drm/etnaviv: move GPU linear window to end of DMA window

If the end of the system DMA window is farther away from the start of
physical RAM than the size of the GPU linear window, move the linear
window so that it ends at the same address than the system DMA window.

This allows to map command buffer from CMA, which is likely to reside
at the end of the system DMA window, while also overlapping as much
RAM as possible, in order to optimize regular buffer mappings through
the linear window.
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>

drivers/gpu/drm/etnaviv/etnaviv_gpu.c

@@ -1563,6 +1563,7 @@ static int etnaviv_gpu_platform_probe(struct platform_device *pdev)
 {
 	struct device *dev = &pdev->dev;
 	struct etnaviv_gpu *gpu;
+	u32 dma_mask;
 	int err = 0;
 
 	gpu = devm_kzalloc(dev, sizeof(*gpu), GFP_KERNEL);
@@ -1573,12 +1574,16 @@ static int etnaviv_gpu_platform_probe(struct platform_device *pdev)
 	mutex_init(&gpu->lock);
 
 	/*
-	 * Set the GPU base address to the start of physical memory.  This
-	 * ensures that if we have up to 2GB, the v1 MMU can address the
-	 * highest memory.  This is important as command buffers may be
-	 * allocated outside of this limit.
+	 * Set the GPU linear window to be at the end of the DMA window, where
+	 * the CMA area is likely to reside. This ensures that we are able to
+	 * map the command buffers while having the linear window overlap as
+	 * much RAM as possible, so we can optimize mappings for other buffers.
 	 */
-	gpu->memory_base = PHYS_OFFSET;
+	dma_mask = (u32)dma_get_required_mask(dev);
+	if (dma_mask < PHYS_OFFSET + SZ_2G)
+		gpu->memory_base = PHYS_OFFSET;
+	else
+		gpu->memory_base = dma_mask - SZ_2G + 1;
 
 	/* Map registers: */
 	gpu->mmio = etnaviv_ioremap(pdev, NULL, dev_name(gpu->dev));