Re: [PATCH v3 04/20] PCI/P2PDMA: introduce helpers for dma_map_sg implementations

[Bjorn Helgaas]

On Thu, Sep 16, 2021 at 05:40:44PM -0600, Logan Gunthorpe wrote:
> Add pci_p2pdma_map_segment() as a helper for simple dma_map_sg()
> implementations. It takes an scatterlist segment that must point to a
> pci_p2pdma struct page and will map it if the mapping requires a bus
> address.
> 
> The return value indicates whether the mapping required a bus address
> or whether the caller still needs to map the segment normally. If the
> segment should not be mapped, -EREMOTEIO is returned.
> 
> This helper uses a state structure to track the changes to the
> pgmap across calls and avoid needing to lookup into the xarray for
> every page.
> 
> Also add pci_p2pdma_map_bus_segment() which is useful for IOMMU
> dma_map_sg() implementations where the sg segment containing the page
> differs from the sg segment containing the DMA address.
> 
> Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>

Acked-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>

Ditto.

> ---
>  drivers/pci/p2pdma.c       | 59 ++++++++++++++++++++++++++++++++++++++
>  include/linux/pci-p2pdma.h | 21 ++++++++++++++
>  2 files changed, 80 insertions(+)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index b656d8c801a7..58c34f1f1473 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -943,6 +943,65 @@ void pci_p2pdma_unmap_sg_attrs(struct device *dev, struct scatterlist *sg,
>  }
>  EXPORT_SYMBOL_GPL(pci_p2pdma_unmap_sg_attrs);
>  
> +/**
> + * pci_p2pdma_map_segment - map an sg segment determining the mapping type
> + * @state: State structure that should be declared outside of the for_each_sg()
> + *	loop and initialized to zero.
> + * @dev: DMA device that's doing the mapping operation
> + * @sg: scatterlist segment to map
> + *
> + * This is a helper to be used by non-iommu dma_map_sg() implementations where
> + * the sg segment is the same for the page_link and the dma_address.

s/non-iommu/non-IOMMU/

> + *
> + * Attempt to map a single segment in an SGL with the PCI bus address.
> + * The segment must point to a PCI P2PDMA page and thus must be
> + * wrapped in a is_pci_p2pdma_page(sg_page(sg)) check.
> + *
> + * Returns the type of mapping used and maps the page if the type is
> + * PCI_P2PDMA_MAP_BUS_ADDR.
> + */
> +enum pci_p2pdma_map_type
> +pci_p2pdma_map_segment(struct pci_p2pdma_map_state *state, struct device *dev,
> +		       struct scatterlist *sg)
> +{
> +	if (state->pgmap != sg_page(sg)->pgmap) {
> +		state->pgmap = sg_page(sg)->pgmap;
> +		state->map = pci_p2pdma_map_type(state->pgmap, dev);
> +		state->bus_off = to_p2p_pgmap(state->pgmap)->bus_offset;
> +	}
> +
> +	if (state->map == PCI_P2PDMA_MAP_BUS_ADDR) {
> +		sg->dma_address = sg_phys(sg) + state->bus_off;
> +		sg_dma_len(sg) = sg->length;
> +		sg_dma_mark_pci_p2pdma(sg);
> +	}
> +
> +	return state->map;
> +}
> +
> +/**
> + * pci_p2pdma_map_bus_segment - map an sg segment pre determined to
> + *	be mapped with PCI_P2PDMA_MAP_BUS_ADDR
> + * @pg_sg: scatterlist segment with the page to map
> + * @dma_sg: scatterlist segment to assign a dma address to

s/dma address/DMA address/, also below

> + *
> + * This is a helper for iommu dma_map_sg() implementations when the
> + * segment for the dma address differs from the segment containing the
> + * source page.
> + *
> + * pci_p2pdma_map_type() must have already been called on the pg_sg and
> + * returned PCI_P2PDMA_MAP_BUS_ADDR.
> + */
> +void pci_p2pdma_map_bus_segment(struct scatterlist *pg_sg,
> +				struct scatterlist *dma_sg)
> +{
> +	struct pci_p2pdma_pagemap *pgmap = to_p2p_pgmap(sg_page(pg_sg)->pgmap);
> +
> +	dma_sg->dma_address = sg_phys(pg_sg) + pgmap->bus_offset;
> +	sg_dma_len(dma_sg) = pg_sg->length;
> +	sg_dma_mark_pci_p2pdma(dma_sg);
> +}
> +
>  /**
>   * pci_p2pdma_enable_store - parse a configfs/sysfs attribute store
>   *		to enable p2pdma
> diff --git a/include/linux/pci-p2pdma.h b/include/linux/pci-p2pdma.h
> index caac2d023f8f..e5a8d5bc0f51 100644
> --- a/include/linux/pci-p2pdma.h
> +++ b/include/linux/pci-p2pdma.h
> @@ -13,6 +13,12 @@
>  
>  #include <linux/pci.h>
>  
> +struct pci_p2pdma_map_state {
> +	struct dev_pagemap *pgmap;
> +	int map;
> +	u64 bus_off;
> +};
> +
>  struct block_device;
>  struct scatterlist;
>  
> @@ -70,6 +76,11 @@ int pci_p2pdma_map_sg_attrs(struct device *dev, struct scatterlist *sg,
>  		int nents, enum dma_data_direction dir, unsigned long attrs);
>  void pci_p2pdma_unmap_sg_attrs(struct device *dev, struct scatterlist *sg,
>  		int nents, enum dma_data_direction dir, unsigned long attrs);
> +enum pci_p2pdma_map_type
> +pci_p2pdma_map_segment(struct pci_p2pdma_map_state *state, struct device *dev,
> +		       struct scatterlist *sg);
> +void pci_p2pdma_map_bus_segment(struct scatterlist *pg_sg,
> +				struct scatterlist *dma_sg);
>  int pci_p2pdma_enable_store(const char *page, struct pci_dev **p2p_dev,
>  			    bool *use_p2pdma);
>  ssize_t pci_p2pdma_enable_show(char *page, struct pci_dev *p2p_dev,
> @@ -135,6 +146,16 @@ static inline void pci_p2pdma_unmap_sg_attrs(struct device *dev,
>  		unsigned long attrs)
>  {
>  }
> +static inline enum pci_p2pdma_map_type
> +pci_p2pdma_map_segment(struct pci_p2pdma_map_state *state, struct device *dev,
> +		       struct scatterlist *sg)
> +{
> +	return PCI_P2PDMA_MAP_NOT_SUPPORTED;
> +}
> +static inline void pci_p2pdma_map_bus_segment(struct scatterlist *pg_sg,
> +					      struct scatterlist *dma_sg)
> +{
> +}
>  static inline int pci_p2pdma_enable_store(const char *page,
>  		struct pci_dev **p2p_dev, bool *use_p2pdma)
>  {
> -- 
> 2.30.2
>