linux-kernel - Re: [PATCH]"drm: change DRM_MIPI_DSI module type from "bool" to "tristate".

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Date:   Tue, 11 Oct 2016 11:53:09 +0200
From:   Andrzej Hajda <a.hajda@...sung.com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        "Sun, Jing A" <jing.a.sun@...el.com>, Takashi Iwai <tiwai@...e.de>
Cc:     "airlied@...ux.ie" <airlied@...ux.ie>,
        "Vetter, Daniel" <daniel.vetter@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Thierry Reding <treding@...dia.com>,
        Emil Velikov <emil.l.velikov@...il.com>
Subject: Re:
 [PATCH]"drm: change DRM_MIPI_DSI module type from "bool" to "tristate".

On 11.10.2016 11:33, Jani Nikula wrote:
> On Tue, 11 Oct 2016, "Sun, Jing A" <jing.a.sun@...el.com> wrote:
>> It's needed that DRM Driver module could be removed and reloaded after
>> kernel booting on the projects that I have been working on, and I hope
>> such module type change could be accepted. Looks like Iwai has similar
>> change request as well. Would you please review it and let us know if
>> any concerns?
> Looking at the Kconfig, selecting CONFIG_DRM_MIPI_DSI is against the
> recommendations of Documentation/kbuild/kconfig-language.txt:
>
> 	select should be used with care. select will force
> 	a symbol to a value without visiting the dependencies.
> 	By abusing select you are able to select a symbol FOO even
> 	if FOO depends on BAR that is not set.
> 	In general use select only for non-visible symbols
> 	(no prompts anywhere) and for symbols with no dependencies.
> 	That will limit the usefulness but on the other hand avoid
> 	the illegal configurations all over.

All existing drivers which selects DRM_MIPI_DSI also depends on DRM.
So the dependency is always true. I am not sure if it could not change in
the future, but in such case mipi_dsi bus should be completely detached from
DRM framework, I hope we have not such case yet :)

>
> Indeed, you may end up with CONFIG_DRM_MIPI_DSI=y and CONFIG_DRM=m,
> which violates DRM_MIPI_DSI dependency on CONFIG_DRM. This is broken and
> should be fixed. The suggested patch does *not* fix this issue.

At the moment it should not be possible.

Regards
Andrzej