[PATCH 5/7] drm/sun4i: backend: Offset layer buffer address by DRAM starting address

[Chen-Yu Tsai <wens@csie.org>]

The display backend, as well as other peripherals that have a DRAM
clock gate and access DRAM directly, bypassing the system bus,
address the DRAM starting from 0x0, while physical addresses the
system uses starts from 0x40000000 (or 0x20000000 in A80's case).

Correct the address configured into the backend layer registers
by PHYS_OFFSET to account for this.

Fixes: 9026e0d122ac ("drm: Add Allwinner A10 Display Engine support")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
---
 drivers/gpu/drm/sun4i/sun4i_backend.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/sun4i/sun4i_backend.c b/drivers/gpu/drm/sun4i/sun4i_backend.c
index 4fefd8add714..d18d7e88d511 100644
--- a/drivers/gpu/drm/sun4i/sun4i_backend.c
+++ b/drivers/gpu/drm/sun4i/sun4i_backend.c
@@ -216,6 +216,13 @@ int sun4i_backend_update_layer_buffer(struct sun4i_backend *backend,
 	paddr = drm_fb_cma_get_gem_addr(fb, state, 0);
 	DRM_DEBUG_DRIVER("Setting buffer address to %pad\n", &paddr);
 
+	/*
+	 * backend DMA accesses DRAM directly, bypassing the system
+	 * bus. As such, the address range is different and the buffer
+	 * address needs to be corrected.
+	 */
+	paddr -= PHYS_OFFSET;
+
 	/* Write the 32 lower bits of the address (in bits) */
 	lo_paddr = paddr << 3;
 	DRM_DEBUG_DRIVER("Setting address lower bits to 0x%x\n", lo_paddr);
-- 
2.14.2