[PATCH] drm: Rationalize DRM_MIPI_DSI inclusion in Kconfig

[Thierry Reding]

On Thu, Aug 16, 2018 at 02:54:14PM -0400, Sean Paul wrote:
> From: Sean Paul <seanpaul at chromium.org>
> 
> DRM_MIPI_DSI is included via both "select" and "depends on", this is
> trouble waiting to happen since this will result in different behavior
> depending on which is used.
> 
> This patch resolves the problem by:
> - Converting all inclusion to "select" since DRM_MIPI_DSI is not a menu
>   item.
> - Remove "depends on DRM" since "select"ing DRM_MIPI_DSI won't include
>   DRM anyways, so this is misleading.

The reason behind the decision to have both select and depends on for
this was, if I recall correctly, so that a driver would select the
symbol if it implemented (i.e. "provided") a DSI controller and panel
drivers (or bridge drivers for that matter) would depend on the symbol
to specify that they want to "consume" a DSI controller. That way the
DSI panels and bridges can't be selected unless there is a driver that
provides a DSI controller. If there's no driver that provides a DSI
controller, there's no use in enabling any panels that need one.

Granted, in retrospect this seems somewhat over-engineered, but the
reason why it works is because thy symbol is not user-visible. That's
one of the, I think even documented, use-cases where it is safe to use
both select and depends.

That said, I'm fine with simplifying this, but if we do, I would prefer
going all the way and just removing the symbol entirely. It's not like
it is a lot of extra infrastructure, at least compared to the rest of
the helpers that we already pull in. There are also quite a few drivers
that support DSI now, so I think there's more justification for
unconditional inclusion than there used to be.

Thierry
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