[Intel-gfx] [PATCH 6/9] drm/i915: don't disable/reenable IVB error interrupts when not needed

[Daniel Vetter]

On Tue, Aug 20, 2013 at 11:43:46AM -0300, Paulo Zanoni wrote:
> 2013/8/20 Daniel Vetter <daniel at ffwll.ch>:
> > On Tue, Aug 06, 2013 at 06:57:16PM -0300, Paulo Zanoni wrote:
> >> From: Paulo Zanoni <paulo.r.zanoni at intel.com>
> >>
> >> If the error interrupts are already disabled, don't disable and
> >> reenable them. This is going to be needed when we're in PC8+, where
> >> all the interrupts are disabled so we won't risk re-enabling
> >> DE_ERR_INT_IVB.
> >>
> >> v2: Use dev_priv->irq_mask (Chris)
> >>
> >> Signed-off-by: Paulo Zanoni <paulo.r.zanoni at intel.com>
> >> ---
> >>  drivers/gpu/drm/i915/i915_irq.c | 7 +++++--
> >>  1 file changed, 5 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
> >> index d96bd1b..5e7e6f6 100644
> >> --- a/drivers/gpu/drm/i915/i915_irq.c
> >> +++ b/drivers/gpu/drm/i915/i915_irq.c
> >> @@ -1373,6 +1373,7 @@ static irqreturn_t ironlake_irq_handler(int irq, void *arg)
> >>       drm_i915_private_t *dev_priv = (drm_i915_private_t *) dev->dev_private;
> >>       u32 de_iir, gt_iir, de_ier, sde_ier = 0;
> >>       irqreturn_t ret = IRQ_NONE;
> >> +     bool err_int_reenable = false;
> >>
> >>       atomic_inc(&dev_priv->irq_received);
> >>
> >> @@ -1401,7 +1402,9 @@ static irqreturn_t ironlake_irq_handler(int irq, void *arg)
> >>        * handler. */
> >>       if (IS_HASWELL(dev)) {
> >>               spin_lock(&dev_priv->irq_lock);
> >> -             ironlake_disable_display_irq(dev_priv, DE_ERR_INT_IVB);
> >> +             err_int_reenable = ~dev_priv->irq_mask & DE_ERR_INT_IVB;
> >> +             if (err_int_reenable)
> >> +                     ironlake_disable_display_irq(dev_priv, DE_ERR_INT_IVB);
> >
> > Hm, that reminds me that this entire logic here is racy wrt concurrent
> > interrupt enabling on a different cpu core (e.g. due to a modeset now
> > again allowing display error interrupts). Do we still need this or could
> > we just ditch this entire complexity?
> 
> Can you please explain more? We still check ivb_can_enable_err_int
> before reenabling.

Yeah, but in-between someone could sneak in and enable the display error
interrupt (since modeset doesn't block it any more), but while the
interrupt is still running. I.e.

CPU 0			CPU 1
			disable DERR due to modeset

start interrupt handler,
check that DERRR is off,
do nothing

			reanable DERR due to modeset done

-> interrupt handler still running, but DERR is enabled

end interrupt handler

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch