Re: [PATCH v4 1/5] dma-mapping: Add dma_opt_mapping_size()

[Robin Murphy]

On 2022-06-27 16:25, John Garry wrote:
> Streaming DMA mapping involving an IOMMU may be much slower for larger
> total mapping size. This is because every IOMMU DMA mapping requires an
> IOVA to be allocated and freed. IOVA sizes above a certain limit are not
> cached, which can have a big impact on DMA mapping performance.
>
> Provide an API for device drivers to know this "optimal" limit, such that
> they may try to produce mapping which don't exceed it.
>
> Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> Reviewed-by: Damien Le Moal <damien.lemoal@xxxxxxxxxxxxxxxxxx>
> ---
>   Documentation/core-api/dma-api.rst |  9 +++++++++
>   include/linux/dma-map-ops.h        |  1 +
>   include/linux/dma-mapping.h        |  5 +++++
>   kernel/dma/mapping.c               | 12 ++++++++++++
>   4 files changed, 27 insertions(+)
>
> diff --git a/Documentation/core-api/dma-api.rst b/Documentation/core-api/dma-api.rst
> index 6d6d0edd2d27..b3cd9763d28b 100644
> --- a/Documentation/core-api/dma-api.rst
> +++ b/Documentation/core-api/dma-api.rst
> @@ -204,6 +204,15 @@ Returns the maximum size of a mapping for the device. The size parameter
>   of the mapping functions like dma_map_single(), dma_map_page() and
>   others should not be larger than the returned value.
>   
> +::
> +
> +	size_t
> +	dma_opt_mapping_size(struct device *dev);
> +
> +Returns the maximum optimal size of a mapping for the device. Mapping large
> +buffers may take longer so device drivers are advised to limit total DMA
> +streaming mappings length to the returned value.

Nit: I'm not sure "advised" is necessarily the right thing to say in 
general - that's only really true for a caller who cares about 
throughput of churning through short-lived mappings more than anything 
else, and doesn't take a significant hit overall from splitting up 
larger requests. I do think it's good to clarify the exact context of 
"optimal" here, but I'd prefer to be objectively clear that it's for 
workloads where the up-front mapping overhead dominates.

Thanks,
Robin.

> +
>   ::
>   
>   	bool
> diff --git a/include/linux/dma-map-ops.h b/include/linux/dma-map-ops.h
> index 0d5b06b3a4a6..98ceba6fa848 100644
> --- a/include/linux/dma-map-ops.h
> +++ b/include/linux/dma-map-ops.h
> @@ -69,6 +69,7 @@ struct dma_map_ops {
>   	int (*dma_supported)(struct device *dev, u64 mask);
>   	u64 (*get_required_mask)(struct device *dev);
>   	size_t (*max_mapping_size)(struct device *dev);
> +	size_t (*opt_mapping_size)(void);
>   	unsigned long (*get_merge_boundary)(struct device *dev);
>   };
>   
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index dca2b1355bb1..fe3849434b2a 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -144,6 +144,7 @@ int dma_set_mask(struct device *dev, u64 mask);
>   int dma_set_coherent_mask(struct device *dev, u64 mask);
>   u64 dma_get_required_mask(struct device *dev);
>   size_t dma_max_mapping_size(struct device *dev);
> +size_t dma_opt_mapping_size(struct device *dev);
>   bool dma_need_sync(struct device *dev, dma_addr_t dma_addr);
>   unsigned long dma_get_merge_boundary(struct device *dev);
>   struct sg_table *dma_alloc_noncontiguous(struct device *dev, size_t size,
> @@ -266,6 +267,10 @@ static inline size_t dma_max_mapping_size(struct device *dev)
>   {
>   	return 0;
>   }
> +static inline size_t dma_opt_mapping_size(struct device *dev)
> +{
> +	return 0;
> +}
>   static inline bool dma_need_sync(struct device *dev, dma_addr_t dma_addr)
>   {
>   	return false;
> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> index db7244291b74..1bfe11b1edb6 100644
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -773,6 +773,18 @@ size_t dma_max_mapping_size(struct device *dev)
>   }
>   EXPORT_SYMBOL_GPL(dma_max_mapping_size);
>   
> +size_t dma_opt_mapping_size(struct device *dev)
> +{
> +	const struct dma_map_ops *ops = get_dma_ops(dev);
> +	size_t size = SIZE_MAX;
> +
> +	if (ops && ops->opt_mapping_size)
> +		size = ops->opt_mapping_size();
> +
> +	return min(dma_max_mapping_size(dev), size);
> +}
> +EXPORT_SYMBOL_GPL(dma_opt_mapping_size);
> +
>   bool dma_need_sync(struct device *dev, dma_addr_t dma_addr)
>   {
>   	const struct dma_map_ops *ops = get_dma_ops(dev);