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

[Chris Wilson]

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?

> ---
>  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