[PATCH v8 12/12] iio: ssp_sensors: reuse embedded RX buffer for SPI transfers

[Sanjay Chitroda]

From: Sanjay Chitroda <sanjayembeddedse@xxxxxxxxx>

Avoid allocating a temporary DMA buffer in the interrupt context when
handling hub-to-AP and AP-to-hub SPI write messages.

Replace the dynamically allocated RX buffer with a fixed-size,
preallocated buffer embedded in the driver structure and reused for all
SPI receive operations. This removes memory allocation from the
IRQ path, simplifies lifetime management.

Signed-off-by: Sanjay Chitroda <sanjayembeddedse@xxxxxxxxx>
---
Changes in v6:
- Address review comment of Jonathan and use __aligned(IIO_DMA_MINALIGN)
  and have buffer assignment specific to block instead of top.
- Regarding DMA and size discussion point are open to check
- v7 change: https://lore.kernel.org/all/20260426091710.3722035-10-sanjayembedded@xxxxxxxxx/
Changes in v6:
- Replace dynamically allocated RX buffer with embedded fixed-size buffer
- Fix struct layout to satisfy DMA alignment constraints comment from David
- v5 change: https://lore.kernel.org/all/20260406080852.2727453-6-sanjayembedded@xxxxxxxxx/
Changes in v5:
- Rebase change on top of latest v5 patch series.
Changes in v4:
- Use preallocated buffer and stash a buffer that gets reused each time instead of a fresh allocation.
- v3 change: https://lore.kernel.org/all/20260315125509.857195-3-sanjayembedded@xxxxxxxxx/
Changes in v3:
- prepare series to have all respective cleanup API support for the ssp_sensors following input from Andy Shevchenko
- v2 change: https://lore.kernel.org/all/20260311174151.3441429-1-sanjayembedded@xxxxxxxxx/
Changes in v2:
- split series to individual patch
- address review comment from Andy Shevchenko
- v1 change: https://lore.kernel.org/all/20260310200513.2162018-3-sanjayembedded@xxxxxxxxx/
---
 drivers/iio/common/ssp_sensors/ssp.h     |  3 +++
 drivers/iio/common/ssp_sensors/ssp_spi.c | 17 +++--------------
 2 files changed, 6 insertions(+), 14 deletions(-)

diff --git a/drivers/iio/common/ssp_sensors/ssp.h b/drivers/iio/common/ssp_sensors/ssp.h
index f649cdecc277..6d7409db014e 100644
--- a/drivers/iio/common/ssp_sensors/ssp.h
+++ b/drivers/iio/common/ssp_sensors/ssp.h
@@ -174,6 +174,7 @@ struct ssp_sensorhub_info {
  * @pending_list:	pending list for messages queued to be sent/read
  * @sensor_devs:	registered IIO devices table
  * @enable_refcount:	enable reference count for wdt (watchdog timer)
+ * @rx_buf:		buffer to receive SPI data
  * @header_buffer:	cache aligned buffer for packet header
  */
 struct ssp_data {
@@ -221,6 +222,8 @@ struct ssp_data {
 	struct iio_dev *sensor_devs[SSP_SENSOR_MAX];
 	atomic_t enable_refcount;
 
+	u8 rx_buf[SSP_DATA_PACKET_SIZE] __aligned(IIO_DMA_MINALIGN);
+
 	__le16 header_buffer[SSP_HEADER_BUFFER_SIZE / sizeof(__le16)] __aligned(IIO_DMA_MINALIGN);
 };
 
diff --git a/drivers/iio/common/ssp_sensors/ssp_spi.c b/drivers/iio/common/ssp_sensors/ssp_spi.c
index 172b4bbd298c..d6919e86aa95 100644
--- a/drivers/iio/common/ssp_sensors/ssp_spi.c
+++ b/drivers/iio/common/ssp_sensors/ssp_spi.c
@@ -396,17 +396,13 @@ int ssp_irq_msg(struct ssp_data *data)
 			 * but the slave should not send such ones - it is to
 			 * check but let's handle this
 			 */
-			buffer = kmalloc(length, GFP_KERNEL | GFP_DMA);
-			if (!buffer)
-				return -ENOMEM;
+			buffer = data->rx_buf;
 
 			/* got dead packet so it is always an error */
 			ret = spi_read(data->spi, buffer, length);
 			if (ret >= 0)
 				ret = -EPROTO;
 
-			kfree(buffer);
-
 			dev_err(SSP_DEV, "No match error %x\n",
 				msg_options);
 
@@ -438,22 +434,15 @@ int ssp_irq_msg(struct ssp_data *data)
 		return ret;
 	}
 	case SSP_HUB2AP_WRITE:
-		buffer = kzalloc(length, GFP_KERNEL | GFP_DMA);
-		if (!buffer)
-			return -ENOMEM;
+		buffer = data->rx_buf;
 
 		ret = spi_read(data->spi, buffer, length);
 		if (ret < 0) {
 			dev_err(SSP_DEV, "spi read fail\n");
-			kfree(buffer);
 			return ret;
 		}
 
-		ret = ssp_parse_dataframe(data, buffer, length);
-
-		kfree(buffer);
-		return ret;
-
+		return ssp_parse_dataframe(data, buffer, length);
 	default:
 		dev_err(SSP_DEV, "unknown msg type\n");
 		return -EPROTO;
-- 
2.34.1