[PATCH v2] drm/sun4i: Align VI buffer addresses for subsampled formats

[Chen-Yu Tsai]

Commit 79ac1c945ab8 ("drm/sun4i: layers: Use drm_fb_dma_get_gem_addr() to
get display memory") dropped the code to calculate the framebuffer's DMA
address in favor of drm_fb_dma_get_gem_addr().

This turned out to be wrong in a couple ways. The hardware can handle
pixel offsets within a sub-sampling block, but the buffer address must
start at the beginning of the subsampled block for all planes. The
hardware then does the interpolation internally. However the helper
gives the address exact pixel for luma, which messes up the result.

Instead of reverting the commit, calculate the intra-block offset and
adjust the buffer address so that it points to the start of the block.

Fixes: 79ac1c945ab8 ("drm/sun4i: layers: Use drm_fb_dma_get_gem_addr() to get display memory")
Cc: <stable at vger.kernel.org> # 7.1+
Signed-off-by: Chen-Yu Tsai <wenst at chromium.org>
---
This is an alternative to Jernej's original revert:
https://lore.kernel.org/all/3980ea1aeb3f7fe8b4700e36560deeba3d050664.1785772659.git.jernej.skrabec@gmail.com/
---
 drivers/gpu/drm/sun4i/sun8i_vi_layer.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/gpu/drm/sun4i/sun8i_vi_layer.c b/drivers/gpu/drm/sun4i/sun8i_vi_layer.c
index 2e9cda45c04e..185e9e5211c1 100644
--- a/drivers/gpu/drm/sun4i/sun8i_vi_layer.c
+++ b/drivers/gpu/drm/sun4i/sun8i_vi_layer.c
@@ -207,6 +207,20 @@ static void sun8i_vi_layer_update_buffer(struct sun8i_layer *layer,
 		/* Get the start of the displayed memory */
 		dma_addr = drm_fb_dma_get_gem_addr(fb, state, i);
 
+		/*
+		 * The mixer can handle odd offsets into sub-sampled YUV
+		 * planes, but needs the address of the first pixel in each
+		 * sub-sampled block. Adjust the luma buffer address backwards.
+		 */
+		if (i == 0) {
+			u32 x_diff, y_diff;
+
+			x_diff = (state->src.x1 >> 16) & (format->hsub - 1);
+			y_diff = (state->src.y1 >> 16) & (format->vsub - 1);
+			dma_addr -= y_diff * fb->pitches[i];
+			dma_addr -= x_diff * format->cpp[i];
+		}
+
 		/* Set the line width */
 		DRM_DEBUG_DRIVER("Layer %d. line width: %d bytes\n",
 				 i + 1, fb->pitches[i]);
-- 
2.55.0.979.g7e5102b832-goog