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).
This issue was witnessed on the Cubietruck, which has 2GB of RAM.
Devices with less RAM function normally due to the DRAM address wrapping around. CMA seems to always allocate its buffer at a very high address, close to the end of DRAM.
On a 1GB RAM device, the physical address would be something like 0x78000000. The DRAM address 0x78000000 would access the same DRAM region as 0x38000000 on a system, as the DRAM address would only span 0x0 ~ 0x3fffffff. The bit 0x40000000 is non-functional in this case.
However on the Cubietruck, the DRAM is 2GB. The physical address is 0x40000000 ~ 0xbfffffff. The buffer would be something like 0xb8000000. But the DRAM address span 0x0 ~ 0x7fffffff, meaning the buffer address wraps around to 0x38000000, which is wrong. The correct DRAM address for it should be 0x78000000.
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);