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

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Message-ID: <877f9dd4kp.fsf@intel.com>
Date:   Wed, 12 Oct 2016 17:28:38 +0300
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Emil Velikov <emil.l.velikov@...il.com>
Cc:     "Sun\, Jing A" <jing.a.sun@...el.com>,
        Takashi Iwai <tiwai@...e.de>,
        "airlied\@linux.ie" <airlied@...ux.ie>,
        "Vetter\, Daniel" <daniel.vetter@...el.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel\@lists.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "a.hajda\@samsung.com" <a.hajda@...sung.com>,
        Thierry Reding <treding@...dia.com>
Subject: Re: [PATCH]"drm: change DRM_MIPI_DSI module type from "bool" to "tristate".

On Wed, 12 Oct 2016, Emil Velikov <emil.l.velikov@...il.com> wrote:
> On 11 October 2016 at 10:33, Jani Nikula <jani.nikula@...ux.intel.com> 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.
>>
>> 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.
>>
> Jani, git log suggests you as the unfortunate author of the select
> DRM_MIPI_DSI/select DRM_PANEL hunks in i915 ;-)

/o\

As much as my present self would like to scold my past self for all his
mistakes, I have to remind myself that it is the mistakes that have
given me invaluable experience that my past self didn't have. I can only
hope my future self will have time to fix even a fraction of the
mistakes.

Anyway, as Andrzej pointed out, all configs that select DRM_MIPI_DSI
also depend on DRM, so this problem can't currently occur. Once dsi bus
un-registration gets addressed, we can turn DRM_MIPI_DSI into a tristate
config (i.e. a loadable module).

BR,
Jani.

-- 
Jani Nikula, Intel Open Source Technology Center