[PATCH] dma/mapping.c: dev_dbg support for dma_addressing_limited

[PATCH] dma/mapping.c: dev_dbg support for dma_addressing_limited

[Balbir Singh]

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 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/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>
Cc: Christoph Hellwig <hch@infradead.org>

Signed-off-by: Balbir Singh <balbirs@nvidia.com>
---
Changelog v2:
- Convert WARN_ONCE to dev_dbg

Testing:
- Limited testing on a VM, could not trigger the debug message on the machine

 kernel/dma/mapping.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index cda127027e48..eea14302fa20 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 +930,15 @@ 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, "might have lower perf due to bounce buffering\n");
+	return true;
+}
 EXPORT_SYMBOL_GPL(dma_addressing_limited);
 
 size_t dma_max_mapping_size(struct device *dev)
-- 
2.49.0

Re: [PATCH] dma/mapping.c: dev_dbg support for dma_addressing_limited

[Christoph Hellwig]

On Mon, Apr 14, 2025 at 09:06:46PM +1000, Balbir Singh wrote:
> +	dev_dbg(dev, "might have lower perf due to bounce buffering\n");

I don't think that's a good message.  Report the actual observation,
i.e. "device is DMA addressing limited" and not a guessed conclusion.

Re: [PATCH] dma/mapping.c: dev_dbg support for dma_addressing_limited

[Balbir Singh]

On 4/14/25 21:14, Christoph Hellwig wrote:
> On Mon, Apr 14, 2025 at 09:06:46PM +1000, Balbir Singh wrote:
>> +	dev_dbg(dev, "might have lower perf due to bounce buffering\n");
> 
> I don't think that's a good message.  Report the actual observation,
> i.e. "device is DMA addressing limited" and not a guessed conclusion.
> 

The bounce buffering remark came from the rest of the mention in code
and I agree the perf is based on my experience and may not be true
in all cases.

I'll respin

Balbir