Re: [PATCH v2 1/4] drm/rcar-du: Use drm_gem_dma_get_sg_table() helper to get scatter gather table

Laurent Pinchart
2026-03-18T16:02:43Z

Hi Chen-Yu,

Thank you for the patch.

On Tue, Mar 17, 2026 at 02:40:43PM +0800, Chen-Yu Tsai wrote:
> The rcar-du driver is directly calling dma_get_sgtable() on a
> drm_gem_dma_object. Not passing the dma_attrs field in may cause
> problems when DMA_ATTR_NO_KERNEL_MAPPING is added to the GEM DMA
> helpers gain support later.
>
> Instead, use the drm_gem_dma_get_sg_table() helper to get the scatter
> gather table.
>
> Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
> ---
> Changes since v1:
> - new patch
>
> Not sure if we should add a helper like drm_fb_dma_get_gem_addr(). This
> seems to be the only driver that is using a scatter gather table to pass
> DMA addresses.

The DU can't access memory directly (at least on Gen3 and newer), and
goes through a separate device called VSP that acts as an external DMA
engine (*) and compositor. This is why buffers need to be mapped
manually to the VSP, the GEM helpers would otherwise use the DU struct
device, which isn't correct. I can't use drm_device.dma_dev as the DMA
initiator can be different per CRTC.

I don't think a separate helper with a single user would be very useful
here, especially given that part of the logic in
drm_fb_dma_get_gem_addr() is in the separate vsp1 driver. Refactoring
may get messy, for little benefit.

* And in some cases the VSP further delegates memory access to the FCP,
which is yet another DMA initiator from an IOMMU point of view.

> ---
> drivers/gpu/drm/renesas/rcar-du/rcar_du_vsp.c | 12 +++++++++---
> 1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/renesas/rcar-du/rcar_du_vsp.c b/drivers/gpu/drm/renesas/rcar-du/rcar_du_vsp.c
> index 94c22d2db197..6a62608ee3a9 100644
> --- a/drivers/gpu/drm/renesas/rcar-du/rcar_du_vsp.c
> +++ b/drivers/gpu/drm/renesas/rcar-du/rcar_du_vsp.c
> @@ -291,10 +291,16 @@ int rcar_du_vsp_map_fb(struct rcar_du_vsp *vsp, struct drm_framebuffer *fb,
> dst = sg_next(dst);
> }
> } else {
> - ret = dma_get_sgtable(rcdu->dev, sgt, gem->vaddr,
> - gem->dma_addr, gem->base.size);

Did you compile the patch ? The rcdu variable is now unused. Apart from
that, it compiles fine and seems to operate as expected.

> - if (ret)
> + struct sg_table *ret_sgt;
> +
> + ret_sgt = drm_gem_dma_get_sg_table(gem);
> + if (IS_ERR(ret_sgt)) {
> + ret = PTR_ERR(ret_sgt);
> goto fail;
> + }
> +
> + memcpy(sgt, ret_sgt, sizeof(*sgt));
> + kfree(ret_sgt);

It's a bit of a shame to kmalloc() a new sg_table and free it right
after :-/ Would it be that bad to switch to dma_get_sgtable_attrs()
here, and pass gem->dma_attrs to the function ?

> }
>
> ret = vsp1_du_map_sg(vsp->vsp, sgt);

--
Regards,

Laurent Pinchart