Re: [PATCH] drm/crtc-helper: don't disable disconnected outputs

[Dave Airlie <airlied@gmail.com>]

On Sun, Jun 16, 2013 at 6:41 AM, Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> This has originally been added in
>
> commit a3a0544b2c84e1d7a2022b558ecf66d8c6a8dd93
> Author: Dave Airlie <airlied@redhat.com>
> Date:   Mon Aug 31 15:16:30 2009 +1000
>
>     drm/kms: add explicit encoder disable function and detach harder.
>
> I think it's the wrong thing to do for a few reasons:
> - It's policy in the kernel. Userspace gets hotplug events when an
>   output disconnects, and it can inquire about all the routing that is
>   already set up. If userspace wants to disable a disconnected output
>   it can simply do so itself.
> - The reason for adding it given in the commit message was to improve
>   use of shared encoders. But the disable_unused_functions call only
>   happens after the modeset has been completed, so it won't help too
>   much in making the modest succeed.
> - We (at least in drm/i915, but iirc all other drivers work the same)
>   ensure that if the user accidentally yanks the cable and then
>   replugs, the same configuration will keep on working without any
>   userspace actions required. For most outputs that's the case by
>   default, and DP can have the same semantics with a simple re-train
>   handler fired off from the hpd replug event. This breaks it since if
>   the user is unlucky and hits the mouse, resulting in a panning of
>   e.g. the integrated panel that modeset might kill the accidentally
>   yanked output. Which isn't too great.
>
> i915 has applied this behaviour change as part of the bit modeset
> review, thus far without resulting in an angry crowd with pitchforks:
>

The reason is that i915 hardware is nearly impossible to ever set
something up that could trigger this on. You don't have
shared encoders in places that could affect this from what I can see.

I know this will break the radeon card I have that I wrote this for
originally, where if you alternate hotplugging a tv-out
and vga that share the same encoder it won't work. I can probably drag
that card out of whatever hole if fell into,
but the given justifications have give me no confidence that this
patch just isn't reverting the original commit without
fixing anything else.

I suspect I was testing hotplugging at fbcon not using X also.

Dave.