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

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