[PATCH v2] dmaengine: pxa: fix double counting of the hw descriptors

[Sascha Hauer]

pxad_alloc_desc() was converted from

        kzalloc(struct_size(sw_desc, hw_desc, nb_hw_desc), GFP_NOWAIT)

to kzalloc_flex(), which sets the __counted_by() counter sw_desc->nb_desc
itself - but only where the compiler has __builtin_counted_by_ref(), so
from gcc 15.1 or clang 22.1 on. The loop below it still increments
nb_desc, which makes it come out doubled there and correct elsewhere.

nb_desc is what pxad_free_desc() iterates over and what
set_updater_desc() indexes from, so set it explicitly and drop the
increment. The error path has to lower it to the number of descriptors
allocated so far, otherwise pxad_free_desc() would free entries that were
never allocated.

Fixes: 69050f8d6d075 ("treewide: Replace kmalloc with kmalloc_obj for non-scalar types")
Assisted-by: Claude:claude-opus-5
Signed-off-by: Sascha Hauer <s.hauer@xxxxxxxxxxxxxx>
---
pxad_alloc_desc() was converted from

        kzalloc(struct_size(sw_desc, hw_desc, nb_hw_desc), GFP_NOWAIT)

to kzalloc_flex(). hw_desc[] is annotated with __counted_by(nb_desc), so
__alloc_flex() now initializes sw_desc->nb_desc to nb_hw_desc itself.
The loop below it still increments nb_desc for every descriptor it
allocates though, so nb_desc ends up being twice the number of
descriptors that are actually there.

Drop the now redundant increment. The error path has to set nb_desc to
the number of descriptors allocated so far, otherwise pxad_free_desc()
would free entries that were never allocated.
---
Changes in v2:
- assign sw_desc->nb_desc manually as it is only automatically assigned
  for compilers supporting __builtin_counted_by_ref()
- Link to v1: https://lore.kernel.org/r/20260817-dmaengine-pxa-v1-1-850c215c1196@xxxxxxxxxxxxxx
---
 drivers/dma/pxa_dma.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/dma/pxa_dma.c b/drivers/dma/pxa_dma.c
index fa2ee0b3e09f8..fc43124fefa89 100644
--- a/drivers/dma/pxa_dma.c
+++ b/drivers/dma/pxa_dma.c
@@ -744,6 +744,7 @@ pxad_alloc_desc(struct pxad_chan *chan, unsigned int nb_hw_desc)
 	sw_desc = kzalloc_flex(*sw_desc, hw_desc, nb_hw_desc, GFP_NOWAIT);
 	if (!sw_desc)
 		return NULL;
+	sw_desc->nb_desc = nb_hw_desc;
 	sw_desc->desc_pool = chan->desc_pool;
 
 	for (i = 0; i < nb_hw_desc; i++) {
@@ -752,10 +753,10 @@ pxad_alloc_desc(struct pxad_chan *chan, unsigned int nb_hw_desc)
 			dev_err(&chan->vc.chan.dev->device,
 				"%s(): Couldn't allocate the %dth hw_desc from dma_pool %p\n",
 				__func__, i, sw_desc->desc_pool);
+			sw_desc->nb_desc = i;
 			goto err;
 		}
 
-		sw_desc->nb_desc++;
 		sw_desc->hw_desc[i] = desc;
 
 		if (i == 0)

---
base-commit: 8d3ae59288f1e7d58d76558a6ee96d533bc5019f
change-id: 20260817-dmaengine-pxa-64152bb34313

Best regards,
-- 
Sascha Hauer <s.hauer@xxxxxxxxxxxxxx>