linux-kernel - Re: [PATCH] drm/bridge: dw_hdmi: add cec notifier support

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Date:   Thu, 6 Jul 2017 12:56:43 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Neil Armstrong <narmstrong@...libre.com>
Cc:     architt@...eaurora.org, a.hajda@...sung.com,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-amlogic@...ts.infradead.org
Subject: Re: [PATCH] drm/bridge: dw_hdmi: add cec notifier support

On Thu, Jul 06, 2017 at 12:45:55PM +0100, Russell King - ARM Linux wrote:
> Well, from what I can see in 4.12, the cec-notifier stuff is rather
> broken (tda998x has stopped working as its stuck with a physical
> address of f.f.f.f) so I think the whole thing is rather moot right
> now.  I don't yet know what's going on with that, other than the
> notifier stuff seems to not be working, despite being enabled in
> the .config.

The problem there appears to be the changes that were made with the
way the config works - which IMHO are totally broken.

Let's take this scenario:

- You have a HDMI bridge, and you build that into the kernel, because you
  want the display to come up early.
- You have a CEC driver, which you build as a module.

If the HDMI bridge driver selects CEC_NOTIFIER and the CEC driver selects
both CEC_NOTIFIER and CEC_CORE, you end up with CEC_NOTIFIER=y and
CEC_CORE=m.

We now come to this:

#if IS_REACHABLE(CONFIG_CEC_CORE) && IS_ENABLED(CONFIG_CEC_NOTIFIER)

The definition of IS_REACHABLE() is that it is false if the config symbol
is selected as a module.  So, in this case, we end up compiling out all
the CEC notifier functions from the HDMI bridge, and building them into
the CEC driver.

The CEC notifier also gets built as a module, meaning that there's no way
for the built-in HDMI bridge could ever call the notifier.

The overall result of this is that such a configuration completely breaks
such a setup - a setup that worked fine before the CEC Kconfig changes.

This isn't limited to tda998x - I'd expect the same to be true of dw-hdmi.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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