Originally by Seth Forshee <seth.forshee@canonical.com>, 2012-10-04:
During graphics driver initialization it's useful to be able to mux
only the DDC to the inactive client in order to read the EDID. Add
a switch_ddc callback to allow capable handlers to provide this
functionality, and add vga_switcheroo_switch_ddc() to allow DRM
to mux only the DDC.
Modified by Dave Airlie <airlied@gmail.com>, 2012-12-22:
I can't figure out why I didn't like this, but I rewrote this [...]
to lock/unlock the ddc lines [...]. I think I'd prefer something
like that otherwise the interface got really ugly.
Modified by Lukas Wunner <lukas@wunner.de>, 2015-04 - 2015-10:
Change semantics of ->switch_ddc handler callback to return previous
DDC owner. Original version tried to determine previous DDC owner
with find_active_client() but this fails if the inactive client
registers before the active client.
Don't lock vgasr_mutex in _lock_ddc() / _unlock_ddc(), it can cause
deadlocks because (a) during switch (with vgasr_mutex already held),
GPU is woken and probes its outputs, tries to re-acquire vgasr_mutex
to lock DDC lines; (b) Likewise during switch, GPU is suspended and
calls cancel_delayed_work_sync() to stop output polling, if poll
task is running at this moment we may wait forever for it to finish.
Instead, lock mux_hw_lock when unregistering the handler because
the only reason why we'd want to lock vgasr_mutex in _lock_ddc() /
_unlock_ddc() is to block the handler from disappearing while DDC
lines are switched.
Also acquire mux_hw_lock in stage2 to avoid race condition where
reading the EDID and switching happens simultaneously. Likewise on
MIGD / MDIS commands and on runtime suspend.
v2.1: Overhaul locking, squash commits (Daniel Vetter)
v2.2: Readability improvements (Thierry Reding)
v2.3: Overhaul locking once more
v2.4: Retain semantics of ->switchto handler callback to switch all
pins, including DDC (Daniel Vetter)
v5: Rename ddc_lock to mux_hw_lock: Since we acquire this both
when calling ->switch_ddc and ->switchto, it protects not just
access to the DDC lines but to the mux in general. This is in
line with the DRM convention to use low-level locks to avoid
concurrent hw access (e.g. i2c, dp_aux) which are often called
hw_lock (Daniel Vetter)
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=88861
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=61115
Tested-by: Lukas Wunner <lukas@wunner.de>
[MBP 9,1 2012 intel IVB + nvidia GK107 pre-retina 15"]
Cc: Seth Forshee <seth.forshee@canonical.com>
Cc: Dave Airlie <airlied@gmail.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/e81ae9722b84c5ed591805fee3ea6dbf5dc6c4b3.1452525860.git.lukas@wunner.de
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