Messages in this thread |  | | Subject | Re: [PATCH 08/16] PCI/P2PDMA: Introduce helpers for dma_map_sg implementations | From | John Hubbard <> | Date | Sun, 2 May 2021 17:50:39 -0700 |
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On 4/8/21 10:01 AM, 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. >
OK, coming back to this patch, after seeing how it is used later in the series...
> 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@deltatee.com> > --- > drivers/pci/p2pdma.c | 65 ++++++++++++++++++++++++++++++++++++++ > include/linux/pci-p2pdma.h | 21 ++++++++++++ > 2 files changed, 86 insertions(+) > > diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c > index 38c93f57a941..44ad7664e875 100644 > --- a/drivers/pci/p2pdma.c > +++ b/drivers/pci/p2pdma.c > @@ -923,6 +923,71 @@ 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 on the stack outside of > + * the for_each_sg() loop and initialized to zero.
Silly fine point for the docs here: it doesn't actually have to be on the stack, so I don't think you need to write that constraint in the documentation. It just has be be somehow allocated and zeroed.
> + * @dev: DMA device that's doing the mapping operation > + * @sg: scatterlist segment to map > + * @attrs: dma mapping attributes > + * > + * 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. > + * > + * 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.
Should this be backed up with actual checks in the function, that the prerequisites are met?
> + * > + * Returns 1 if the segment was mapped, 0 if the segment should be mapped > + * directly (or through the IOMMU) and -EREMOTEIO if the segment should not > + * be mapped at all. > + */ > +int pci_p2pdma_map_segment(struct pci_p2pdma_map_state *state, > + struct device *dev, struct scatterlist *sg, > + unsigned long dma_attrs) > +{ > + if (state->pgmap != sg_page(sg)->pgmap) { > + state->pgmap = sg_page(sg)->pgmap; > + state->map = pci_p2pdma_map_type(state->pgmap, dev, dma_attrs); > + state->bus_off = to_p2p_pgmap(state->pgmap)->bus_offset; > + }
I'll quote myself from patch 9, because I had a comment there that actually was meant for this patch:
Is it worth putting this stuff on the caller's stack? I mean, is there a noticeable performance improvement from caching the state? Because if it's invisible, then simplicity is better. I suspect you're right, and that it *is* worth it, but it's good to know for real.
> + > + switch (state->map) { > + case PCI_P2PDMA_MAP_BUS_ADDR: > + sg->dma_address = sg_phys(sg) + state->bus_off; > + sg_dma_len(sg) = sg->length; > + sg_mark_pci_p2pdma(sg); > + return 1; > + case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE: > + return 0; > + default: > + return -EREMOTEIO; > + } > +} > + > +/** > + * pci_p2pdma_map_bus_segment - map an sg segment pre determined to > + * be mapped with PCI_P2PDMA_MAP_BUS_ADDR
Or:
* pci_p2pdma_map_bus_segment - map an SG segment that is already known * to be mapped with PCI_P2PDMA_MAP_BUS_ADDR
Also, should that prerequisite be backed up with checks in the function?
> + * @pg_sg: scatterlist segment with the page to map > + * @dma_sg: scatterlist segment to assign a dma address to > + * > + * 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.
Another prerequisite, so same question: do you think that the code should also check that this prerequisite is met?
thanks, -- John Hubbard NVIDIA
> + */ > +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_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 a06072ac3a52..49e7679403cf 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; > > @@ -43,6 +49,11 @@ 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); > +int pci_p2pdma_map_segment(struct pci_p2pdma_map_state *state, > + struct device *dev, struct scatterlist *sg, > + unsigned long dma_attrs); > +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, > @@ -109,6 +120,16 @@ static inline void pci_p2pdma_unmap_sg_attrs(struct device *dev, > unsigned long attrs) > { > } > +static inline int pci_p2pdma_map_segment(struct pci_p2pdma_map_state *state, > + struct device *dev, struct scatterlist *sg, > + unsigned long dma_attrs) > +{ > + return 0; > +} > +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) > { >
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