Re: [PATCH v6 03/13] PCI/P2PDMA: Add PCI p2pmem DMA mappings to adjust the bus offset

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, Sep 12, 2018 at 06:11:46PM -0600, Logan Gunthorpe wrote:
> The DMA address used when mapping PCI P2P memory must be the PCI bus
> address. Thus, introduce pci_p2pmem_map_sg() to map the correct
> addresses when using P2P memory. Memory mapped in this way does not
> need to be unmapped.

I think the use of "map" in this context is slightly confusing because the
general expectation is that map/unmap must be balanced.

I assume it's because the "mapping" consumes no resources, e.g., requires
no page table entries.  Possibly there's a better verb than "map", e.g.,
"convert", "convert_to_p2pdma", etc?

If you keep "map", maybe add a sentence or two about why there's no
corresponding unmap?

> For this, we assume that an SGL passed to these functions contain all
> P2P memory or no P2P memory.
> 
> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>

Acked-by: Bjorn Helgaas <bhelgaas@google.com>

> ---
>  drivers/pci/p2pdma.c       | 43 ++++++++++++++++++++++++++++++++++++++
>  include/linux/memremap.h   |  1 +
>  include/linux/pci-p2pdma.h |  7 +++++++
>  3 files changed, 51 insertions(+)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 67c1daf1189e..29bd40a87768 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -191,6 +191,8 @@ int pci_p2pdma_add_resource(struct pci_dev *pdev, int bar, size_t size,
>  	pgmap->res.flags = pci_resource_flags(pdev, bar);
>  	pgmap->ref = &pdev->p2pdma->devmap_ref;
>  	pgmap->type = MEMORY_DEVICE_PCI_P2PDMA;
> +	pgmap->pci_p2pdma_bus_offset = pci_bus_address(pdev, bar) -
> +		pci_resource_start(pdev, bar);
>  
>  	addr = devm_memremap_pages(&pdev->dev, pgmap);
>  	if (IS_ERR(addr)) {
> @@ -813,3 +815,44 @@ void pci_p2pmem_publish(struct pci_dev *pdev, bool publish)
>  	pdev->p2pdma->p2pmem_published = publish;
>  }
>  EXPORT_SYMBOL_GPL(pci_p2pmem_publish);
> +
> +/**
> + * pci_p2pdma_map_sg - map a PCI peer-to-peer scatterlist for DMA
> + * @dev: device doing the DMA request
> + * @sg: scatter list to map
> + * @nents: elements in the scatterlist
> + * @dir: DMA direction
> + *
> + * Scatterlists mapped with this function should not be unmapped in any way.
> + *
> + * Returns the number of SG entries mapped or 0 on error.
> + */
> +int pci_p2pdma_map_sg(struct device *dev, struct scatterlist *sg, int nents,
> +		      enum dma_data_direction dir)
> +{
> +	struct dev_pagemap *pgmap;
> +	struct scatterlist *s;
> +	phys_addr_t paddr;
> +	int i;
> +
> +	/*
> +	 * p2pdma mappings are not compatible with devices that use
> +	 * dma_virt_ops. If the upper layers do the right thing
> +	 * this should never happen because it will be prevented
> +	 * by the check in pci_p2pdma_add_client()
> +	 */
> +	if (WARN_ON_ONCE(IS_ENABLED(CONFIG_DMA_VIRT_OPS) &&
> +			 dev->dma_ops == &dma_virt_ops))
> +		return 0;
> +
> +	for_each_sg(sg, s, nents, i) {
> +		pgmap = sg_page(s)->pgmap;
> +		paddr = sg_phys(s);
> +
> +		s->dma_address = paddr - pgmap->pci_p2pdma_bus_offset;
> +		sg_dma_len(s) = s->length;
> +	}
> +
> +	return nents;
> +}
> +EXPORT_SYMBOL_GPL(pci_p2pdma_map_sg);
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index 9553370ebdad..0ac69ddf5fc4 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -125,6 +125,7 @@ struct dev_pagemap {
>  	struct device *dev;
>  	void *data;
>  	enum memory_type type;
> +	u64 pci_p2pdma_bus_offset;
>  };
>  
>  #ifdef CONFIG_ZONE_DEVICE
> diff --git a/include/linux/pci-p2pdma.h b/include/linux/pci-p2pdma.h
> index 7b2b0f547528..2f03dbbf5af6 100644
> --- a/include/linux/pci-p2pdma.h
> +++ b/include/linux/pci-p2pdma.h
> @@ -36,6 +36,8 @@ struct scatterlist *pci_p2pmem_alloc_sgl(struct pci_dev *pdev,
>  					 unsigned int *nents, u32 length);
>  void pci_p2pmem_free_sgl(struct pci_dev *pdev, struct scatterlist *sgl);
>  void pci_p2pmem_publish(struct pci_dev *pdev, bool publish);
> +int pci_p2pdma_map_sg(struct device *dev, struct scatterlist *sg, int nents,
> +		      enum dma_data_direction dir);
>  #else /* CONFIG_PCI_P2PDMA */
>  static inline int pci_p2pdma_add_resource(struct pci_dev *pdev, int bar,
>  		size_t size, u64 offset)
> @@ -98,5 +100,10 @@ static inline void pci_p2pmem_free_sgl(struct pci_dev *pdev,
>  static inline void pci_p2pmem_publish(struct pci_dev *pdev, bool publish)
>  {
>  }
> +static inline int pci_p2pdma_map_sg(struct device *dev,
> +		struct scatterlist *sg, int nents, enum dma_data_direction dir)
> +{
> +	return 0;
> +}
>  #endif /* CONFIG_PCI_P2PDMA */
>  #endif /* _LINUX_PCI_P2P_H */
> -- 
> 2.19.0
>