Re: [PATCH 12/12] dma-mapping: move the dma_declare_coherent_memory documentation

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On 2020-09-08 17:47, Christoph Hellwig wrote:
dma_declare_coherent_memory should not be in a DMA API guide aimed
at driver writers (that is consumers of the API).  Move it to a comment
near the function instead.

I still think there might be an occasional valid use for device-local memory outside the scope of platform code without the driver having to go full ZONE_DEVICE/HMM/TTM, e.g. with stuff like PCIe-based FPGA prototyping cards, but the kind of driver I'm imagining for that case would never be upstream anyway (if it were even written, rather than just using hard-coded hacks), so meh.

Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>

Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
  Documentation/core-api/dma-api.rst | 24 ------------------------
  kernel/dma/coherent.c              | 17 +++++++++++++++++
  2 files changed, 17 insertions(+), 24 deletions(-)

diff --git a/Documentation/core-api/dma-api.rst b/Documentation/core-api/dma-api.rst
index 3b3abbbb4b9a6f..90239348b30f6f 100644
--- a/Documentation/core-api/dma-api.rst
+++ b/Documentation/core-api/dma-api.rst
@@ -586,30 +586,6 @@ the DMA_ATTR_NON_CONSISTENT flag starting at virtual address vaddr and
  continuing on for size.  Again, you *must* observe the cache line
  boundaries when doing this.
-::
-
-	int
-	dma_declare_coherent_memory(struct device *dev, phys_addr_t phys_addr,
-				    dma_addr_t device_addr, size_t size);
-
-Declare region of memory to be handed out by dma_alloc_coherent() when
-it's asked for coherent memory for this device.
-
-phys_addr is the CPU physical address to which the memory is currently
-assigned (this will be ioremapped so the CPU can access the region).
-
-device_addr is the DMA address the device needs to be programmed
-with to actually address this memory (this will be handed out as the
-dma_addr_t in dma_alloc_coherent()).
-
-size is the size of the area (must be multiples of PAGE_SIZE).
-
-As a simplification for the platforms, only *one* such region of
-memory may be declared per device.
-
-For reasons of efficiency, most platforms choose to track the declared
-region only at the granularity of a page.  For smaller allocations,
-you should use the dma_pool() API.
Part III - Debug drivers use of the DMA-API
  -------------------------------------------
diff --git a/kernel/dma/coherent.c b/kernel/dma/coherent.c
index 2a0c4985f38e41..f85d14bbfcbe03 100644
--- a/kernel/dma/coherent.c
+++ b/kernel/dma/coherent.c
@@ -107,6 +107,23 @@ static int dma_assign_coherent_memory(struct device *dev,
  	return 0;
  }
+/*
+ * Declare a region of memory to be handed out by dma_alloc_coherent() when it
+ * is asked for coherent memory for this device.  This shall only be used
+ * from platform code, usually based on the device tree description.
+ *
+ * phys_addr is the CPU physical address to which the memory is currently
+ * assigned (this will be ioremapped so the CPU can access the region).
+ *
+ * device_addr is the DMA address the device needs to be programmed with to
+ * actually address this memory (this will be handed out as the dma_addr_t in
+ * dma_alloc_coherent()).
+ *
+ * size is the size of the area (must be a multiple of PAGE_SIZE).
+ *
+ * As a simplification for the platforms, only *one* such region of memory may
+ * be declared per device.
+ */
  int dma_declare_coherent_memory(struct device *dev, phys_addr_t phys_addr,
  				dma_addr_t device_addr, size_t size)
  {




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