Re: [PATCH 06/10] drm/i915: Stop using AGP layer for GEN6+

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

On Mon, 22 Oct 2012 18:34:11 -0700, Ben Widawsky <ben@bwidawsk.net> wrote:
> +/*
> + * Binds an object into the global gtt with the specified cache level. The object
> + * will be accessible to the GPU via commands whose operands reference offsets
> + * within the global GTT as well as accessible by the GPU through the GMADR
> + * mapped BAR (dev_priv->mm.gtt->gtt).
> + */
> +static void gen6_ggtt_bind_object(struct drm_i915_gem_object *obj,
> +				  enum i915_cache_level level)
> +{
> +	struct drm_device *dev = obj->base.dev;
> +	struct drm_i915_private *dev_priv = dev->dev_private;
> +	struct sg_table *st = obj->pages;
> +	struct scatterlist *sg = st->sgl;
> +	const int first_entry = obj->gtt_space->start >> PAGE_SHIFT;
> +	const int max_entries = dev_priv->mm.gtt->gtt_total_entries - first_entry;
> +	gtt_pte_t __iomem *gtt_entries = dev_priv->mm.gtt->gtt + first_entry;
> +	int unused, i = 0;
> +	unsigned int len, m = 0;
> +
> +	for_each_sg(st->sgl, sg, st->nents, unused) {
> +		len = sg_dma_len(sg) >> PAGE_SHIFT;
> +		for (m = 0; m < len; m++) {
> +			dma_addr_t addr = sg_dma_address(sg) + (m << PAGE_SHIFT);
> +			gtt_entries[i] = pte_encode(dev, addr, level);
> +			i++;
> +			if (WARN_ON(i > max_entries))
> +				goto out;
> +		}
> +	}
> +
> +out:
> +	/* XXX: This serves as a posting read preserving the way the old code
> +	 * works. It's not clear if this is strictly necessary or just voodoo
> +	 * based on what I've tried to gather from the docs.
> +	 */
> +	readl(&gtt_entries[i-1]);

It will be required until we replace the voodoo with more explicit mb().
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre