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

[Boris Brezillon <boris.brezillon@collabora.com>]

On Thu, 27 Nov 2025 09:58:55 +0100
Thomas Zimmermann <tzimmermann@suse.de> wrote:

> Hi
> 
> Am 27.11.25 um 09:42 schrieb Thomas Zimmermann:
> > Hi
> >
> > Am 27.11.25 um 09:34 schrieb Thomas Zimmermann:  
> >> 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;  
> >
> > On a second thought, we probably cannot be sure that dma_buf->priv 
> > really is a GEM object until we tested the ops field. :/  IIRC that's 
> > why the ops test goes first and the test for obj->dev goes second. So 
> > neither solution works.  
> 
> I think, instead of looking at the ops field, the test could look at 
> dma_buf->owner == dev->driver->fops->owner.  This will tell if the 
> dma_buf comes from the same driver and hence is a GEM object. In the 
> next step, do obj->dev == dev as before.  This will also allow drivers 
> like amdgpu to use the helper for testing. See [1].

Except this doesn't work when the driver is linked statically (not
enabled as a module), because THIS_MODULE is NULL in that case.