lib/scatterlist: add flag for indicating P2PDMA segments in an SGL

Introduce a dma_flags field in struct scatterlist. These flags will be
used by dma_[un]map_sg_p2pdma() to determine when a given SGL segments
dma_address points to a PCI bus address. dma_unmap_sg_p2pdma() will need
to perform different cleanup when a segment is marked as a bus address.

The dma_flags field will fit in the existing padding on 64BIT systems
(assuming CONFIG_NEED_SG_DMA_LENGTH is also set).

The new bit will only be used when CONFIG_PCI_P2PDMA is set; this means
PCI P2PDMA will require CONFIG_64BIT. This should be acceptable as the
majority of P2PDMA use cases are restricted to newer root complexes and
roughly require the extra address space for memory BARs used in the
transactions.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/pci/Kconfig

@@ -164,6 +164,11 @@ config PCI_PASID
 config PCI_P2PDMA
 	bool "PCI peer-to-peer transfer support"
 	depends on ZONE_DEVICE
+	#
+	# The need for the scatterlist DMA bus address flag means PCI P2PDMA
+	# requires 64bit
+	#
+	depends on 64BIT
 	select GENERIC_ALLOCATOR
 	help
 	  Enableѕ drivers to do PCI peer-to-peer transactions to and from

include/linux/scatterlist.h

@@ -16,6 +16,9 @@ struct scatterlist {
 #ifdef CONFIG_NEED_SG_DMA_LENGTH
 	unsigned int	dma_length;
 #endif
+#ifdef CONFIG_PCI_P2PDMA
+	unsigned int    dma_flags;
+#endif
 };
 
 /*
@@ -245,6 +248,72 @@ static inline void sg_unmark_end(struct scatterlist *sg)
 	sg->page_link &= ~SG_END;
 }
 
+/*
+ * CONFGI_PCI_P2PDMA depends on CONFIG_64BIT which means there is 4 bytes
+ * in struct scatterlist (assuming also CONFIG_NEED_SG_DMA_LENGTH is set).
+ * Use this padding for DMA flags bits to indicate when a specific
+ * dma address is a bus address.
+ */
+#ifdef CONFIG_PCI_P2PDMA
+
+#define SG_DMA_BUS_ADDRESS (1 << 0)
+
+/**
+ * sg_dma_is_bus address - Return whether a given segment was marked
+ *			   as a bus address
+ * @sg:		 SG entry
+ *
+ * Description:
+ *   Returns true if sg_dma_mark_bus_address() has been called on
+ *   this segment.
+ **/
+static inline bool sg_is_dma_bus_address(struct scatterlist *sg)
+{
+	return sg->dma_flags & SG_DMA_BUS_ADDRESS;
+}
+
+/**
+ * sg_dma_mark_bus address - Mark the scatterlist entry as a bus address
+ * @sg:		 SG entry
+ *
+ * Description:
+ *   Marks the passed in sg entry to indicate that the dma_address is
+ *   a bus address and doesn't need to be unmapped. This should only be
+ *   used by dma_map_sg() implementations to mark bus addresses
+ *   so they can be properly cleaned up in dma_unmap_sg().
+ **/
+static inline void sg_dma_mark_bus_address(struct scatterlist *sg)
+{
+	sg->dma_flags |= SG_DMA_BUS_ADDRESS;
+}
+
+/**
+ * sg_unmark_bus_address - Unmark the scatterlist entry as a bus address
+ * @sg:		 SG entry
+ *
+ * Description:
+ *   Clears the bus address mark.
+ **/
+static inline void sg_dma_unmark_bus_address(struct scatterlist *sg)
+{
+	sg->dma_flags &= ~SG_DMA_BUS_ADDRESS;
+}
+
+#else
+
+static inline bool sg_is_dma_bus_address(struct scatterlist *sg)
+{
+	return false;
+}
+static inline void sg_dma_mark_bus_address(struct scatterlist *sg)
+{
+}
+static inline void sg_dma_unmark_bus_address(struct scatterlist *sg)
+{
+}
+
+#endif
+
 /**
  * sg_phys - Return physical address of an sg entry
  * @sg:	     SG entry