[PATCH v2] spi: stm32: fix pointer-to-pointer variables usage

[Antonio Quartulli <antonio@mandelbit.com>]

In stm32_spi_prepare_rx_dma_mdma_chaining() both rx_dma_desc
and rx_mdma_desc are passed as pointer-to-pointer arguments.

The goal is to pass back to the caller the value returned
by dmaengine_prep_slave_sg(), when it is not NULL.

However, these variables are wrongly handled as simple pointers
during later assignments and checks.

Fix this behaviour by introducing two pointer variables
which can then be treated accordingly.

Fixes: d17dd2f1d8a1 ("spi: stm32: use STM32 DMA with STM32 MDMA to enhance DDR use")
Addresses-Coverity-ID: 1644715 ("Null pointer dereferences (REVERSE_INULL)")
Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>

---
Changes from v1:
* introduce *_mdma_desc and *_dma_desc for better readability
* fix another instance of rx_dma_desc bogus assignment in case of
  failure of sg_alloc_table()
* commit title/message reworded accordingly to the previous point
---
 drivers/spi/spi-stm32.c | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/drivers/spi/spi-stm32.c b/drivers/spi/spi-stm32.c
index 3d20f09f1ae7..4b7f074fdba9 100644
--- a/drivers/spi/spi-stm32.c
+++ b/drivers/spi/spi-stm32.c
@@ -1474,6 +1474,8 @@ static int stm32_spi_prepare_rx_dma_mdma_chaining(struct stm32_spi *spi,
 						  struct dma_async_tx_descriptor **rx_dma_desc,
 						  struct dma_async_tx_descriptor **rx_mdma_desc)
 {
+	struct dma_async_tx_descriptor *_mdma_desc = *rx_mdma_desc;
+	struct dma_async_tx_descriptor *_dma_desc = *rx_dma_desc;
 	struct dma_slave_config rx_mdma_conf = {0};
 	u32 sram_period, nents = 0, spi_s_len;
 	struct sg_table dma_sgt, mdma_sgt;
@@ -1524,18 +1526,18 @@ static int stm32_spi_prepare_rx_dma_mdma_chaining(struct stm32_spi *spi,
 		}
 	}
 
-	*rx_dma_desc = dmaengine_prep_slave_sg(spi->dma_rx, dma_sgt.sgl,
-					       dma_sgt.nents, rx_dma_conf->direction,
-					       DMA_PREP_INTERRUPT);
+	_dma_desc = dmaengine_prep_slave_sg(spi->dma_rx, dma_sgt.sgl,
+					    dma_sgt.nents, rx_dma_conf->direction,
+					    DMA_PREP_INTERRUPT);
 	sg_free_table(&dma_sgt);
 
-	if (!rx_dma_desc)
+	if (!_dma_desc)
 		return -EINVAL;
 
 	/* Prepare MDMA slave_sg transfer MEM_TO_MEM (SRAM>DDR) */
 	ret = sg_alloc_table(&mdma_sgt, nents, GFP_ATOMIC);
 	if (ret) {
-		rx_dma_desc = NULL;
+		_dma_desc = NULL;
 		return ret;
 	}
 
@@ -1558,13 +1560,13 @@ static int stm32_spi_prepare_rx_dma_mdma_chaining(struct stm32_spi *spi,
 		}
 	}
 
-	*rx_mdma_desc = dmaengine_prep_slave_sg(spi->mdma_rx, mdma_sgt.sgl,
-						mdma_sgt.nents, rx_mdma_conf.direction,
-						DMA_PREP_INTERRUPT);
+	_mdma_desc = dmaengine_prep_slave_sg(spi->mdma_rx, mdma_sgt.sgl,
+					     mdma_sgt.nents, rx_mdma_conf.direction,
+					     DMA_PREP_INTERRUPT);
 	sg_free_table(&mdma_sgt);
 
-	if (!rx_mdma_desc) {
-		rx_dma_desc = NULL;
+	if (!_mdma_desc) {
+		_dma_desc = NULL;
 		return -EINVAL;
 	}
 
-- 
2.49.0