dma/mapping.c: dev_dbg support for dma_addressing_limited

JIRA: https://issues.redhat.com/browse/RHEL-117031

commit 2042c35
Author: Balbir Singh <balbirs@nvidia.com>
Date:   Mon Apr 14 21:37:52 2025 +1000

    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/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: Prarit Bhargava <prarit@redhat.com>

Author: prarit

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)