Re: [PATCH v1 08/17] dma-mapping: add a dma_need_unmap helper

[Robin Murphy <robin.murphy@arm.com>]

On 30/10/2024 3:12 pm, Leon Romanovsky wrote:
> From: Christoph Hellwig <hch@lst.de>
> 
> Add helper that allows a driver to skip calling dma_unmap_*
> if the DMA layer can guarantee that they are no-nops.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> ---
>   include/linux/dma-mapping.h |  5 +++++
>   kernel/dma/mapping.c        | 20 ++++++++++++++++++++
>   2 files changed, 25 insertions(+)
> 
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 8074a3b5c807..6906edde505d 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -410,6 +410,7 @@ static inline bool dma_need_sync(struct device *dev, dma_addr_t dma_addr)
>   {
>   	return dma_dev_need_sync(dev) ? __dma_need_sync(dev, dma_addr) : false;
>   }
> +bool dma_need_unmap(struct device *dev);
>   #else /* !CONFIG_HAS_DMA || !CONFIG_DMA_NEED_SYNC */
>   static inline bool dma_dev_need_sync(const struct device *dev)
>   {
> @@ -435,6 +436,10 @@ static inline bool dma_need_sync(struct device *dev, dma_addr_t dma_addr)
>   {
>   	return false;
>   }
> +static inline bool dma_need_unmap(struct device *dev)
> +{
> +	return false;
> +}
>   #endif /* !CONFIG_HAS_DMA || !CONFIG_DMA_NEED_SYNC */
>   
>   struct page *dma_alloc_pages(struct device *dev, size_t size,
> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> index 864a1121bf08..daa97a650778 100644
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -442,6 +442,26 @@ bool __dma_need_sync(struct device *dev, dma_addr_t dma_addr)
>   }
>   EXPORT_SYMBOL_GPL(__dma_need_sync);
>   
> +/**
> + * dma_need_unmap - does this device need dma_unmap_* operations
> + * @dev: device to check
> + *
> + * If this function returns %false, drivers can skip calling dma_unmap_* after
> + * finishing an I/O.  This function must be called after all mappings that might
> + * need to be unmapped have been performed.

In terms of the unmap call itself, why don't we just use dma_skip_sync 
to short-cut dma_direct_unmap_*() and make sure it's as cheap as possible?

In terms of not having to unmap implying not having to store addresses 
at all, it doesn't seem super-useful when you still have to store them 
for long enough to find out that you don't :/

Thanks,
Robin.

> + */
> +bool dma_need_unmap(struct device *dev)
> +{
> +	if (!dma_map_direct(dev, get_dma_ops(dev)))
> +		return true;
> +#ifdef CONFIG_DMA_NEED_SYNC
> +	if (!dev->dma_skip_sync)
> +		return true;
> +#endif
> +	return IS_ENABLED(CONFIG_DMA_API_DEBUG);
> +}
> +EXPORT_SYMBOL_GPL(dma_need_unmap);
> +
>   static void dma_setup_need_sync(struct device *dev)
>   {
>   	const struct dma_map_ops *ops = get_dma_ops(dev);