ntb: Force physically contiguous allocation of rx ring buffers

Physical addresses under IOVA on x86 platform are mapped contiguously
as a side effect before the patch that removed CONFIG_DMA_REMAP. The
NTB rx buffer ring is a single chunk DMA buffer that is allocated
against the NTB PCI device. If the receive side is using a DMA device,
then the buffers are remapped against the DMA device before being
submitted via the dmaengine API. This scheme becomes a problem when
the physical memory is discontiguous. When dma_map_page() is called
on the kernel virtual address from the dma_alloc_coherent() call, the
new IOVA mapping no longer points to all the physical memory allocated
due to being discontiguous. Change dma_alloc_coherent() to dma_alloc_attrs()
in order to force DMA_ATTR_FORCE_CONTIGUOUS attribute. This is the best
fix for the circumstance. A potential future solution may be having the DMA
mapping API providing a way to alias an existing IOVA mapping to a new
device perhaps.

This fix is not to fix the patch pointed to by the fixes tag, but to fix
the issue arised in the ntb_transport driver on x86 platforms after the
said patch is applied.

Reported-by: Jerry Dai <[email protected]>
Fixes: f5ff79fddf0e ("dma-mapping: remove CONFIG_DMA_REMAP")
Tested-by: Jerry Dai <[email protected]>
Signed-off-by: Dave Jiang <[email protected]>
Signed-off-by: Jon Mason <[email protected]>

diff --git a/drivers/ntb/ntb_transport.c b/drivers/ntb/ntb_transport.c
index 392a5952afd51cd249fa2f621688347dce26b262..a22ea4a4b202bd63fe5e027a69d540551e15a80a 100644 (file)
--- a/drivers/ntb/ntb_transport.c
+++ b/drivers/ntb/ntb_transport.c
@@ -809,16 +809,29 @@ static void ntb_free_mw(struct ntb_transport_ctx *nt, int num_mw)
 }
 
 static int ntb_alloc_mw_buffer(struct ntb_transport_mw *mw,
-                              struct device *dma_dev, size_t align)
+                              struct device *ntb_dev, size_t align)
 {
        dma_addr_t dma_addr;
        void *alloc_addr, *virt_addr;
        int rc;
 
-       alloc_addr = dma_alloc_coherent(dma_dev, mw->alloc_size,
-                                       &dma_addr, GFP_KERNEL);
+       /*
+        * The buffer here is allocated against the NTB device. The reason to
+        * use dma_alloc_*() call is to allocate a large IOVA contiguous buffer
+        * backing the NTB BAR for the remote host to write to. During receive
+        * processing, the data is being copied out of the receive buffer to
+        * the kernel skbuff. When a DMA device is being used, dma_map_page()
+        * is called on the kvaddr of the receive buffer (from dma_alloc_*())
+        * and remapped against the DMA device. It appears to be a double
+        * DMA mapping of buffers, but first is mapped to the NTB device and
+        * second is to the DMA device. DMA_ATTR_FORCE_CONTIGUOUS is necessary
+        * in order for the later dma_map_page() to not fail.
+        */
+       alloc_addr = dma_alloc_attrs(ntb_dev, mw->alloc_size,
+                                    &dma_addr, GFP_KERNEL,
+                                    DMA_ATTR_FORCE_CONTIGUOUS);
        if (!alloc_addr) {
-               dev_err(dma_dev, "Unable to alloc MW buff of size %zu\n",
+               dev_err(ntb_dev, "Unable to alloc MW buff of size %zu\n",
                        mw->alloc_size);
                return -ENOMEM;
        }
@@ -847,7 +860,7 @@ static int ntb_alloc_mw_buffer(struct ntb_transport_mw *mw,
        return 0;
 
 err:
-       dma_free_coherent(dma_dev, mw->alloc_size, alloc_addr, dma_addr);
+       dma_free_coherent(ntb_dev, mw->alloc_size, alloc_addr, dma_addr);
 
        return rc;
 }