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From:   Daniel G <dagmcr@gmail.com>
Date:   Fri, 22 May 2020 12:36:44 +0200
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Subject: Re: [PATCH] drm: rcar-du: Fix build error
To:     kieran.bingham+renesas@ideasonboard.com
Cc:     Daniel Vetter <daniel@ffwll.ch>, airlied@linux.ie,
        laurent.pinchart@ideasonboard.com, dri-devel@lists.freedesktop.org,
        linux-renesas-soc@vger.kernel.org,
        open list <linux-kernel@vger.kernel.org>
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Hi Kieran and Emil,

On Fri, 22 May 2020 at 11:43, Kieran Bingham
<kieran.bingham+renesas@ideasonboard.com> wrote:
>
> Hi Daniel,
>
> On 18/05/2020 21:16, Daniel Gomez wrote:
> > Select DRM_KMS_HELPER dependency.
> >
> > Build error when DRM_KMS_HELPER is not selected:
> >
> > drivers/gpu/drm/rcar-du/rcar_lvds.o:(.rodata+0xd48): undefined reference to `drm_atomic_helper_bridge_duplicate_state'
> > drivers/gpu/drm/rcar-du/rcar_lvds.o:(.rodata+0xd50): undefined reference to `drm_atomic_helper_bridge_destroy_state'
> > drivers/gpu/drm/rcar-du/rcar_lvds.o:(.rodata+0xd70): undefined reference to `drm_atomic_helper_bridge_reset'
> > drivers/gpu/drm/rcar-du/rcar_lvds.o:(.rodata+0xdc8): undefined reference to `drm_atomic_helper_connector_reset'
> > drivers/gpu/drm/rcar-du/rcar_lvds.o:(.rodata+0xde0): undefined reference to `drm_helper_probe_single_connector_modes'
> > drivers/gpu/drm/rcar-du/rcar_lvds.o:(.rodata+0xe08): undefined reference to `drm_atomic_helper_connector_duplicate_state'
> > drivers/gpu/drm/rcar-du/rcar_lvds.o:(.rodata+0xe10): undefined reference to `drm_atomic_helper_connector_destroy_state'
> >
>
> Looking at the files in rcar_du that utilise drm_atomic_helpers...
>
> git grep -l drm_atomic_helper_ drivers/gpu/drm/rcar-du/
>  drivers/gpu/drm/rcar-du/rcar_du_crtc.c
>  drivers/gpu/drm/rcar-du/rcar_du_kms.c
>  drivers/gpu/drm/rcar-du/rcar_du_plane.c
>  drivers/gpu/drm/rcar-du/rcar_du_vsp.c
>  drivers/gpu/drm/rcar-du/rcar_du_writeback.c
>  drivers/gpu/drm/rcar-du/rcar_lvds.c
>
> of those, these are configurable:
>
>  rcar_du_vsp.c          # DRM_RCAR_VSP
>  rcar_du_writeback.c    # DRM_RCAR_WRITEBACK
>  rcar_lvds.c            # DRM_RCAR_LVDS
>
> But VSP and WRITEBACK are already implicitly dependant upon DRM_RCAR_DU
> because they get built into the DU module.
>
> So indeed, only the RCAR_LVDS is a separate module, using those helpers,
> so I think a select is a reasonable fix.
>
> I would also ask whether DRM_RCAR_LVDS should depend upon DRM_RCAR_DU
> though as well.
>
> There is no linkage requirement, as it's a standalone bridge driver from
> what I can see, but I don't think it serves much purpose without the DU?

I have actually spotted when using arch/arm/configs/multi_v7_defconfig where
DRM_RCAR_DU is built as module. But when I was reviewing it I was able to
compile RCAR_LVDS=y with DRM_RCAR_DU=n. Also, according to
https://patchwork.kernel.org/patch/10159063/, the LVDS encoders used to be
described as part of the DU but not after the patch. So, I assume it can
be used without the DU but not completely sure.

>
> Anyway, even if it's just for compile testing maybe, the select here
> should be fine.
>
> Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
>
> > Signed-off-by: Daniel Gomez <dagmcr@gmail.com>
> > ---
> >  drivers/gpu/drm/rcar-du/Kconfig | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/gpu/drm/rcar-du/Kconfig b/drivers/gpu/drm/rcar-du/Kconfig
> > index 0919f1f159a4..f65d1489dc50 100644
> > --- a/drivers/gpu/drm/rcar-du/Kconfig
> > +++ b/drivers/gpu/drm/rcar-du/Kconfig
> > @@ -31,6 +31,7 @@ config DRM_RCAR_DW_HDMI
> >  config DRM_RCAR_LVDS
> >       tristate "R-Car DU LVDS Encoder Support"
> >       depends on DRM && DRM_BRIDGE && OF
> > +     select DRM_KMS_HELPER
> >       select DRM_PANEL
> >       select OF_FLATTREE
> >       select OF_OVERLAY
> >
>