[Intel-gfx] [PATCH 3/3] drm/i915: touch VGA MSR after we enable the power well

[Daniel Vetter]

On Thu, Dec 12, 2013 at 11:36:09AM +0000, Damien Lespiau wrote:
> On Wed, Dec 11, 2013 at 06:50:10PM -0200, Paulo Zanoni wrote:
> > From: Paulo Zanoni <paulo.r.zanoni at intel.com>
> > 
> > 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 at intel.com>
> 
> The commit message is convincing!
> 
> Reviewed-by: Damien Lespiau <damien.lespiau at intel.com>

All three merged, thanks for review&patches.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch