[PATCH v2] dma-mapping: don't trace the DMA address when the allocation fails

dma_alloc_attrs() passes *dma_handle to trace_dma_alloc() without
checking whether the allocation succeeded. No backend writes it on
failure: dma_direct_alloc(), iommu_dma_alloc() and the dma_map_ops
instances assign it only on the path that returns a buffer. Callers
usually pass an uninitialized automatic variable, so a failed allocation
records whatever the stack held, next to the virt_addr=(null) that marks
the record as an error:

 dma_alloc: dmatrace dir=BIDIRECTIONAL dma_addr=deadbeefdeadbeef
 size=1099511627776 virt_addr=0000000000000000

The device coherent pool path reaches the same call: a non-zero return
from dma_alloc_from_dev_coherent() means the request was handled, not
that it succeeded, so cpu_addr is NULL and dma_handle is untouched once
the pool runs out.

For an allocation event a NULL virt_addr already means the request
failed, so the address field carries nothing. Report 0 for it in the
event class rather than at each call site, which covers dma_alloc_pages()
and dma_alloc_sgt_err() as well.

Fixes: 038eb433dc14 ("dma-mapping: add tracing for dma-mapping API calls")
Fixes: 68b6dbf1f441 ("dma-mapping: trace more error paths")
Suggested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Donggeun Yoo <redacted>
---
v2:
- report 0 in the event class instead of splitting the two call sites,
  as suggested by Marek. kernel/dma/mapping.c is untouched now, so
  debug_dma_alloc_coherent() keeps being called for a failed allocation
  and decides for itself what to do with it.
- tested on x86_64 rather than compile-tested only.
v1: https://lore.kernel.org/linux-iommu/20260905071919.30784-1-donggeunyoo.kernel@gmail.com/ (local)

Tested with a module that puts a known value in the caller's handle and
then asks for 1 TiB from a 32-bit capable device, with the dma_alloc
event enabled. Before, the record carries the caller's stale value; after,
it carries 0:

 -dma_addr=deadbeefdeadbeef size=1099511627776 virt_addr=0000000000000000
 +dma_addr=0 size=1099511627776 virt_addr=0000000000000000

A PAGE_SIZE request from the same device, as a control, reports its real
address on both kernels, matching the handle the caller got back.

This changes what the record contains, not what the caller does:
*dma_handle is still read to build the tracepoint arguments, since those
are evaluated before the static branch.

 include/trace/events/dma.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/trace/events/dma.h b/include/trace/events/dma.h
index 9df02c1511de..b06d8f99922d 100644
--- a/include/trace/events/dma.h
+++ b/include/trace/events/dma.h

@@ -134,7 +134,7 @@ DECLARE_EVENT_CLASS(dma_alloc_class,
 	TP_fast_assign(
 		__assign_str(device);
 		__entry->virt_addr = virt_addr;
-		__entry->dma_addr = dma_addr;
+		__entry->dma_addr = virt_addr ? dma_addr : 0;
 		__entry->size = size;
 		__entry->flags = flags;
 		__entry->dir = dir;

base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
-- 
2.53.0