[Intel-gfx] [PATCH] drm/i915/crt: Do not rely upon the HPD presence pin

[Chris Wilson]

On Mon, 11 Jun 2012 16:38:40 +0200, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> From: Chris Wilson <chris at chris-wilson.co.uk>
> 
> VGA hotplug detection "works" by measuring the resistance across
> certain pins. A lot of kvm switches fumble this and wire up cheap
> resistors with the wrong resistance or don't bother at all.
> 
> To accomodate these, also try to detect a connected monitor by trying
> to grab the edid. Contrary to !HAS_HOTPLUG platforms we don't bother
> with an actual load-detection cycle when the output is life - that
> would be actual work to implement because things moved around. This is
> the big difference to Chris Wilson's original approach:
> 
> commit 9e612a008fa7fe493a473454def56aa321479495
> Author: Chris Wilson <chris at chris-wilson.co.uk>
> Date:   Thu May 31 13:08:53 2012 +0100
> 
>     drm/i915/crt: Do not rely upon the HPD presence pin
> 
> This blew up on Linus' machine because it errornously detected a vga
> screen (without and edid and hence only the default modes), leading to
> it's prompt removal:
> 
> commit 8f53369b753f5f4c7684c2eb0b592152abb1dd00
> Author: Linus Torvalds <torvalds at linux-foundation.org>
> Date:   Fri Jun 8 14:53:06 2012 -0700
> 
>     Revert "drm/i915/crt: Do not rely upon the HPD presence pin"
> 
> Some digging around in Bspec shows the reason why load detect doesn't work on
> newer chips - the legacy VGA load detect bit isn't wired up any longer:
> 
> Public Snb Bspec, Vol3 Part1, 1.1.1 ST00 Input Status 0, bit4:
> 
> "RGB Comparator / Sense. This bit is here for compatibility and will
> always return one. Monitor detection must be done be done through the
> programming of registers in the MMIO space.
> 0 = Below threshold
> 1 = Above threshold"
> 
> v2: Add a comment in the code that load detect on hotplug capable
> machines is broken and pimp the commit message with a quote of Bspec
> to show why.
> 
> Reported-and-tested-by: Matthieu LAVIE <boiteamadmax at hotmail.com>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=50501
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>

Just missing your s-o-b, and you can add my reviewed-by to your
amendment. :)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre