On Tue, 23 Oct 2012 07:57:12 -0700
Ben Widawsky <ben at bwidawsk.net> wrote:
> On 2012-10-23 02:59, Chris Wilson wrote:
> > On Mon, 22 Oct 2012 18:34:11 -0700, Ben Widawsky <ben at 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(>t_entries[i-1]);
> >
> > It will be required until we replace the voodoo with more explicit
> > mb().
> > -Chris
>
> Actually, after we introduce the FLSH_CNTL patch from Jesse/me later in
> the series, I think we just want a POSTING_READ on that register. It is
> technically "required" by our desire to some day WC the registers, and
> should synchronize everything else for us.
>
> After a quick read of memory_barriers.txt (again), I think mmiowb is
> actually what we might want in addition to the POSTING_READ I'd add.
On a big NUMA system maybe (i.e. on nothing we run on yet), but on x86
mmiowb doesn't do anything other than act as a compiler optimization
barrier.
--
Jesse Barnes, Intel Open Source Technology Center
