Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct device

[Alexey Kardashevskiy]

On 21/03/2020 01:16, Christoph Hellwig wrote:
> Several IOMMU drivers have a bypass mode where they can use a direct
> mapping if the devices DMA mask is large enough.  Add generic support
> to the core dma-mapping code to do that to switch those drivers to
> a common solution.
> 
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
>  include/linux/device.h      |  6 ++++++
>  include/linux/dma-mapping.h | 30 ++++++++++++++++++------------
>  kernel/dma/mapping.c        | 36 +++++++++++++++++++++++++++---------
>  3 files changed, 51 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 0cd7c647c16c..09be8bb2c4a6 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -525,6 +525,11 @@ struct dev_links_info {
>   *		  sync_state() callback.
>   * @dma_coherent: this particular device is dma coherent, even if the
>   *		architecture supports non-coherent devices.
> + * @dma_ops_bypass: If set to %true then the dma_ops are bypassed for the
> + *		streaming DMA operations (->map_* / ->unmap_* / ->sync_*),
> + *		and optionall (if the coherent mask is large enough) also
> + *		for dma allocations.  This flag is managed by the dma ops
> + *		instance from ->dma_supported.
>   *
>   * At the lowest level, every device in a Linux system is represented by an
>   * instance of struct device. The device structure contains the information
> @@ -625,6 +630,7 @@ struct device {
>      defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU_ALL)
>  	bool			dma_coherent:1;
>  #endif
> +	bool			dma_ops_bypass : 1;
>  };
>  
>  static inline struct device *kobj_to_dev(struct kobject *kobj)
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 330ad58fbf4d..c3af0cf5e435 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -188,9 +188,15 @@ static inline int dma_mmap_from_global_coherent(struct vm_area_struct *vma,
>  }
>  #endif /* CONFIG_DMA_DECLARE_COHERENT */
>  
> -static inline bool dma_is_direct(const struct dma_map_ops *ops)
> +/*
> + * Check if the devices uses a direct mapping for streaming DMA operations.
> + * This allows IOMMU drivers to set a bypass mode if the DMA mask is large
> + * enough.
> + */
> +static inline bool dma_map_direct(struct device *dev,
> +		const struct dma_map_ops *ops)
>  {
> -	return likely(!ops);
> +	return likely(!ops) || dev->dma_ops_bypass;
>  }
>  
>  /*
> @@ -279,7 +285,7 @@ static inline dma_addr_t dma_map_page_attrs(struct device *dev,
>  	dma_addr_t addr;
>  
>  	BUG_ON(!valid_dma_direction(dir));
> -	if (dma_is_direct(ops))
> +	if (dma_map_direct(dev, ops))
>  		addr = dma_direct_map_page(dev, page, offset, size, dir, attrs);
>  	else
>  		addr = ops->map_page(dev, page, offset, size, dir, attrs);
> @@ -294,7 +300,7 @@ static inline void dma_unmap_page_attrs(struct device *dev, dma_addr_t addr,
>  	const struct dma_map_ops *ops = get_dma_ops(dev);
>  
>  	BUG_ON(!valid_dma_direction(dir));
> -	if (dma_is_direct(ops))
> +	if (dma_map_direct(dev, ops))
>  		dma_direct_unmap_page(dev, addr, size, dir, attrs);
>  	else if (ops->unmap_page)
>  		ops->unmap_page(dev, addr, size, dir, attrs);
> @@ -313,7 +319,7 @@ static inline int dma_map_sg_attrs(struct device *dev, struct scatterlist *sg,
>  	int ents;
>  
>  	BUG_ON(!valid_dma_direction(dir));
> -	if (dma_is_direct(ops))
> +	if (dma_map_direct(dev, ops))
>  		ents = dma_direct_map_sg(dev, sg, nents, dir, attrs);
>  	else
>  		ents = ops->map_sg(dev, sg, nents, dir, attrs);
