From: Balbir Singh <balbirs@nvidia.com>
To: iommu@lists.linux.dev
Cc: linux-kernel@vger.kernel.org, "Balbir Singh" <balbirs@nvidia.com>,
	"Marek Szyprowski" <m.szyprowski@samsung.com>,
	"Robin Murphy" <robin.murphy@arm.com>,
	"Christian König" <christian.koenig@amd.com>,
	"Ingo Molnar" <mingo@kernel.org>, "Kees Cook" <kees@kernel.org>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Alex Deucher" <alexander.deucher@amd.com>,
	"Bert Karwatzki" <spasswolf@web.de>,
	"Christoph Hellwig" <hch@infradead.org>
Subject: [v2] dma/mapping.c: dev_dbg support for dma_addressing_limited
Date: Mon, 14 Apr 2025 21:37:52 +1000	[thread overview]
Message-ID: <20250414113752.3298276-1-balbirs@nvidia.com> (raw)

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:
- Change the debug message to be factual
- 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..67da08fa6723 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, "device is DMA addressing limited\n");
+	return true;
+}
 EXPORT_SYMBOL_GPL(dma_addressing_limited);
 
 size_t dma_max_mapping_size(struct device *dev)
-- 
2.49.0

next             reply	other threads:[~2025-04-14 11:38 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CGME20250414113810eucas1p2d3b2f60d567a27a2c22930229b4bccfc@eucas1p2.samsung.com>
2025-04-14 11:37 ` Balbir Singh [this message]
2025-04-14 11:42   ` [v2] dma/mapping.c: dev_dbg support for dma_addressing_limited Christoph Hellwig
2025-04-15  8:12   ` Marek Szyprowski

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