Re: [PATCH 08/16] PCI/P2PDMA: Introduce helpers for dma_map_sg implementations
From: John Hubbard
Date: Sun May 02 2021 - 20:52:00 EST
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@xxxxxxxxxxxx>
---
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)
{