Re: [PATCH v2 1/9] dma: contiguous: Turn heap registration logic around

[Albert Esteve <aesteve@redhat.com>]

On Fri, Feb 27, 2026 at 2:23 PM Maxime Ripard <mripard@kernel.org> wrote:
>
> The CMA heap instantiation was initially developed by having the
> contiguous DMA code call into the CMA heap to create a new instance
> every time a reserved memory area is probed.
>
> Turning the CMA heap into a module would create a dependency of the
> kernel on a module, which doesn't work.
>
> Let's turn the logic around and do the opposite: store all the reserved
> memory CMA regions into the contiguous DMA code, and provide an iterator
> for the heap to use when it probes.
>
> Reviewed-by: T.J. Mercier <tjmercier@google.com>
> Signed-off-by: Maxime Ripard <mripard@kernel.org>

Reviewed-by: Albert Esteve <aesteve@redhat.com>

> ---
>  drivers/dma-buf/heaps/cma_heap.c | 18 ++----------------
>  include/linux/dma-map-ops.h      |  5 +++++
>  kernel/dma/contiguous.c          | 26 ++++++++++++++++++++++++--
>  3 files changed, 31 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/dma-buf/heaps/cma_heap.c b/drivers/dma-buf/heaps/cma_heap.c
> index bd3370b9a3f6d4e18885a1d0e8ba3f659b85ef47..f8a3d87f3ccee9630383ba28502eb40b10671cc2 100644
> --- a/drivers/dma-buf/heaps/cma_heap.c
> +++ b/drivers/dma-buf/heaps/cma_heap.c
> @@ -28,23 +28,10 @@
>  #include <linux/slab.h>
>  #include <linux/vmalloc.h>
>
>  #define DEFAULT_CMA_NAME "default_cma_region"
>
> -static struct cma *dma_areas[MAX_CMA_AREAS] __initdata;
> -static unsigned int dma_areas_num __initdata;
> -
> -int __init dma_heap_cma_register_heap(struct cma *cma)
> -{
> -       if (dma_areas_num >= ARRAY_SIZE(dma_areas))
> -               return -EINVAL;
> -
> -       dma_areas[dma_areas_num++] = cma;
> -
> -       return 0;
> -}
> -
>  struct cma_heap {
>         struct dma_heap *heap;
>         struct cma *cma;
>  };
>
> @@ -412,22 +399,21 @@ static int __init __add_cma_heap(struct cma *cma, const char *name)
>  }
>
>  static int __init add_cma_heaps(void)
>  {
>         struct cma *default_cma = dev_get_cma_area(NULL);
> +       struct cma *cma;
>         unsigned int i;
>         int ret;
>
>         if (default_cma) {
>                 ret = __add_cma_heap(default_cma, DEFAULT_CMA_NAME);
>                 if (ret)
>                         return ret;
>         }
>
> -       for (i = 0; i < dma_areas_num; i++) {
> -               struct cma *cma = dma_areas[i];
> -
> +       for (i = 0; (cma = dma_contiguous_get_reserved_region(i)) != NULL; i++) {
>                 ret = __add_cma_heap(cma, cma_get_name(cma));
>                 if (ret) {
>                         pr_warn("Failed to add CMA heap %s", cma_get_name(cma));
>                         continue;
>                 }
> diff --git a/include/linux/dma-map-ops.h b/include/linux/dma-map-ops.h
> index 60b63756df821d839436618f1fca2bfa3eabe075..3007c68a8ec5b85990d1938d04a2f05c1a71acdb 100644
> --- a/include/linux/dma-map-ops.h
> +++ b/include/linux/dma-map-ops.h
> @@ -110,10 +110,11 @@ bool dma_release_from_contiguous(struct device *dev, struct page *pages,
>                                  int count);
>  struct page *dma_alloc_contiguous(struct device *dev, size_t size, gfp_t gfp);
>  void dma_free_contiguous(struct device *dev, struct page *page, size_t size);
>
>  void dma_contiguous_early_fixup(phys_addr_t base, unsigned long size);
> +struct cma *dma_contiguous_get_reserved_region(unsigned int idx);
>  #else /* CONFIG_DMA_CMA */
>  static inline struct cma *dev_get_cma_area(struct device *dev)
>  {
>         return NULL;
>  }
> @@ -148,10 +149,14 @@ static inline void dma_free_contiguous(struct device *dev, struct page *page,
>         __free_pages(page, get_order(size));
>  }
>  static inline void dma_contiguous_early_fixup(phys_addr_t base, unsigned long size)
>  {
>  }
> +static inline struct cma *dma_contiguous_get_reserved_region(unsigned int idx)
> +{
> +       return NULL;
> +}
>  #endif /* CONFIG_DMA_CMA*/
>
>  #ifdef CONFIG_DMA_DECLARE_COHERENT
>  int dma_declare_coherent_memory(struct device *dev, phys_addr_t phys_addr,
>                 dma_addr_t device_addr, size_t size);
> diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
> index c56004d314dc2e436cddf3b20a4ee6ce8178bee4..14bd54fb758537f01a6fe27318e7b683964e20b1 100644
> --- a/kernel/dma/contiguous.c
> +++ b/kernel/dma/contiguous.c
> @@ -456,10 +456,32 @@ void dma_free_contiguous(struct device *dev, struct page *page, size_t size)
>  #include <linux/of_reserved_mem.h>
>
>  #undef pr_fmt
>  #define pr_fmt(fmt) fmt
>
> +static struct cma *rmem_cma_areas[MAX_CMA_AREAS];
> +static unsigned int rmem_cma_areas_num;
> +
> +static int rmem_cma_insert_area(struct cma *cma)
> +{
> +       if (rmem_cma_areas_num >= ARRAY_SIZE(rmem_cma_areas))
> +               return -EINVAL;
> +
> +       rmem_cma_areas[rmem_cma_areas_num++] = cma;
> +
> +       return 0;
> +}
> +
> +struct cma *dma_contiguous_get_reserved_region(unsigned int idx)
> +{
> +       if (idx >= rmem_cma_areas_num)
> +               return NULL;
> +
> +       return rmem_cma_areas[idx];
> +}
> +EXPORT_SYMBOL_GPL(dma_contiguous_get_reserved_region);
> +
>  static int rmem_cma_device_init(struct reserved_mem *rmem, struct device *dev)
>  {
>         dev->cma_area = rmem->priv;
>         return 0;
>  }
> @@ -504,13 +526,13 @@ static int __init rmem_cma_setup(struct reserved_mem *rmem)
>         rmem->priv = cma;
>
>         pr_info("Reserved memory: created CMA memory pool at %pa, size %ld MiB\n",
>                 &rmem->base, (unsigned long)rmem->size / SZ_1M);
>
> -       err = dma_heap_cma_register_heap(cma);
> +       err = rmem_cma_insert_area(cma);
>         if (err)
> -               pr_warn("Couldn't register CMA heap.");
> +               pr_warn("Couldn't store CMA reserved area.");
>
>         return 0;
>  }
>  RESERVEDMEM_OF_DECLARE(cma, "shared-dma-pool", rmem_cma_setup);
>  #endif
>
> --
> 2.53.0
>
>