[PATCH 2/5] dmaengine: xilinx_dma: Fix cyclic period count truncation

["Jeff C. Jensen" <kernel@nullptr.com>]

xilinx_dma_prep_dma_cyclic() stores buf_len / period_len, a size_t
quotient, in an unsigned int. On 64-bit builds the count can be
truncated: a buffer of 2^32 + 1 bytes split into 1-byte periods becomes
a single period, and the transfer covers one byte of the buffer the
client asked for.

Compute the quotient as a size_t and reject it if it exceeds
XILINX_DMA_NUM_DESCS before narrowing it. Each period needs at least one
hardware descriptor, so a larger count could never be prepared from the
pool anyway.

Fixes: 92d794dfb699 ("dmaengine: vdma: Add support for cyclic dma mode")
Assisted-by: Claude:claude-fable-5-1
Signed-off-by: Jeff C. Jensen <kernel@nullptr.com>
---
 drivers/dma/xilinx/xilinx_dma.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/dma/xilinx/xilinx_dma.c b/drivers/dma/xilinx/xilinx_dma.c
index 5ccc9f9d58a3..aa714effd10d 100644
--- a/drivers/dma/xilinx/xilinx_dma.c
+++ b/drivers/dma/xilinx/xilinx_dma.c
@@ -2454,7 +2454,7 @@ static struct dma_async_tx_descriptor *xilinx_dma_prep_dma_cyclic(
 	struct xilinx_dma_chan *chan = to_xilinx_chan(dchan);
 	struct xilinx_dma_tx_descriptor *desc;
 	struct xilinx_axidma_tx_segment *segment, *head_segment, *prev = NULL;
-	size_t copy, sg_used;
+	size_t copy, sg_used, periods;
 	unsigned int num_periods;
 	int i;
 	u32 reg;
@@ -2462,10 +2462,11 @@ static struct dma_async_tx_descriptor *xilinx_dma_prep_dma_cyclic(
 	if (!period_len)
 		return NULL;
 
-	num_periods = buf_len / period_len;
-
-	if (!num_periods)
+	/* a period takes at least one descriptor */
+	periods = buf_len / period_len;
+	if (!periods || periods > XILINX_DMA_NUM_DESCS)
 		return NULL;
+	num_periods = periods;
 
 	if (!is_slave_direction(direction))
 		return NULL;

-- 
2.43.0