> @@ -331,7 +337,7 @@ static inline void dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sg
>  
>  	BUG_ON(!valid_dma_direction(dir));
>  	debug_dma_unmap_sg(dev, sg, nents, dir);
> -	if (dma_is_direct(ops))
> +	if (dma_map_direct(dev, ops))
>  		dma_direct_unmap_sg(dev, sg, nents, dir, attrs);
>  	else if (ops->unmap_sg)
>  		ops->unmap_sg(dev, sg, nents, dir, attrs);
> @@ -352,7 +358,7 @@ static inline dma_addr_t dma_map_resource(struct device *dev,
>  	if (WARN_ON_ONCE(pfn_valid(PHYS_PFN(phys_addr))))
>  		return DMA_MAPPING_ERROR;
>  
> -	if (dma_is_direct(ops))
> +	if (dma_map_direct(dev, ops))
>  		addr = dma_direct_map_resource(dev, phys_addr, size, dir, attrs);
>  	else if (ops->map_resource)
>  		addr = ops->map_resource(dev, phys_addr, size, dir, attrs);
> @@ -368,7 +374,7 @@ static inline void dma_unmap_resource(struct device *dev, dma_addr_t addr,
>  	const struct dma_map_ops *ops = get_dma_ops(dev);
>  
>  	BUG_ON(!valid_dma_direction(dir));
> -	if (!dma_is_direct(ops) && ops->unmap_resource)
> +	if (!dma_map_direct(dev, ops) && ops->unmap_resource)
>  		ops->unmap_resource(dev, addr, size, dir, attrs);
>  	debug_dma_unmap_resource(dev, addr, size, dir);
>  }
> @@ -380,7 +386,7 @@ static inline void dma_sync_single_for_cpu(struct device *dev, dma_addr_t addr,
>  	const struct dma_map_ops *ops = get_dma_ops(dev);
>  
>  	BUG_ON(!valid_dma_direction(dir));
> -	if (dma_is_direct(ops))
> +	if (dma_map_direct(dev, ops))
>  		dma_direct_sync_single_for_cpu(dev, addr, size, dir);
>  	else if (ops->sync_single_for_cpu)
>  		ops->sync_single_for_cpu(dev, addr, size, dir);
> @@ -394,7 +400,7 @@ static inline void dma_sync_single_for_device(struct device *dev,
>  	const struct dma_map_ops *ops = get_dma_ops(dev);
>  
>  	BUG_ON(!valid_dma_direction(dir));
> -	if (dma_is_direct(ops))
> +	if (dma_map_direct(dev, ops))
>  		dma_direct_sync_single_for_device(dev, addr, size, dir);
>  	else if (ops->sync_single_for_device)
>  		ops->sync_single_for_device(dev, addr, size, dir);
> @@ -408,7 +414,7 @@ dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg,
>  	const struct dma_map_ops *ops = get_dma_ops(dev);
>  
>  	BUG_ON(!valid_dma_direction(dir));
> -	if (dma_is_direct(ops))
> +	if (dma_map_direct(dev, ops))
>  		dma_direct_sync_sg_for_cpu(dev, sg, nelems, dir);
>  	else if (ops->sync_sg_for_cpu)
>  		ops->sync_sg_for_cpu(dev, sg, nelems, dir);
> @@ -422,7 +428,7 @@ dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg,
>  	const struct dma_map_ops *ops = get_dma_ops(dev);
>  
>  	BUG_ON(!valid_dma_direction(dir));
> -	if (dma_is_direct(ops))
> +	if (dma_map_direct(dev, ops))
>  		dma_direct_sync_sg_for_device(dev, sg, nelems, dir);
>  	else if (ops->sync_sg_for_device)
>  		ops->sync_sg_for_device(dev, sg, nelems, dir);
> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> index 12ff766ec1fa..fdea45574345 100644
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -105,6 +105,24 @@ void *dmam_alloc_attrs(struct device *dev, size_t size, dma_addr_t *dma_handle,
>  }
>  EXPORT_SYMBOL(dmam_alloc_attrs);
>  
> +static bool dma_alloc_direct(struct device *dev, const struct dma_map_ops *ops)
> +{
> +	if (!ops)
> +		return true;
> +
> +	/*
> +	 * Allows IOMMU drivers to bypass dynamic translations if the DMA mask
> +	 * is large enough.
> +	 */
> +	if (dev->dma_ops_bypass) {
> +		if (min_not_zero(dev->coherent_dma_mask, dev->bus_dma_limit) >=
> +				dma_direct_get_required_mask(dev))
> +			return true;
> +	}


