drm/i915: Limit the ioremap of the PCI bar to the registers

In the future we may like to experiment with using a WC map of the GTT
portion. However, that will conflict with i915.ko mapping the entire bar
as UC in order to access the GPU registers. Instead we can shrink the
register ioremap to only map the register block.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by (IVB): Ben Widawsky <ben@bwidawsk.net>
Acked-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Squashed-in follow-up fix for gen2/3 registers file size from
Chris Wilson.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_dma.c

@@ -1461,7 +1461,7 @@ int i915_driver_load(struct drm_device *dev, unsigned long flags)
 {
 	struct drm_i915_private *dev_priv;
 	struct intel_device_info *info;
-	int ret = 0, mmio_bar;
+	int ret = 0, mmio_bar, mmio_size;
 	uint32_t aperture_size;
 
 	info = (struct intel_device_info *) flags;
@@ -1526,7 +1526,19 @@ int i915_driver_load(struct drm_device *dev, unsigned long flags)
 		dma_set_coherent_mask(&dev->pdev->dev, DMA_BIT_MASK(32));
 
 	mmio_bar = IS_GEN2(dev) ? 1 : 0;
-	dev_priv->regs = pci_iomap(dev->pdev, mmio_bar, 0);
+	/* Before gen4, the registers and the GTT are behind different BARs.
+	 * However, from gen4 onwards, the registers and the GTT are shared
+	 * in the same BAR, so we want to restrict this ioremap from
+	 * clobbering the GTT which we want ioremap_wc instead. Fortunately,
+	 * the register BAR remains the same size for all the earlier
+	 * generations up to Ironlake.
+	 */
+	if (info->gen < 5)
+		mmio_size = 512*1024;
+	else
+		mmio_size = 2*1024*1024;
+
+	dev_priv->regs = pci_iomap(dev->pdev, mmio_bar, mmio_size);
 	if (!dev_priv->regs) {
 		DRM_ERROR("failed to map registers\n");
 		ret = -EIO;