[RFC] dma/mapping.c: WARN_ONCE on dma_addressing_limited() being true

[Balbir Singh <balbirs@nvidia.com>]

In the debug and resolution of an issue involving forced use of bounce
buffers, 7170130e4c72 ("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 a warning about a
device not being able to address all of memory and thus forcing all
accesses through a bounce buffer. Please see[2].

A warning would have let the user of the system know that in their
particular case, use_dma32 is set due to the addressing limitation
and this would impact performance of the driver in use.

Implement a WARN_ONCE() to point to 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 warning could be
triggered in that check as well.

Link: https://lore.kernel.org/lkml/20250401000752.249348-1-balbirs@nvidia.com/ [1]
Link: https://lore.kernel.org/lkml/20250310112206.4168-1-spasswolf@web.de/ [2]

Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Kees Cook <kees@kernel.org>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Bert Karwatzki <spasswolf@web.de>

Signed-off-by: Balbir Singh <balbirs@nvidia.com>
---
Testing: Tested lightly with on a virtual machine, I do not have access
to a device where dma_addressing_limited() is true


 kernel/dma/mapping.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index cda127027e48..0f0455fa5bc8 100644
--- a/kernel/dma/mapping.c
+++ b/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 +930,17 @@ bool dma_addressing_limited(struct device *dev)
 		return false;
 	return !dma_direct_all_ram_mapped(dev);
 }
+
+bool dma_addressing_limited(struct device *dev)
+{
+	bool ret = __dma_addressing_limited(dev);
+
+	WARN_ONCE((ret == true),
+		"%s might have lower performance due to bounce buffering",
+		dev_name(dev));
+
+	return ret;
+}
 EXPORT_SYMBOL_GPL(dma_addressing_limited);
 
 size_t dma_max_mapping_size(struct device *dev)
-- 
2.49.0