On 2021-05-02 6:32 p.m., John Hubbard wrote:

On 4/8/21 10:01 AM, Logan Gunthorpe wrote:

quoted

Add a flags member to the dma_map_ops structure with one flag to
indicate support for PCI P2PDMA.

Also, add a helper to check if a device supports PCI P2PDMA.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
---
  include/linux/dma-map-ops.h |  3 +++
  include/linux/dma-mapping.h |  5 +++++
  kernel/dma/mapping.c        | 18 ++++++++++++++++++
  3 files changed, 26 insertions(+)

diff --git a/include/linux/dma-map-ops.h b/include/linux/dma-map-ops.h
index 51872e736e7b..481892822104 100644
--- a/include/linux/dma-map-ops.h
+++ b/include/linux/dma-map-ops.h

@@ -12,6 +12,9 @@
  struct cma;
  
  struct dma_map_ops {
+	unsigned int flags;
+#define DMA_F_PCI_P2PDMA_SUPPORTED     (1 << 0)
+

Can we move this up and out of the struct member area, so that it looks
more like this:

/*
  * Values for struct dma_map_ops.flags:
  *
  * DMA_F_PCI_P2PDMA_SUPPORTED: <documentation here...this is a good place to
  * explain exactly what this flag is for.>
  */
#define DMA_F_PCI_P2PDMA_SUPPORTED     (1 << 0)

struct dma_map_ops {
	unsigned int flags;

Sure, I don't care that much. I was just following the style in nvme.h.

quoted

  	void *(*alloc)(struct device *dev, size_t size,
  			dma_addr_t *dma_handle, gfp_t gfp,
  			unsigned long attrs);

diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index 50b8f586cf59..c31980ecca62 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h

@@ -146,6 +146,7 @@ int dma_mmap_attrs(struct device *dev, struct vm_area_struct *vma,
  		unsigned long attrs);
  bool dma_can_mmap(struct device *dev);
  int dma_supported(struct device *dev, u64 mask);
+bool dma_pci_p2pdma_supported(struct device *dev);
  int dma_set_mask(struct device *dev, u64 mask);
  int dma_set_coherent_mask(struct device *dev, u64 mask);
  u64 dma_get_required_mask(struct device *dev);

@@ -247,6 +248,10 @@ static inline int dma_supported(struct device *dev, u64 mask)
  {
  	return 0;
  }
+static inline bool dma_pci_p2pdma_supported(struct device *dev)
+{
+	return 0;

Should be:
	
	return false;

Yup, will fix.

quoted

+}
  static inline int dma_set_mask(struct device *dev, u64 mask)
  {
  	return -EIO;

diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index 923089c4267b..ce44a0fcc4e5 100644
--- a/kernel/dma/mapping.c
+++ b/kernel/dma/mapping.c

@@ -573,6 +573,24 @@ int dma_supported(struct device *dev, u64 mask)
  }
  EXPORT_SYMBOL(dma_supported);
  
+bool dma_pci_p2pdma_supported(struct device *dev)
+{
+	const struct dma_map_ops *ops = get_dma_ops(dev);
+
+	/* if ops is not set, dma direct will be used which supports P2PDMA */
+	if (!ops)
+		return true;
+
+	/*
+	 * Note: dma_ops_bypass is not checked here because P2PDMA should
+	 * not be used with dma mapping ops that do not have support even
+	 * if the specific device is bypassing them.
+	 */
+
+	return ops->flags & DMA_F_PCI_P2PDMA_SUPPORTED;

Wow, rather unusual combination of things in order decide this. It feels
a bit over-complicated to have flags and ops and a bool function all
dealing with the same 1-bit answer, but there is no caller shown here,
so I'll have to come back to this after reviewing subsequent patches.

Yeah, I originally had it much simpler, but it confused Ira and it was
clear it had to be written more explicitly and commented better.

Logan