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

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Message-ID: <cdad47d0-939d-9eda-feae-78cf204a0ae6@suse.de>
Date:   Thu, 28 Apr 2022 10:04:32 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Javier Martinez Canillas <javierm@...hat.com>,
        linux-kernel@...r.kernel.org
Cc:     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

Hi

Am 28.04.22 um 09:52 schrieb Javier Martinez Canillas:
> On 4/28/22 09:45, Thomas Zimmermann wrote:
> 
> [snip]
> 
>>>> 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.
>>
>> It was a rhetorical only. I didn't mean to actually set DISPLAY_HELPER.
>>
> 
> Ah, sorry for misunderstanding.
> 
>>>    
>>>> 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.
>>
>> Yes please.  These options enable features of the DP code. If there's no
>> driver with DP, it doesn't make sense to allow them.
>>
>> I know that there could be an odd situation where userspace might not
>> have DP, but still wants the chardev file of aux bus.  But that
>> situation existed already when the code was located within KMS helpers.
>>
> 
> Agreed.
> 
>>>
>>> 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 ?
>>
>> That's not considered good style. Select should not be used for anything
>> that is user-configurable. [1]
>>
> 
> Right. So giving even more thought to this, now I think that we should just include
> drm_dp_aux_dev.o, drm_dp_cec.o (and probably drm_dp_aux_bus.o?) unconditionally to
> drm_display_helper-$(CONFIG_DRM_DISPLAY_DP_HELPER).
> 
> After all, these are not big objects and drm_display_helper can now be built as module.
> 
> I don't see that much value to have separate user-configurable config options...
> 

I don't know the side effects of this. We're exporting another device 
file after all.

For know I'd make them depend on DRM_DISPLAY_DP_HELPER. If someone 
complains we can revert and fix the linker error by adding stub 
functions for the missing symbols.

Best regards
Thomas

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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