[PATCH v2 2/4] drm: Introduce DRM_MODE_DUMB_KERNEL_MAP flag

Chen-Yu Tsai
2026-03-17T06:41:25Z

From: Rob Herring <robh@kernel.org>

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
CMA 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. Several drivers are using CMA, but not the CMA helpers because
they distinguish between kernel and userspace allocations to create a
kernel mapping or not.

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@kernel.org>
[wenst@chromium.org: Emit warning if args->flags is not zero]
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
---
Changes since v1:
- Emit warning if args->flags is not zero
---
drivers/gpu/drm/drm_client.c | 1 +
drivers/gpu/drm/drm_dumb_buffers.c | 4 ++++
include/uapi/drm/drm_mode.h | 2 ++
3 files changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/drm_client.c b/drivers/gpu/drm/drm_client.c
index 46c465bce98c..1a57e7eadc0f 100644
--- a/drivers/gpu/drm/drm_client.c
+++ b/drivers/gpu/drm/drm_client.c
@@ -404,6 +404,7 @@ 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..ccdcc401d4dc 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(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/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index 3693d82b5279..7d5f50e4af70 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -1272,6 +1272,8 @@ struct drm_mode_create_dumb {
__u64 size;
};

+#define DRM_MODE_DUMB_KERNEL_MAP (1<<0) /* For internal kernel use */
+
/* set up for mmap of a dumb scanout buffer */
struct drm_mode_map_dumb {
/** Handle for the object being mapped. */
--
2.53.0.851.ga537e3e6e9-goog