Why not do this in dma_map_direct() as well?
Or simply have just one dma_map_direct()?

And one more general question - we need a way to use non-direct IOMMU
for RAM above certain limit.

Let's say we have a system with:
0 .. 0x1.0000.0000
0x100.0000.0000 .. 0x101.0000.0000

2x4G, each is 1TB aligned. And we can map directly only the first 4GB
(because of the maximum IOMMU table size) but not the other. And 1:1 on
that "pseries" is done with offset=0x0800.0000.0000.0000.

So we want to check every bus address against dev->bus_dma_limit, not
dev->coherent_dma_mask. In the example above I'd set bus_dma_limit to
0x0800.0001.0000.0000 and 1:1 mapping for the second 4GB would not be
tried. Does this sound reasonable? Thanks,


> +
> +	return false;
> +}
> +
>  /*
>   * Create scatter-list for the already allocated DMA buffer.
>   */
> @@ -138,7 +156,7 @@ int dma_get_sgtable_attrs(struct device *dev, struct sg_table *sgt,
>  {
>  	const struct dma_map_ops *ops = get_dma_ops(dev);
>  
> -	if (dma_is_direct(ops))
> +	if (dma_alloc_direct(dev, ops))
>  		return dma_direct_get_sgtable(dev, sgt, cpu_addr, dma_addr,
>  				size, attrs);
>  	if (!ops->get_sgtable)
> @@ -206,7 +224,7 @@ bool dma_can_mmap(struct device *dev)
>  {
>  	const struct dma_map_ops *ops = get_dma_ops(dev);
>  
> -	if (dma_is_direct(ops))
> +	if (dma_alloc_direct(dev, ops))
>  		return dma_direct_can_mmap(dev);
>  	return ops->mmap != NULL;
>  }
> @@ -231,7 +249,7 @@ int dma_mmap_attrs(struct device *dev, struct vm_area_struct *vma,
>  {
>  	const struct dma_map_ops *ops = get_dma_ops(dev);
>  
> -	if (dma_is_direct(ops))
> +	if (dma_alloc_direct(dev, ops))
>  		return dma_direct_mmap(dev, vma, cpu_addr, dma_addr, size,
>  				attrs);
>  	if (!ops->mmap)
> @@ -244,7 +262,7 @@ u64 dma_get_required_mask(struct device *dev)
>  {
>  	const struct dma_map_ops *ops = get_dma_ops(dev);
>  
> -	if (dma_is_direct(ops))
> +	if (dma_map_direct(dev, ops))
>  		return dma_direct_get_required_mask(dev);
>  	if (ops->get_required_mask)
>  		return ops->get_required_mask(dev);
> @@ -275,7 +293,7 @@ void *dma_alloc_attrs(struct device *dev, size_t size, dma_addr_t *dma_handle,
>  	/* let the implementation decide on the zone to allocate from: */
>  	flag &= ~(__GFP_DMA | __GFP_DMA32 | __GFP_HIGHMEM);
>  
> -	if (dma_is_direct(ops))
> +	if (dma_alloc_direct(dev, ops))
>  		cpu_addr = dma_direct_alloc(dev, size, dma_handle, flag, attrs);
>  	else if (ops->alloc)
>  		cpu_addr = ops->alloc(dev, size, dma_handle, flag, attrs);
> @@ -307,7 +325,7 @@ void dma_free_attrs(struct device *dev, size_t size, void *cpu_addr,
>  		return;
>  
>  	debug_dma_free_coherent(dev, size, cpu_addr, dma_handle);
> -	if (dma_is_direct(ops))
> +	if (dma_alloc_direct(dev, ops))
>  		dma_direct_free(dev, size, cpu_addr, dma_handle, attrs);
>  	else if (ops->free)
>  		ops->free(dev, size, cpu_addr, dma_handle, attrs);
> @@ -318,7 +336,7 @@ int dma_supported(struct device *dev, u64 mask)
>  {
>  	const struct dma_map_ops *ops = get_dma_ops(dev);
>  
> -	if (dma_is_direct(ops))
> +	if (!ops)
>  		return dma_direct_supported(dev, mask);
>  	if (!ops->dma_supported)
>  		return 1;
> @@ -374,7 +392,7 @@ void dma_cache_sync(struct device *dev, void *vaddr, size_t size,
>  
>  	BUG_ON(!valid_dma_direction(dir));
>  
> -	if (dma_is_direct(ops))
> +	if (dma_alloc_direct(dev, ops))
>  		arch_dma_cache_sync(dev, vaddr, size, dir);
>  	else if (ops->cache_sync)
>  		ops->cache_sync(dev, vaddr, size, dir);
> @@ -386,7 +404,7 @@ size_t dma_max_mapping_size(struct device *dev)
>  	const struct dma_map_ops *ops = get_dma_ops(dev);
>  	size_t size = SIZE_MAX;
>  
> -	if (dma_is_direct(ops))
> +	if (dma_map_direct(dev, ops))
>  		size = dma_direct_max_mapping_size(dev);
>  	else if (ops && ops->max_mapping_size)
>  		size = ops->max_mapping_size(dev);
> 

-- 
Alexey