Re: [PATCH 1/2] gpu: host1x: Allow entries in BO caches to be freed

[Thomas Huth <thuth@redhat.com>]

On 15/05/2026 04.34, Mikko Perttunen wrote:
> When a buffer object is pinned via host1x_bo_pin() with a cache, the
> resulting mapping is kept in the cache so it can be reused on subsequent
> pins. Each mapping held a reference to the underlying host1x_bo (taken
> in tegra_bo_pin / gather_bo_pin), so as long as a mapping was cached,
> the bo itself could not be freed.
> 
> However, the only way to remove the cached mapping was through the free
> path of the buffer object. This meant that if a bo got cached, it could
> never get freed again.
> 
> Resolve the circularity by holding a weak reference to the bo from the
> cache side. This is done by having the .pin callbacks not bump the bo's
> refcount -- instead the common Host1x bo code does so, except for the
> cache reference.
> 
> Also move the remove-cache-mapping-on-free code into a common function
> inside Host1x code. This is only called from the TegraDRM GEM buffers
> since those are the only ones that can be cached at the moment.
> 
> Reported-by: Aaron Kling <webgeek1234@gmail.com>
> Fixes: 1f39b1dfa53c ("drm/tegra: Implement buffer object cache")
> Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
> ---
...
> diff --git a/include/linux/host1x.h b/include/linux/host1x.h
> index 5e7a63143a4a..d8f052a85b75 100644
> --- a/include/linux/host1x.h
> +++ b/include/linux/host1x.h
> @@ -143,6 +143,12 @@ static inline struct host1x_bo_mapping *to_host1x_bo_mapping(struct kref *ref)
>   	return container_of(ref, struct host1x_bo_mapping, ref);
>   }
>   
> +/**
> + * struct host1x_bo_ops - operations implemented by a host1x_bo provider
> + *
> + * @pin: create a DMA mapping. Implementation must not touch the bo's refcount.
> + * @unpin: destroy a DMA mapping. Implementation must not touch the bo's refcount.
> + */
>   struct host1x_bo_ops {
>   	struct host1x_bo *(*get)(struct host1x_bo *bo);
>   	void (*put)(struct host1x_bo *bo);

  Hi Mikko!

FYI, this now causes some warnings during "make htmldocs":

WARNING: .../linux/include/linux/host1x.h:159 struct member 'get' not 
described in 'host1x_bo_ops'
WARNING: .../linux/include/linux/host1x.h:159 struct member 'put' not 
described in 'host1x_bo_ops'
WARNING: .../linux/include/linux/host1x.h:159 struct member 'mmap' not 
described in 'host1x_bo_ops'
WARNING: .../linux/include/linux/host1x.h:159 struct member 'munmap' not 
described in 'host1x_bo_ops'

If you've got some spare time, could you maybe send a patch to fix it?

  Thanks,
   Thomas