Re: [RESEND PATCH v3] drm/cma-helper: Describe what a "contiguous chunk" actually means

[Lucas Stach]

Am Mittwoch, dem 27.04.2022 um 15:09 +0100 schrieb Daniel Thompson:
> Since it's inception in 2012 it has been understood that the DRM GEM CMA
> helpers do not depend on CMA as the backend allocator. In fact the first
> bug fix to ensure the cma-helpers work correctly with an IOMMU backend
> appeared in 2014. However currently the documentation for
> drm_gem_cma_create() talks about "a contiguous chunk of memory" without
> making clear which address space it will be a contiguous part of.
> Additionally the CMA introduction is actively misleading because it only
> contemplates the CMA backend.
> 
> This matters because when the device accesses the bus through an IOMMU
> (and don't use the CMA backend) then the allocated memory is contiguous
> only in the IOVA space. This is a significant difference compared to the
> CMA backend and the behaviour can be a surprise even to someone who does
> a reasonable level of code browsing (but doesn't find all the relevant
> function pointers ;-) ).
> 
> Improve the kernel doc comments accordingly.
> 
> Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>

Reviewed-by: Lucas Stach <l.stach@xxxxxxxxxxxxxx>

> ---
> 
> Notes:
>     RESEND is unaltered but rebased on v5.18-rc3.
>     
>     Changes in v3:
>     - Rebased on v5.17-rc2
>     - Minor improvements to wording.
>     
>     Changes in v2:
>     - Oops. I did a final proof read and accidentally committed these
>       changes as a seperate patch. This means that v1 contains only
>       one tenth of the actual patch. This is fixed in v2. Many apologies
>       for the noise!
> 
>  drivers/gpu/drm/drm_gem_cma_helper.c | 39 +++++++++++++++++++++-------
>  1 file changed, 29 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_cma_helper.c b/drivers/gpu/drm/drm_gem_cma_helper.c
> index f36734c2c9e1..42abee9a0f4f 100644
> --- a/drivers/gpu/drm/drm_gem_cma_helper.c
> +++ b/drivers/gpu/drm/drm_gem_cma_helper.c
> @@ -26,12 +26,22 @@
>  /**
>   * DOC: cma helpers
>   *
> - * The Contiguous Memory Allocator reserves a pool of memory at early boot
> - * that is used to service requests for large blocks of contiguous memory.
> + * The DRM GEM/CMA helpers are a means to provide buffer objects that are
> + * presented to the device as a contiguous chunk of memory. This is useful
> + * for devices that do not support scatter-gather DMA (either directly or
> + * by using an intimately attached IOMMU).
>   *
> - * The DRM GEM/CMA helpers use this allocator as a means to provide buffer
> - * objects that are physically contiguous in memory. This is useful for
> - * display drivers that are unable to map scattered buffers via an IOMMU.
> + * Despite the name, the DRM GEM/CMA helpers are not hardwired to use the
> + * Contiguous Memory Allocator (CMA).
> + *
> + * For devices that access the memory bus through an (external) IOMMU then
> + * the buffer objects are allocated using a traditional page-based
> + * allocator and may be scattered through physical memory. However they
> + * are contiguous in the IOVA space so appear contiguous to devices using
> + * them.
> + *
> + * For other devices then the helpers rely on CMA to provide buffer
> + * objects that are physically contiguous in memory.
>   *
>   * For GEM callback helpers in struct &drm_gem_object functions, see likewise
>   * named functions with an _object_ infix (e.g., drm_gem_cma_object_vmap() wraps
> @@ -111,8 +121,14 @@ __drm_gem_cma_create(struct drm_device *drm, size_t size, bool private)
>   * @drm: DRM device
>   * @size: size of the object to allocate
>   *
> - * This function creates a CMA GEM object and allocates a contiguous chunk of
> - * memory as backing store.
> + * This function creates a CMA GEM object and allocates memory as backing store.
> + * The allocated memory will occupy a contiguous chunk of bus address space.
> + *
> + * For devices that are directly connected to the memory bus then the allocated
> + * memory will be physically contiguous. For devices that access through an
> + * IOMMU, then the allocated memory is not expected to be physically contiguous
> + * because having contiguous IOVAs is sufficient to meet a devices DMA
> + * requirements.
>   *
>   * Returns:
>   * A struct drm_gem_cma_object * on success or an ERR_PTR()-encoded negative
> @@ -162,9 +178,12 @@ EXPORT_SYMBOL_GPL(drm_gem_cma_create);
>   * @size: size of the object to allocate
>   * @handle: return location for the GEM handle
>   *
> - * This function creates a CMA GEM object, allocating a physically contiguous
> - * chunk of memory as backing store. The GEM object is then added to the list
> - * of object associated with the given file and a handle to it is returned.
> + * This function creates a CMA GEM object, allocating a chunk of memory as
> + * backing store. The GEM object is then added to the list of object associated
> + * with the given file and a handle to it is returned.
> + *
> + * The allocated memory will occupy a contiguous chunk of bus address space.
> + * See drm_gem_cma_create() for more details.
>   *
>   * Returns:
>   * A struct drm_gem_cma_object * on success or an ERR_PTR()-encoded negative
> 
> base-commit: b2d229d4ddb17db541098b83524d901257e93845
> --
> 2.35.1
>