dma/mapping.c: dev_dbg support for dma_addressing_limited

In the debug and resolution of an issue involving forced use of bounce
buffers, 7170130 ("x86/mm/init: Handle the special case of device
private pages in add_pages(), to not increase max_pfn and trigger
dma_addressing_limited() bounce buffers"). It would have been easier
to debug the issue if dma_addressing_limited() had debug information
about the device not being able to address all of memory and thus forcing
all accesses through a bounce buffer. Please see[2]

Implement dev_dbg to debug the potential use of bounce buffers
when we hit the condition. When swiotlb is used,
dma_addressing_limited() is used to determine the size of maximum dma
buffer size in dma_direct_max_mapping_size(). The debug prints could be
triggered in that check as well (when enabled).

Link: https://lore.kernel.org/lkml/[email protected]/ [1]
Link: https://lore.kernel.org/lkml/[email protected]/ [2]

Cc: Marek Szyprowski <[email protected]>
Cc: Robin Murphy <[email protected]>
Cc: "Christian König" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Alex Deucher <[email protected]>
Cc: Bert Karwatzki <[email protected]>
Cc: Christoph Hellwig <[email protected]>

Signed-off-by: Balbir Singh <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Marek Szyprowski <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: balbirs-nv

kernel/dma/mapping.c

@@ -918,7 +918,7 @@ EXPORT_SYMBOL(dma_set_coherent_mask);
  * the system, else %false.  Lack of addressing bits is the prime reason for
  * bounce buffering, but might not be the only one.
  */
-bool dma_addressing_limited(struct device *dev)
+static bool __dma_addressing_limited(struct device *dev)
 {
 	const struct dma_map_ops *ops = get_dma_ops(dev);
 
@@ -930,6 +930,15 @@ bool dma_addressing_limited(struct device *dev)
 		return false;
 	return !dma_direct_all_ram_mapped(dev);
 }
+
+bool dma_addressing_limited(struct device *dev)
+{
+	if (!__dma_addressing_limited(dev))
+		return false;
+
+	dev_dbg(dev, "device is DMA addressing limited\n");
+	return true;
+}
 EXPORT_SYMBOL_GPL(dma_addressing_limited);
 
 size_t dma_max_mapping_size(struct device *dev)