linux-kernel - Re: [PATCH] drm/bridge: Select DRM_KMS_HELPER for DRM_PANEL

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Message-Id: <CZO9HDL1N45S.1JIXM2QUFTQCS@fairphone.com>
Date: Fri, 08 Mar 2024 10:29:42 +0100
From: "Luca Weiss" <luca.weiss@...rphone.com>
To: "Dmitry Baryshkov" <dmitry.baryshkov@...aro.org>
Cc: <neil.armstrong@...aro.org>, "Andrzej Hajda" <andrzej.hajda@...el.com>,
 "Robert Foss" <rfoss@...nel.org>, "Laurent Pinchart"
 <Laurent.pinchart@...asonboard.com>, "Jonas Karlman" <jonas@...boo.se>,
 "Jernej Skrabec" <jernej.skrabec@...il.com>, "Maarten Lankhorst"
 <maarten.lankhorst@...ux.intel.com>, "Maxime Ripard" <mripard@...nel.org>,
 "Thomas Zimmermann" <tzimmermann@...e.de>, "David Airlie"
 <airlied@...il.com>, "Daniel Vetter" <daniel@...ll.ch>,
 <~postmarketos/upstreaming@...ts.sr.ht>, <phone-devel@...r.kernel.org>,
 <dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/bridge: Select DRM_KMS_HELPER for DRM_PANEL_BRIDGE

On Sun Mar 3, 2024 at 9:37 PM CET, Dmitry Baryshkov wrote:
> On Thu, 29 Feb 2024 at 11:27, Luca Weiss <luca.weiss@...rphone.com> wrote:
> >
> > On Wed Jan 17, 2024 at 9:59 AM CET, Luca Weiss wrote:
> > > On Mon Jan 15, 2024 at 9:43 AM CET, Neil Armstrong wrote:
> > > > Hi Luca,
> > > >
> > > > On 11/01/2024 13:38, Luca Weiss wrote:
> > > > > Since the kconfig symbol of DRM_PANEL_BRIDGE is only adding
> > > > > bridge/panel.o to drm_kms_helper object, we need to select
> > > > > DRM_KMS_HELPER to make sure the file is actually getting built.
> > > > >
> > > > > Otherwise with certain defconfigs e.g. devm_drm_of_get_bridge will not
> > > > > be properly available:
> > > > >
> > > > >    aarch64-linux-gnu-ld: drivers/phy/qualcomm/phy-qcom-qmp-combo.o: in function `qmp_combo_bridge_attach':
> > > > >    drivers/phy/qualcomm/phy-qcom-qmp-combo.c:3204:(.text+0x8f4): undefined reference to `devm_drm_of_get_bridge'
> > > > >
> > > > > Signed-off-by: Luca Weiss <luca.weiss@...rphone.com>
> > > > > ---
> > > > > I can see "depends on DRM_KMS_HELPER" was removed with commit
> > > > > 3c3384050d68 ("drm: Don't make DRM_PANEL_BRIDGE dependent on DRM_KMS_HELPERS")
>
> Could you please make sure that the usecase described in the mentioned
> commit message doesn't get broken by your change?

Hi Neil,

The problem fixed in that linked patch (3c3384050d68) is about fixing
undefined reference errors with specific .config setups - similar to
this patch.

Since we're only adding a 'select' and not removing anything I don't see
how it could cause new errors like that, and it does fix the one I'm
describing.

And also I checked again and I don't see any circular dependencies
(something that was also mentioned in the linked patch), so apart from
what I mentioned with that I'm not too familiar when 'select' should be
used and when 'depend' should be used, it's good from my perspective.

Regards
Luca

>
> > > > >
> > > > > I'm not too familiar with Kconfig but it feels more correct if
> > > > > PHY_QCOM_QMP_COMBO selects DRM_PANEL_BRIDGE that that's enough; and it
> > > > > doesn't also has to explicitly select DRM_KMS_HELPER because of how the
> > > > > objects are built in the Makefile.
> > > > >
> > > > > Alternatively solution to this patch could be adjusting this line in
> > > > > include/drm/drm_bridge.h:
> > > > >
> > > > >    -#if defined(CONFIG_OF) && defined(CONFIG_DRM_PANEL_BRIDGE)
> > > > >    +#if defined(CONFIG_OF) && defined(CONFIG_DRM_PANEL_BRIDGE) && defined(CONFIG_DRM_KMS_HELPER)
> > > > >     struct drm_bridge *devm_drm_of_get_bridge(struct device *dev, struct device_node *node,
> > > > >                                              u32 port, u32 endpoint);
> > > > >
> > > > > .. and then selecting DRM_KMS_HELPER for PHY_QCOM_QMP_COMBO.
> > > > >
> > > > > But I think the solution in this patch is better. Let me know what you
> > > > > think.
> > > >
> > > > I think this is no more the case after on linux-next:
> > > > 35921910bbd0 phy: qcom: qmp-combo: switch to DRM_AUX_BRIDGE
> > > >
> > > > But could you still check ?
> > >
> > > On next-20240117 the error happens in the aux-bridge file instead then.
> > >
> > > aarch64-linux-gnu-ld: drivers/gpu/drm/bridge/aux-bridge.o: in function `drm_aux_bridge_probe':
> > > drivers/gpu/drm/bridge/aux-bridge.c:115:(.text+0xe0): undefined reference to `devm_drm_of_get_bridge'
> > >
> > > I'm attaching the defconfig with which I can reproduce this but it's
> > > really just DRM_KMS_HELPER=n and PHY_QCOM_QMP_COMBO=y I believe.
> >
> > Hi Neil,
> >
> > Ping on this patch
> >
> > Regards
> > Luca
> >
> > >
> > > Regards
> > > Luca
> > >
> > >
> > > >
> > > > Neil
> > > >
> > > > > ---
> > > > >   drivers/gpu/drm/bridge/Kconfig | 1 +
> > > > >   1 file changed, 1 insertion(+)
> > > > >
> > > > > diff --git a/drivers/gpu/drm/bridge/Kconfig b/drivers/gpu/drm/bridge/Kconfig
> > > > > index ac9ec5073619..ae782b427829 100644
> > > > > --- a/drivers/gpu/drm/bridge/Kconfig
> > > > > +++ b/drivers/gpu/drm/bridge/Kconfig
> > > > > @@ -8,6 +8,7 @@ config DRM_BRIDGE
> > > > >   config DRM_PANEL_BRIDGE
> > > > >           def_bool y
> > > > >           depends on DRM_BRIDGE
> > > > > + select DRM_KMS_HELPER
> > > > >           select DRM_PANEL
> > > > >           help
> > > > >             DRM bridge wrapper of DRM panels
> > > > >
> > > > > ---
> > > > > base-commit: b9c3a1fa6fb324e691a03cf124b79f4842e65d76
> > > > > change-id: 20240111-drm-panel-bridge-fixup-5c2977fb969f
> > > > >
> > > > > Best regards,
> >