Re: [PATCH v3 1/3] drm: Introduce DRM_MODE_DUMB_KERNEL_MAP flag

[Chen-Yu Tsai]

On Fri, Mar 27, 2026 at 3:21 PM Thomas Zimmermann <tzimmermann@xxxxxxx> wrote:
>
> Hi
>
> Am 26.03.26 um 11:01 schrieb Chen-Yu Tsai:
> > From: Rob Herring <robh@xxxxxxxxxx>
> >
> > Introduce a new flag, DRM_MODE_DUMB_KERNEL_MAP, for struct
> > drm_mode_create_dumb. This flag is for internal kernel use to indicate
> > if dumb buffer allocation needs a kernel mapping. This is needed only for
> > GEM DMA where creating a kernel mapping or not has to be decided at
> > allocation time because creating a mapping on demand (with vmap()) is not
> > guaranteed to work.
>
> I still don't understand what you're trying to achieve. As I pointed
> out, ever driver's memory manager potentially requires a vmap. Passing
> around flags will not solve that problem. If vmap is not possible, the
> driver should not provide the vmap callbacks in the first place.

I'm trying to convert the Rockchip and Exynos drivers to the GEM DMA
helpers. Both have their reasons for not using the helpers, one of them
being that the DRM device is not the DMA device. This is already fixed
by my other series. Both drivers also set DMA_ATTR_NO_KERNEL_MAPPING
for buffer allocations other than the one used by fb_helper.

Exynos's GEM implementation also sets extra DMA attributes at the
allocation phase for their custom GEM create ioctl. But otherwise
exynos is just reimplementing all the GEM DMA helpers in almost
identical fashion. And to your point, exynos does not provide the
vmap callback.

Rockchip's GEM implementation splits the code path between DMA and
IOMMU usage as it needs to attach and detach the DMA device from the
IOMMU domain on the fly. This predates the DMA IOMMU framework. This
GEM implementation provides the vmap callback, which will fail for
direct DMA allocations without the kernel mapping.

There's also something fishy about Rockchip's DRM driver. It's using
fbdev-dma, even though it's not based on the GEM DMA helpers or even
drm_gem_dma_object.

As you are against changing the existing *_dumb_create() interface,
I will look into reusing drm_gem_dma_object with driver-specific
functions. I believe I will need to keep patch 2 adding dma_attrs
though. Would that be acceptable?

Once again, thank you for your patience.


ChenYu


> Best regards
> Thomas
>
> >
> > Several drivers are using reimplementing the GEM DMA helpers because
> > they distinguish between kernel and userspace allocations to create a
> > kernel mapping or not. Adding a flag allows migrating these drivers
> > to the helpers while preserving their existing behavior. These include
> > exynos, rockchip, and previously mediatek.
> >
> > Update the callers of drm_mode_dumb_create() to set
> > drm_mode_dumb_create.flags to appropriate defaults. Currently, flags can
> > be set to anything by userspace, but is unused within the kernel. Let's
> > force flags to zero (no kernel mapping) for userspace callers by default.
> > For in kernel clients, set DRM_MODE_DUMB_KERNEL_MAP by default. Drivers
> > can override this as needed.
> >
> > Signed-off-by: Rob Herring <robh@xxxxxxxxxx>
> > [wenst@xxxxxxxxxxxx: Emit warning (once) if args->flags is not zero]
> > [wenst@xxxxxxxxxxxx: Moved flag def. to include/drm/drm_dumb_buffers.h]
> > Signed-off-by: Chen-Yu Tsai <wenst@xxxxxxxxxxxx>
> > ---
> > Changes since v2:
> > - Switched to drm_warn_once()
> > - Moved flag definition from include/uapi/ to include/drm/drm_dumb_buffers.h
> > - Reworded commit message
> >
> > Changes since v1:
> > - Emit warning if args->flags is not zero
> > ---
> >   drivers/gpu/drm/drm_client.c       | 2 ++
> >   drivers/gpu/drm/drm_dumb_buffers.c | 4 ++++
> >   include/drm/drm_dumb_buffers.h     | 3 +++
> >   3 files changed, 9 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/drm_client.c b/drivers/gpu/drm/drm_client.c
> > index 46c465bce98c..3d3e61823cc1 100644
> > --- a/drivers/gpu/drm/drm_client.c
> > +++ b/drivers/gpu/drm/drm_client.c
> > @@ -14,6 +14,7 @@
> >   #include <drm/drm_client_event.h>
> >   #include <drm/drm_device.h>
> >   #include <drm/drm_drv.h>
> > +#include <drm/drm_dumb_buffers.h>
> >   #include <drm/drm_file.h>
> >   #include <drm/drm_fourcc.h>
> >   #include <drm/drm_framebuffer.h>
> > @@ -404,6 +405,7 @@ drm_client_buffer_create_dumb(struct drm_client_dev *client, u32 width, u32 heig
> >       dumb_args.width = width;
> >       dumb_args.height = height;
> >       dumb_args.bpp = drm_format_info_bpp(info, 0);
> > +     dumb_args.flags = DRM_MODE_DUMB_KERNEL_MAP;
> >       ret = drm_mode_create_dumb(dev, &dumb_args, client->file);
> >       if (ret)
> >               return ERR_PTR(ret);
> > diff --git a/drivers/gpu/drm/drm_dumb_buffers.c b/drivers/gpu/drm/drm_dumb_buffers.c
> > index e2b62e5fb891..60f4c2d08641 100644
> > --- a/drivers/gpu/drm/drm_dumb_buffers.c
> > +++ b/drivers/gpu/drm/drm_dumb_buffers.c
> > @@ -233,6 +233,10 @@ int drm_mode_create_dumb_ioctl(struct drm_device *dev,
> >       struct drm_mode_create_dumb *args = data;
> >       int err;
> >
> > +     if (args->flags)
> > +             drm_warn_once(dev, "drm_mode_create_dumb.flags is not zero.\n");
> > +     args->flags = 0;
> > +
> >       err = drm_mode_create_dumb(dev, args, file_priv);
> >       if (err) {
> >               args->handle = 0;
> > diff --git a/include/drm/drm_dumb_buffers.h b/include/drm/drm_dumb_buffers.h
> > index 1f3a8236fb3d..4657e44533f4 100644
> > --- a/include/drm/drm_dumb_buffers.h
> > +++ b/include/drm/drm_dumb_buffers.h
> > @@ -6,6 +6,9 @@
> >   struct drm_device;
> >   struct drm_mode_create_dumb;
> >
> > +/* drm_mode_create_dumb flags for internal use */
> > +#define DRM_MODE_DUMB_KERNEL_MAP     (1<<0)
> > +
> >   int drm_mode_size_dumb(struct drm_device *dev,
> >                      struct drm_mode_create_dumb *args,
> >                      unsigned long hw_pitch_align,
>
> --
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
> GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)
>
>