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drm/i915/bxt: map GTT as uncached

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On Broxton per specification the GTT has to be mapped as uncached.
This was caught by the PTE write readback warning, which showed a
corrupted PTE value with using the current write-combine mapping.

v2:
- add comment explaining how the problem with WC mapping manifests
  (Daniel)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Antti Koskipää <antti.koskipaa@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
This commit is contained in:
Imre Deak 2015-03-27 13:07:33 +02:00 committed by Daniel Vetter
parent 8232edb5f7
commit 2a073f8943
1 changed files with 11 additions and 1 deletions
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12
drivers/gpu/drm/i915/i915_gem_gtt.c
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@ -2253,7 +2253,17 @@ static int ggtt_probe_common(struct drm_device *dev,
gtt_phys_addr = pci_resource_start(dev->pdev, 0) +
(pci_resource_len(dev->pdev, 0) / 2);
dev_priv->gtt.gsm = ioremap_wc(gtt_phys_addr, gtt_size);
/*
* On BXT writes larger than 64 bit to the GTT pagetable range will be
* dropped. For WC mappings in general we have 64 byte burst writes
* when the WC buffer is flushed, so we can't use it, but have to
* resort to an uncached mapping. The WC issue is easily caught by the
* readback check when writing GTT PTE entries.
*/
if (IS_BROXTON(dev))
dev_priv->gtt.gsm = ioremap_nocache(gtt_phys_addr, gtt_size);
else
dev_priv->gtt.gsm = ioremap_wc(gtt_phys_addr, gtt_size);
if (!dev_priv->gtt.gsm) {
DRM_ERROR("Failed to map the gtt page table\n");
return -ENOMEM;