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

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Date: Mon, 18 Mar 2024 10:04:28 +0100
From: neil.armstrong@...aro.org
To: Luca Weiss <luca.weiss@...rphone.com>,
 Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc: 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 08/03/2024 10:29, Luca Weiss wrote:
> 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.

Sure, LGTM:

Reviewed-by: Neil Armstrong <neil.armstrong@...aro.org>

> 
> 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,
>>>
>