linux-kernel - Re: [PATCH v2] drm/display: Select DP helper for DRM_DP_AUX_CHARDEV and DRM_DP

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Date:   Thu, 28 Apr 2022 09:26:10 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        linux-kernel@...r.kernel.org
Cc:     Lyude Paul <lyude@...hat.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2] drm/display: Select DP helper for DRM_DP_AUX_CHARDEV
 and DRM_DP_CEC

Hello Thomas,

Thanks for your feedback.

On 4/28/22 09:02, Thomas Zimmermann wrote:

[snip]

>> Changes in v2:
>> - Explain better the issue in the change description.
>> - Only select DRM_DISPLAY_DP_HELPER and not DRM_DISPLAY_HELPER.
>>
>>   drivers/gpu/drm/display/Kconfig | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/display/Kconfig b/drivers/gpu/drm/display/Kconfig
>> index f84f1b0cd23f..9fe80c4e555c 100644
>> --- a/drivers/gpu/drm/display/Kconfig
>> +++ b/drivers/gpu/drm/display/Kconfig
>> @@ -32,6 +32,7 @@ config DRM_DISPLAY_HDMI_HELPER
>>   config DRM_DP_AUX_CHARDEV
>>   	bool "DRM DP AUX Interface"
>>   	depends on DRM
>> +	select DRM_DISPLAY_DP_HELPER
> 
> You cannot select DISPLAY_DP_HELPER without DISPLAY_HELPER.
>

That was my original thought as well and what did in v1, but then I noticed
that doing that it would force DRM_DISPLAY_HELPER to be set as built-in and
not allow to be built as a module.

> Can't you simply make it depend on DISPLAY_DP_HELPER.  The menu entry 
> will show up as soon as there's a driver that selcets DISPLAY_DP_HELPER.
>

I could but then that means that once won't be able to select these two config
options unless some enable symbol selects DRM_DISPLAY_DP_HELPER.

In my opinion, DRM_DP_AUX_CHARDEV and DRM_DP_CEC are different than all other
options that select DRM_DISPLAY_DP_HELPER, since those are drivers and want to
have both DRM_DISPLAY_DP_HELPER and DRM_DISPLAY_HELPER set.

But DRM_DP_AUX_CHARDEV and DRM_DP_CEC are just included in drm_display_helper.o
if enabled, and depend on symbols that are present if CONFIG_DRM_DISPLAY_DP_HELPER
is enabled. So just need the latter, if DRM_DISPLAY_HELPER is not enabled then it
will just be a no-op.

Having written that though I noticed that a "depends on DRM_DISPLAY_HELPER" makes
sense. If you agree I can add it and post a v3.

Now, pondering more about this issue, probably the most correct thing to do is for
the drivers that make use of the symbols exported by DRM_DP_{AUX_CHARDEV,CEC} to
select these. What do you think ?
 -- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat