[Intel-gfx] [PATCH] drm/i915: Include a note about the dangers of I915_READ64/I915_WRITE64

[Daniel Vetter]

On Fri, Mar 21, 2014 at 01:16:43PM +0000, Chris Wilson wrote:
> It is important that the user is fully aware that the seemingly atomic
> read/write of a 64-bit value from MMIO space, may in fact be 2 separate
> reads of 32-bits. This can lead to hilarity, such as

s/reads/operations/

> 
> commit d18b9619034230b6f945e215276425636ca401fe
> Author: Chris Wilson <chris at chris-wilson.co.uk>
> Date:   Wed Jul 10 13:36:23 2013 +0100
> 
>     drm/i915: Fix incoherence with fence updates on Sandybridge+
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>

Indeed, there's no stern enough warning for this!

Queued for -next, thanks for the patch.
-Daniel

> ---
>  drivers/gpu/drm/i915/i915_drv.h | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 4c09fb2ff1b5..a7a3e9ed10d4 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2783,6 +2783,12 @@ void vlv_force_wake_put(struct drm_i915_private *dev_priv, int fw_engine);
>  #define I915_READ_NOTRACE(reg)		dev_priv->uncore.funcs.mmio_readl(dev_priv, (reg), false)
>  #define I915_WRITE_NOTRACE(reg, val)	dev_priv->uncore.funcs.mmio_writel(dev_priv, (reg), (val), false)
>  
> +/* Be very careful with read/write 64-bit values. On 32-bit machines, they
> + * will be implemented using 2 32-bit writes in an arbitrary order with
> + * an arbitrary delay between them. This can cause the hardware to
> + * act upon the intermediate value, possibly leading to corruption and
> + * machine death. You have been warned.
> + */
>  #define I915_WRITE64(reg, val)	dev_priv->uncore.funcs.mmio_writeq(dev_priv, (reg), (val), true)
>  #define I915_READ64(reg)	dev_priv->uncore.funcs.mmio_readq(dev_priv, (reg), true)
>  
> -- 
> 1.9.1
> 
> _______________________________________________
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> Intel-gfx at lists.freedesktop.org
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch