[Intel-gfx] [PATCH] drm/i915: don't disable disconnected outputs

[Daniel Vetter]

On Fri, Oct 12, 2012 at 11:02 AM, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> On Thu, 11 Oct 2012 19:46:27 +0200, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
>> This piece of neat lore has been ported painstakingly and bug-for-bug
>> compatible from the old crtc helper code.
>>
>> Imo it's utter nonsense.
>>
>> If you have disconnect a cable and before you reconnect it, userspace
>> (or the kernel) does an set_crtc call, this will result in that
>> connector getting disable. Which will result in a nice black screen
>> when plugging in the cable again.
>>
>> There's absolutely no reason the kernel does such policy changes - if
>> userspace tries to set up a mode on something disconnected we might
>> fail loudly (since the dp link training fails), but silently adjusting
>> the output configuration behind userspace's back is a recipe for
>> disaster. Specifically I think that this could explain some of our
>> MI_WAIT hangs around suspend, where userspace issues a scanline wait
>> on a disable pipe. This mechanisims here could explain how that pipe
>> got disabled without userspace noticing.
>
> Indeed, it might just explain all those funky disables on other crtc
> after a setcrtc.

Yeah, those are pretty much the reason why I want to see this dead.

> Won't a side effect of this be that the system continues to power a
> disconnected output if userspace is asleep and fails to notice the
> disconnect?

Well, we _only_ disable disconnected outputs at setcrtc time. Which
means as long as userspace is sound asleep, nothing will happen at
all. Note that the crtc helper only calls disable_unused_functions
when doing a full modeset (and not just a call to ->mode_set_base). So
vt-switching or panning won't be good enough. Hence I think we can
safely ignore power consumption concerns, this wont' make it worse.

One thing I've noticed though is that some graphical xrandr tools do
not allow you to disable disconnected outputs. Which is imo again
totally brain-dead - you have to either disable the output before
unplugging (but the gui only pops up at the unplug event, so a neat
chicken/egg userinterface fail). Or you change some unrelated crtc's
mode ...

I've just looked at the drm helper code, and this logic has been added in

commit a3a0544b2c84e1d7a2022b558ecf66d8c6a8dd93
Author: Dave Airlie <airlied at redhat.com>
Date:   Mon Aug 31 15:16:30 2009 +1000

    drm/kms: add explicit encoder disable function and detach harder.

    For shared tv-out and VGA encoders, we really need to know if
    the encoder is just being switched off temporarily in blanking
    or if we are really disabling it hard.

    Also we need to try harder to disconnect encoders from unused
    connectors so we can share more efficently.

    (shared encoders stuff is coming in radeon tv-out support)

    Signed-off-by: Dave Airlie <airlied at redhat.com>

tbh I have no idea why we want this behaviour (besides that some GUI
xrandr interfaces seem to be broken like described). Adding Dave.

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch