Re: [PATCH 10/16] dma-mapping: Add flags to dma_map_ops to indicate PCI P2PDMA support

[Don Dutile <ddutile@redhat.com>]

On 4/8/21 1:01 PM, Logan Gunthorpe wrote:
> Add a flags member to the dma_map_ops structure with one flag to
> indicate support for PCI P2PDMA.
>
> Also, add a helper to check if a device supports PCI P2PDMA.
>
> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> ---
>   include/linux/dma-map-ops.h |  3 +++
>   include/linux/dma-mapping.h |  5 +++++
>   kernel/dma/mapping.c        | 18 ++++++++++++++++++
>   3 files changed, 26 insertions(+)
>
> diff --git a/include/linux/dma-map-ops.h b/include/linux/dma-map-ops.h
> index 51872e736e7b..481892822104 100644
> --- a/include/linux/dma-map-ops.h
> +++ b/include/linux/dma-map-ops.h
> @@ -12,6 +12,9 @@
>   struct cma;
>   
>   struct dma_map_ops {
> +	unsigned int flags;
> +#define DMA_F_PCI_P2PDMA_SUPPORTED     (1 << 0)
> +
I'm not a fan of in-line define's; if we're going to add a flags field to the dma-ops
-- and logically it'd be good to have p2pdma go through the dma-ops struct --
then let's move this up in front of the dma-ops description.

And now that the dma-ops struct is being 'opened' for p2pdma, should p2pdma ops be added
to this struct, so all this work can be mimic'd/reflected/leveraged/refactored for CXL, GenZ, etc. p2pdma in (the near?) future?

>   	void *(*alloc)(struct device *dev, size_t size,
>   			dma_addr_t *dma_handle, gfp_t gfp,
>   			unsigned long attrs);
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 50b8f586cf59..c31980ecca62 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -146,6 +146,7 @@ int dma_mmap_attrs(struct device *dev, struct vm_area_struct *vma,
>   		unsigned long attrs);
>   bool dma_can_mmap(struct device *dev);
>   int dma_supported(struct device *dev, u64 mask);
> +bool dma_pci_p2pdma_supported(struct device *dev);
>   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);
> @@ -247,6 +248,10 @@ static inline int dma_supported(struct device *dev, u64 mask)
>   {
>   	return 0;
>   }
> +static inline bool dma_pci_p2pdma_supported(struct device *dev)
> +{
> +	return 0;
> +}
>   static inline int dma_set_mask(struct device *dev, u64 mask)
>   {
>   	return -EIO;
> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> index 923089c4267b..ce44a0fcc4e5 100644
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -573,6 +573,24 @@ int dma_supported(struct device *dev, u64 mask)
>   }
>   EXPORT_SYMBOL(dma_supported);
>   
> +bool dma_pci_p2pdma_supported(struct device *dev)
> +{
> +	const struct dma_map_ops *ops = get_dma_ops(dev);
> +
> +	/* if ops is not set, dma direct will be used which supports P2PDMA */
> +	if (!ops)
> +		return true;
So, this means one cannot have p2pdma with IOMMU's? ...
-- or is this 'for now' and this may change?  if it may change, add a note here.

> +
> +	/*
> +	 * Note: dma_ops_bypass is not checked here because P2PDMA should
> +	 * not be used with dma mapping ops that do not have support even
> +	 * if the specific device is bypassing them.
> +	 */
> +
> +	return ops->flags & DMA_F_PCI_P2PDMA_SUPPORTED;
that's a bool?

> +}
> +EXPORT_SYMBOL_GPL(dma_pci_p2pdma_supported);
> +
>   #ifdef CONFIG_ARCH_HAS_DMA_SET_MASK
>   void arch_dma_set_mask(struct device *dev, u64 mask);
>   #else