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

Laurent Pinchart
2026-03-18T16:27:16Z

On Thu, Mar 19, 2026 at 12:21:49AM +0800, Chen-Yu Tsai wrote:
> On Thu, Mar 19, 2026 at 12:02 AM Laurent Pinchart wrote:
> > 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.
>
> I see. That's probably the most complicated hardware I've seen so far.

I felt similar when I found out Gen3 dropped the DMA engines and
compositor previously part of the DU :-)

> > > ---
> > > 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.
>
> I probably missed it in the logs. Thanks for catching it.
>
> > > - 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 ?
>
> That's another option. I was trying to isolate the DMA API calls to
> just the GEM DMA helpers, but as you said, it's a bit wasteful.

I like the goal of avoiding direct DMA mapping API calls, but I'm
worried it will cause more harm than good in this case :-/

> I'll just switch to dma_get_sgtable_attrs() and squash the change into
> the other dma_*_attrs() conversion patch.

OK.

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

--
Regards,

Laurent Pinchart