Re: [PATCH v6 01/16] drm/prime: Simplify life of drivers needing custom dma_buf_ops

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

Am 26.11.25 um 13:44 schrieb Boris Brezillon:
> drm_gem_is_prime_exported_dma_buf() checks the dma_buf->ops against
> drm_gem_prime_dmabuf_ops, which makes it impossible to use if the
> driver implements custom dma_buf_ops. Instead of duplicating a bunch
> of helpers to work around it, let's provide a way for drivers to
> expose their custom dma_buf_ops so the core prime helpers can rely on
> that instead of hardcoding &drm_gem_prime_dmabuf_ops.

This can't go in as-is. I've spent an awful amount of patches on 
removing buffer callbacks from struct drm_driver. Let's please not go 
back to that.

>
> v5:
> - New patch
>
> v6:
> - Pass custom dma_buf_ops directly instead of through a getter
>
> Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> ---
>   drivers/gpu/drm/drm_prime.c | 10 ++++++++--
>   include/drm/drm_drv.h       |  8 ++++++++
>   2 files changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index 21809a82187b..86fd95f0c105 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -904,6 +904,12 @@ unsigned long drm_prime_get_contiguous_size(struct sg_table *sgt)
>   }
>   EXPORT_SYMBOL(drm_prime_get_contiguous_size);
>   
> +static const struct dma_buf_ops *
> +drm_gem_prime_get_dma_buf_ops(struct drm_device *dev)
> +{
> +	return dev->driver->dma_buf_ops ?: &drm_gem_prime_dmabuf_ops;
> +}
> +
>   /**
>    * drm_gem_prime_export - helper library implementation of the export callback
>    * @obj: GEM object to export
> @@ -920,7 +926,7 @@ struct dma_buf *drm_gem_prime_export(struct drm_gem_object *obj,
>   	struct dma_buf_export_info exp_info = {
>   		.exp_name = KBUILD_MODNAME, /* white lie for debug */
>   		.owner = dev->driver->fops->owner,
> -		.ops = &drm_gem_prime_dmabuf_ops,
> +		.ops = drm_gem_prime_get_dma_buf_ops(dev),

Rather provide a new function drm_gem_prime_export_with_ops() that takes 
an additional dma_ops instance. The current drm_gem_prime_export() would 
call it with &drm_gem_prime_dmabuf_ops.

If this really does not work, you could add a pointer to dma_buf_ops to 
drm_gem_object_funcs and fetch that from drm_gem_prime_export(). We 
already vm_ops there.

Other drivers, such as amdgpu, would also benefit from such a change

>   		.size = obj->size,
>   		.flags = flags,
>   		.priv = obj,
> @@ -947,7 +953,7 @@ bool drm_gem_is_prime_exported_dma_buf(struct drm_device *dev,
>   {
>   	struct drm_gem_object *obj = dma_buf->priv;
>   
> -	return (dma_buf->ops == &drm_gem_prime_dmabuf_ops) && (obj->dev == dev);
> +	return dma_buf->ops == drm_gem_prime_get_dma_buf_ops(dev) && obj->dev == dev;

This is a bit more complicated and the test has been a pain point 
before. For this case, I think we should add a GEM callback for this

struct drm_gem_object_funcs {
     bool (*exported_by)(struct drm_gem_object *obj, struct drm_device *dev)
}

next to the existing export callback.

And drm_gem_is_prime_exported_dma_buf would then do

{
     if (obj->funcs->exported_by)
        return obj->funcs-<exported_by(obj, dev)

    return /* what we currently test */
}

IIRC amdgpu would again benefit from this.

These changes will isolate dma_buf handling near GEM code, keep 
drm_driver clean, and even allow for a driver to have different 
implementations of dma_buf_ops.

Best regards
Thomas

>   }
>   EXPORT_SYMBOL(drm_gem_is_prime_exported_dma_buf);
>   
> diff --git a/include/drm/drm_drv.h b/include/drm/drm_drv.h
> index 42fc085f986d..1c6dae60d523 100644
> --- a/include/drm/drm_drv.h
> +++ b/include/drm/drm_drv.h
> @@ -431,6 +431,14 @@ struct drm_driver {
>   	 * some examples.
>   	 */
>   	const struct file_operations *fops;
> +
> +	/**
> +	 * @dma_buf_ops:
> +	 *
> +	 * dma_buf_ops to use for buffers exported by this driver. When NULL,
> +	 * the drm_prime logic defaults to &drm_gem_prime_dmabuf_ops.
> +	 */
> +	const struct dma_buf_ops *dma_buf_ops;
>   };
>   
>   void *__devm_drm_dev_alloc(struct device *parent,

-- 
--
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)