Re: [PATCH 2/4] drm/i915: protect backlight registers and data with a spinlock

[Chris Wilson <chris@chris-wilson.co.uk>]

On Mon, Apr 15, 2013 at 06:59:58PM +0200, Daniel Vetter wrote:
> On Mon, Apr 15, 2013 at 6:38 PM, Jani Nikula <jani.nikula@intel.com> wrote:
> > On Mon, 15 Apr 2013, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> >> On Fri, Apr 12, 2013 at 03:18:37PM +0300, Jani Nikula wrote:
> >>> Backlight data and registers are fiddled through LVDS/eDP modeset
> >>> enable/disable hooks, backlight sysfs files, asle interrupts, and register
> >>> save/restore. Protect the backlight related registers and driver private
> >>> fields using a spinlock.
> >>>
> >>> The locking in register save/restore covers a little more than is strictly
> >>> necessary, including non-modeset case, for simplicity.
> >>>
> >>> v2: Cover register access, save/restore, i915_read_blc_pwm_ctl() and code
> >>>     paths leading there.
> >>>
> >>> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
> >>
> >> Looks reasonable.
> >>
> >> intel_panel_actually_set_backlight() should have a WARN_ON(!spinlocked);
> >>
> >> The irqness of the register writes scares me slightly - since the IRQ in
> >> question is from ACPI and we have a few bug reports along the lines of
> >> "backlight makes the entire system sluggish" i.e. commonly associated
> >> with bad interrupt handling. Whilst you are looking at updating the
> >> backlight programming, can you look at pushing the writes from out
> >> of the interrupt handler?
> >
> > So, add a work to do the register writes, and change the spinlock into a
> > mutex while at it? Should be fairly simple, if you think that's the way
> > to go.
> 
> I think I'll go ahead with the spinlock fix here for 3.10 and we can
> look into offloading this all for 3.11. Also, Chris do you remember
> one of these reports - I've kinda never noticed that particular kind
> of suck?

I maybe way off the mark and the cause is likely just to be a sucky ACPI
firmware:
https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/1019370

spinlock -> mutex + workqueue would mitigate against any bad firmware
being run under irq context.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre