drm/i915: touch VGA MSR after we enable the power well

drm/i915: touch VGA MSR after we enable the power well

Fixes regression introduced by:
    commit bf51d5e2cda5d36d98e4b46ac7fca9461e512c41
    Author: Paulo Zanoni <paulo.r.zanoni at intel.com>
    Date:   Wed Jul 3 17:12:13 2013 -0300
        drm/i915: switch disable_power_well default value to 1

The bug I'm seeing can be reproduced with:
  - Have vgacon configured/enabled
  - Make sure the power well gets disabled, then enabled. You can
    check this by seeing the messages print by hsw_set_power_well
  - Stop your display manager
  - echo 0 > /sys/class/vtconsole/vtcon1/bind

I can easily reproduce this by blacklising snd_hda_intel and booting
with eDP+HDMI.

If you do this and then look at dmesg, you'll see we're printing
infinite "Unclaimed register" messages. This is happening because
we're stuck on an infinite loop inside console_unlock(), which is
calling many functions from vgacon.c. And the code that's triggering
the error messages is from vgacon_set_cursor_size().

After we re-enable the power well, every time we read/write the VGA
address 0x3d5 we get an "unclaimed register" interrupt (ERR_INT) and
print error messages. If we write anything to the VGA MSR register (it
doesn't really matter which value you write to bit 0), any
reads/writes to 0x3d5 _don't_ trigger the "unclaimed register" errors
anymore (even if MSR bit 0 is zero). So what happens with the current
code is that when we unbind i915 and bind vgacon, we call
console_unlock(). Function console_unlock() is responsible for
printing any messages that were supposed to be print when the console
was locked, so it calls the TTY layer, which calls the console layer,
which calls vgacon to print the messages. At this point, vgacon
eventually calls vgacon_set_cursor_size(), which touches 0x3d5, which
triggers unclaimed register interrupts. The problem is that when we
get these interrupts, we print the error messages, so we add more work
to console_unlock(), which will try to print it again, and then call
vgacon again, trigger a new interrupt, which will put more stuff to
the buffer, and then we'll be stuck at console_unlock() forever.

If you patch intel_uncore.c to not print anything when we detect
unclaimed registers, we won't get into the console_unlock() infinite
loop and the driver unbind will work just fine. We will still be
getting interrupts every time vgacon touches those registers, but we
will survive. This is a valid experiment, but IMHO it's not the real
fix: if we don't print any error messages we will still keep getting
the interrupts, and if we disable ERR_INT we won't get the interrupt
anymore, but we will also stop getting all the other error interrupts.

I talked about this problem with the HW engineer and his
recommendation is "So don't do any VGA I/O or memory access while the
power well is disabled, and make to re-program MSR after enabling the
power well and before using VGA I/O or memory accesses.".

Notice that this is just a partial fix to fd.o #67813. This fixes the
case where the power well is already enabled when we unbind, not when
it's disabled when we unbind.

V2: - Rebase (first version was sent in September).
V3: - Complete rewrite of the same fix: smaller implementation,
      improved commit message.

Testcase: igt/drv_module_reload
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67813
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>