From: Balbir Singh balbirs@nvidia.com
[ Upstream commit 2042c352e21d19eaf5f9e22fb6afce72293ef28c ]
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 Reviewed-by: Christoph Hellwig hch@lst.de Signed-off-by: Marek Szyprowski m.szyprowski@samsung.com Link: https://lore.kernel.org/r/20250414113752.3298276-1-balbirs@nvidia.com Signed-off-by: Sasha Levin sashal@kernel.org --- 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 864a1121bf086..f7366083b4d00 100644 --- a/kernel/dma/mapping.c +++ b/kernel/dma/mapping.c @@ -905,7 +905,7 @@ 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);
@@ -917,6 +917,15 @@ 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)