel-samsung-atna33xc20.c:(.text+0x744): undefined reference to
    > `drm_panel_dp_aux_backlight'
    > make: *** [vmlinux] Error 1
    >
    > Let CONFIG_DRM_PANEL_SAMSUNG_ATNA33XC20 select DRM_DISPLAY_DP_HELPER and
    > CONFIG_DRM_DISPLAY_HELPER to fix this error.
    >
    > Fixes: 32ce3b320343 ("drm/panel: atna33xc20: Introduce the Samsung ATNA33XC20 panel")
    > Reported-by: Hulk Robot <hulkci@huawei.com>
    > Signed-off-by: Gao Chao <gaochao49@huawei.com>
    > Reviewed-by: Douglas Anderson <dianders@chromium.org>
    > Signed-off-by: Douglas Anderson <dianders@chromium.org>
    > Link: https://patchwork.freedesktop.org/patch/msgid/20220524024551.539-1-gaochao49@huawei.com
    > Signed-off-by: Sasha Levin <sashal@kernel.org>
    > ---
    > drivers/gpu/drm/panel/Kconfig | 2 ++
    > 1 file changed, 2 insertions(+)

    While it doesn't hurt to land this patch as-is on 5.18 and older
    kernels, it's not quite right. The symbols that this patch 'select'
    don't actually exist on 5.18. ;-) Doing a `git grep` of
    `DRM_DISPLAY_DP_HELPER` shows no hits except the one introduced in
    this patch...

    If you want the equivalent fix for v5.18 and older, I believe you'd want:

    select DRM_DP_HELPER
    select DRM_KMS_HELPER

    See commit 3755d35ee1d2 ("drm/panel: Select DRM_DP_HELPER for
    DRM_PANEL_EDP") and commit 3c3384050d68 ("drm: Don't make
    DRM_PANEL_BRIDGE dependent on DRM_KMS_HELPERS") which added those for
    the (very similar) panel-edp.

    The first of those is what got changed in v5.19 in commit 1e0f66420b13
    ("drm/display: Introduce a DRM display-helper module")

    So I guess the tl;dr:

    * If you leave this patch in 5.18 (and 5.15), nothing bad will happen
    but the broken "randconfig" won't be fixed.

    * If you revert this patch in 5.18 (and 5.15) also nothing bad will
    happen but also the broken "randconfig" won't be fixed.

    * If someone cares about the randconfig on 5.15 / 5.18, we need a
    backport that adapts what's selected to the old symbol names.

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