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

On Tue, Aug 4, 2026 at 1:25 AM Chen-Yu Tsai <[email protected]> wrote:
>
> 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?

After some headbanging on my end I see that the offset for the Y plane
needs to be rounded down.

But instead of reverting the whole thing and open-coding the helper
again, could you adjust the address returned by the helper for odd
offsets?

And just a heads up, this also needs a clipped version of
drm_fb_dma_get_gem_addr() as sun8i_ui_layer_update_coord() uses the
clipped dimensions. I am currently working on this part.


ChenYu

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