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@vger.kernel.org> # 7.1+
Signed-off-by: Chen-Yu Tsai <wenst@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

On Tue, Sep 8, 2026 at 1:02 PM Chen-Yu Tsai <wenst@chromium.org> wrote:
>
> 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@vger.kernel.org> # 7.1+
> Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>

Please ignore this one. As Sashiko pointed out, this could underflow the
buffer address.

Instead this should be sent along with another fix I have that switches
this back to the clipped buffer address.


Sorry for the noise.

> ---
> 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
>