Re: [RFC 3/7] dma: Add function for drivers to know if allowing blocking is useful

["Petr Tesařík" <petr@tesarici.cz>]

On Thu, 22 Aug 2024 11:37:14 -0700
mhkelley58@gmail.com wrote:

> From: Michael Kelley <mhklinux@outlook.com>
> 
> With the addition of swiotlb throttling functionality, storage
> device drivers may want to know whether using the DMA_ATTR_MAY_BLOCK
> attribute is useful. In a CoCo VM or environment where swiotlb=force
> is used, the MAY_BLOCK attribute enables swiotlb throttling. But if
> throttling is not enable or useful, storage device drivers probably
> do not want to set BLK_MQ_F_BLOCKING at the blk-mq request queue level.
> 
> Add function dma_recommend_may_block() that indicates whether
> the underlying implementation of the DMA map calls would benefit
> from allowing blocking. If the kernel was built with
> CONFIG_SWIOTLB_THROTTLE, and swiotlb=force is set (on the kernel
> command line or due to being a CoCo VM), this function returns
> true. Otherwise it returns false.
> 
> Signed-off-by: Michael Kelley <mhklinux@outlook.com>

LGTM.

Reviewed-by: Petr Tesarik <ptesarik@suse.com>

Petr T

> ---
>  include/linux/dma-mapping.h |  5 +++++
>  kernel/dma/direct.c         |  6 ++++++
>  kernel/dma/direct.h         |  1 +
>  kernel/dma/mapping.c        | 10 ++++++++++
>  4 files changed, 22 insertions(+)
> 
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 7b78294813be..ec2edf068218 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -145,6 +145,7 @@ 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);
>  bool dma_addressing_limited(struct device *dev);
> +bool dma_recommend_may_block(struct device *dev);
>  size_t dma_max_mapping_size(struct device *dev);
>  size_t dma_opt_mapping_size(struct device *dev);
>  unsigned long dma_get_merge_boundary(struct device *dev);
> @@ -252,6 +253,10 @@ static inline bool dma_addressing_limited(struct device *dev)
>  {
>  	return false;
>  }
> +static inline bool dma_recommend_may_block(struct device *dev)
> +{
> +	return false;
> +}
>  static inline size_t dma_max_mapping_size(struct device *dev)
>  {
>  	return 0;
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 80e03c0838d4..34d14e4ace64 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -649,6 +649,12 @@ bool dma_direct_all_ram_mapped(struct device *dev)
>  				      check_ram_in_range_map);
>  }
>  
> +bool dma_direct_recommend_may_block(struct device *dev)
> +{
> +	return IS_ENABLED(CONFIG_SWIOTLB_THROTTLE) &&
> +			is_swiotlb_force_bounce(dev);
> +}
> +
>  size_t dma_direct_max_mapping_size(struct device *dev)
>  {
>  	/* If SWIOTLB is active, use its maximum mapping size */
> diff --git a/kernel/dma/direct.h b/kernel/dma/direct.h
> index d2c0b7e632fc..63516a540276 100644
> --- a/kernel/dma/direct.h
> +++ b/kernel/dma/direct.h
> @@ -21,6 +21,7 @@ bool dma_direct_need_sync(struct device *dev, dma_addr_t dma_addr);
>  int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents,
>  		enum dma_data_direction dir, unsigned long attrs);
>  bool dma_direct_all_ram_mapped(struct device *dev);
> +bool dma_direct_recommend_may_block(struct device *dev);
>  size_t dma_direct_max_mapping_size(struct device *dev);
>  
>  #if defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_DEVICE) || \
> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> index b1c18058d55f..832982bafd5a 100644
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -858,6 +858,16 @@ bool dma_addressing_limited(struct device *dev)
>  }
>  EXPORT_SYMBOL_GPL(dma_addressing_limited);
>  
> +bool dma_recommend_may_block(struct device *dev)
> +{
> +	const struct dma_map_ops *ops = get_dma_ops(dev);
> +
> +	if (dma_map_direct(dev, ops))
> +		return dma_direct_recommend_may_block(dev);
> +	return false;
> +}
> +EXPORT_SYMBOL_GPL(dma_recommend_may_block);
> +
>  size_t dma_max_mapping_size(struct device *dev)
>  {
>  	const struct dma_map_ops *ops = get_dma_ops(dev);