Re: [PATCH 13/13] drm/sun4i: Align VI buffer addresses for subsampled formats

On Tue, Aug 4, 2026 at 12:11 AM Jernej Skrabec <[email protected]> wrote:
>
> This is a partial revert of commit 79ac1c945ab8 ("drm/sun4i: layers:
> Use drm_fb_dma_get_gem_addr() to get display memory").
>
> Chroma must start at the beginning of a subsampling block, for example
> chroma start address for NV12 must be aligned to 2 pixels.
> drm_fb_dma_get_gem_addr() offsets luma by the exact source coordinates
> and chroma by the coordinates divided by the subsampling factor, so for
> odd offsets both planes no longer describe the same pixel, which the
> Display Engine scaler can't handle.
>
> Align source coordinates down for all planes instead. Remaining shift
> of one pixel is already compensated with scaler phase shift in
> sun8i_vi_layer_update_coord().

Well I think this applies to the format in general, and probably should
be fixed in drm_fb_dma_get_gem_addr() instead?

> Fixes: 79ac1c945ab8 ("drm/sun4i: layers: Use drm_fb_dma_get_gem_addr() to get display memory")
> Signed-off-by: Jernej Skrabec <[email protected]>
> ---
>  drivers/gpu/drm/sun4i/sun8i_vi_layer.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/sun4i/sun8i_vi_layer.c b/drivers/gpu/drm/sun4i/sun8i_vi_layer.c
> index 09f668c8af24..ad036cb9d88e 100644
> --- a/drivers/gpu/drm/sun4i/sun8i_vi_layer.c
> +++ b/drivers/gpu/drm/sun4i/sun8i_vi_layer.c
> @@ -197,15 +197,31 @@ static void sun8i_vi_layer_update_buffer(struct sun8i_layer *layer,
>         struct drm_plane_state *state = plane->state;
>         struct drm_framebuffer *fb = state->fb;
>         const struct drm_format_info *format = fb->format;
> +       struct drm_gem_dma_object *gem;
> +       u32 dx, dy, src_x, src_y;
>         dma_addr_t dma_addr;
>         u32 ch_base;
>         int i;
>
>         ch_base = sun8i_channel_base(layer);
>
> +       /* Adjust x and y to be divisible by subsampling factor */
> +       src_x = (state->src.x1 >> 16) & ~(format->hsub - 1);
> +       src_y = (state->src.y1 >> 16) & ~(format->vsub - 1);

AFAICT the only difference compared to drm_fb_dma_get_gem_addr()
is the masking here, i.e. round_down().

> +
>         for (i = 0; i < format->num_planes; i++) {
> -               /* Get the start of the displayed memory */
> -               dma_addr = drm_fb_dma_get_gem_addr(fb, state, i);
> +               gem = drm_fb_dma_get_gem_obj(fb, i);
> +               dma_addr = gem->dma_addr + fb->offsets[i];
> +
> +               dx = src_x;
> +               dy = src_y;
> +               if (i > 0) {
> +                       dx /= format->hsub;
> +                       dy /= format->vsub;
> +               }
> +
> +               dma_addr += dx * format->cpp[i];
> +               dma_addr += dy * fb->pitches[i];


Where as the helper has (or used to have before the blocksize stuff):

    paddr += (format->cpp[plane] * (state->src_x >> 16)) / fb->format->hsub;
    paddr += (fb->pitches[plane] * (state->src_y >> 16)) / fb->format->vsub;

Am I missing something?


ChenYu

>
>                 /* Set the line width */
>                 DRM_DEBUG_DRIVER("Layer %d. line width: %d bytes\n",
> --
> 2.43.0
>