[Intel-gfx] [PATCH] drm/i915: Replace WARN inside I915_READ64_2x32 with retry loop

[Daniel Vetter]

On Tue, Jul 28, 2015 at 02:53:16PM +0100, Chris Wilson wrote:
> On Wed, Jul 15, 2015 at 09:50:42AM +0100, Chris Wilson wrote:
> > Since we may conceivably encounter situations where the upper part of the
> > 64bit register changes between reads, for example when a timestamp
> > counter overflows, change the WARN into a retry loop.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Michał Winiarski <michal.winiarski at intel.com>
> 
> Michał, as you correctly predicted this WARN will be hit in the wild
> after adjusting reg_read_ioctl(TIMESTAMP).
> 
> Anyone care to review this patch?

Picked up for -fixes, thanks for the patch.
-Daniel

> 
> > ---
> >  drivers/gpu/drm/i915/i915_drv.h | 17 ++++++++---------
> >  1 file changed, 8 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> > index ee53873ab252..cfddd69bd06c 100644
> > --- a/drivers/gpu/drm/i915/i915_drv.h
> > +++ b/drivers/gpu/drm/i915/i915_drv.h
> > @@ -3484,15 +3484,14 @@ int intel_freq_opcode(struct drm_i915_private *dev_priv, int val);
> >  #define I915_READ64(reg)	dev_priv->uncore.funcs.mmio_readq(dev_priv, (reg), true)
> >  
> >  #define I915_READ64_2x32(lower_reg, upper_reg) ({			\
> > -		u32 upper = I915_READ(upper_reg);			\
> > -		u32 lower = I915_READ(lower_reg);			\
> > -		u32 tmp = I915_READ(upper_reg);				\
> > -		if (upper != tmp) {					\
> > -			upper = tmp;					\
> > -			lower = I915_READ(lower_reg);			\
> > -			WARN_ON(I915_READ(upper_reg) != upper);		\
> > -		}							\
> > -		(u64)upper << 32 | lower; })
> > +	u32 upper, lower, tmp;						\
> > +	tmp = I915_READ(upper_reg);					\
> > +	do {								\
> > +		upper = tmp;						\
> > +		lower = I915_READ(lower_reg);				\
> > +		tmp = I915_READ(upper_reg);				\
> > +	} while (upper != tmp);						\
> > +	(u64)upper << 32 | lower; })
> >  
> >  #define POSTING_READ(reg)	(void)I915_READ_NOTRACE(reg)
> >  #define POSTING_READ16(reg)	(void)I915_READ16_NOTRACE(reg)
> 
> -- 
> Chris Wilson, Intel Open Source Technology Centre
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> Intel-gfx at lists.freedesktop.org
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch