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

Hi

Am 17.03.26 um 11:58 schrieb Chen-Yu Tsai:

On Tue, Mar 17, 2026 at 4:12 PM Thomas Zimmermann [off-list ref] wrote:

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Hi

Am 17.03.26 um 07:40 schrieb Chen-Yu Tsai:

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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.

Any dumb allocation might require vmap. This is not limited to internal
buffers.  Because if the dumb buffer is shared with another driver, that
driver might require the vmap for displaying the buffer content. There
are enough of such drivers.

By sharing, do you mean exported via PRIME then imported by another device?
Or just passing around GEM objects within the kernel?

I mean PRIME/dma-buf sharing.

And could you provide an example?

Create a dumb GEM buffer on one of those CMA devices and mirror it to a 
display or udl device.  These devices have no means of scanning out the 
provided buffer directly, so their drivers vmap the buffer and copy the 
pixel data into their internal memory (via a peripheral bus). The vmap 
call happens via the kernel's internal dma-buf interfaces. Hence any 
driver needs support for vmap if it wants to support this use case.

Best regards
Thomas

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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");

At most use drm_warn_once() to not let this spam the kernel log.

Sure.

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+     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 */
+

If this is an internal flag, it should not live in the UAPI header.
include/drm/drm_dumb_buffers.h would be a good place.

Makes sense.


Thanks
ChenYu

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Best regards
Thomas

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   /* set up for mmap of a dumb scanout buffer */
   struct drm_mode_map_dumb {
       /** Handle for the object being mapped. */

--
--
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)

-- 
--
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)