Re: [PATCH v5 5/5] iio: ssp_sensors: reuse preallocated RX buffer for SPI transfers

[David Lechner <dlechner@baylibre.com>]

On 4/6/26 3:08 AM, Sanjay Chitroda wrote:
> From: Sanjay Chitroda <sanjayembeddedse@gmail.com>
> 
> Avoid allocating a temporary DMA buffer in the interrupt context when
> handling hub-to-AP and AP-to-hub SPI write messages.
> 
> Preallocate RX buffer during probe and reuse it for SPI receive
> operations. This removes repeated kzalloc() calls from the IRQ
> path, reduces allocation overhead, and avoids potential allocation
> failures under memory pressure.
> 
> The RX buffer size is tracked and allocated using devm_kzalloc(), ensuring
> proper lifetime management tied to the device.
> 
> No functional change intended; this is an internal optimization and
> robustness improvement.

If we are going to claim this is an optimization, we should have
some measurements to back that up.

> 
> Signed-off-by: Sanjay Chitroda <sanjayembeddedse@gmail.com>
> ---
> 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.
> - Link to v3: https://lore.kernel.org/all/20260315125509.857195-3-sanjayembedded@gmail.com/
> Changes in v3:
> - prepare series to have all respective cleanup API support for the ssp_sensors following input from Andy Shevchenko
> - Link to v2 https://lore.kernel.org/all/20260311174151.3441429-1-sanjayembedded@gmail.com/
> Changes in v2:
> - split series to individual patch
> - address review comment from Andy Shevchenko
> - Link to v1 https://lore.kernel.org/all/20260310200513.2162018-3-sanjayembedded@gmail.com/
> ---
>  drivers/iio/common/ssp_sensors/ssp.h     |  5 +++++
>  drivers/iio/common/ssp_sensors/ssp_dev.c | 12 ++++++++++++
>  drivers/iio/common/ssp_sensors/ssp_spi.c | 19 +++----------------
>  3 files changed, 20 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/iio/common/ssp_sensors/ssp.h b/drivers/iio/common/ssp_sensors/ssp.h
> index f649cdecc277..aa125fd1bed5 100644
> --- a/drivers/iio/common/ssp_sensors/ssp.h
> +++ b/drivers/iio/common/ssp_sensors/ssp.h
> @@ -175,6 +175,8 @@ struct ssp_sensorhub_info {
>   * @sensor_devs:	registered IIO devices table
>   * @enable_refcount:	enable reference count for wdt (watchdog timer)
>   * @header_buffer:	cache aligned buffer for packet header
> + * @rx_buf:		buffer to receive SPI data
> + * @rx_buf_size:	allocated size of rx_buf
>   */
>  struct ssp_data {
>  	struct spi_device *spi;
> @@ -222,6 +224,9 @@ struct ssp_data {
>  	atomic_t enable_refcount;
>  
>  	__le16 header_buffer[SSP_HEADER_BUFFER_SIZE / sizeof(__le16)] __aligned(IIO_DMA_MINALIGN);
> +
> +	u8 *rx_buf;
> +	size_t rx_buf_size;

No, these can't be after _aligned(IIO_DMA_MINALIGN); without causing problems.

What would work here though is:

	u8 rx_buf[SSP_DATA_PACKET_SIZE];

>  };
>  
>  void ssp_clean_pending_list(struct ssp_data *data);
> diff --git a/drivers/iio/common/ssp_sensors/ssp_dev.c b/drivers/iio/common/ssp_sensors/ssp_dev.c
> index aab28f2a0f75..2a8d6f040ae4 100644
> --- a/drivers/iio/common/ssp_sensors/ssp_dev.c
> +++ b/drivers/iio/common/ssp_sensors/ssp_dev.c
> @@ -516,6 +516,18 @@ static int ssp_probe(struct spi_device *spi)
>  		goto err_setup_spi;
>  	}
>  
> +	data->rx_buf_size = SSP_DATA_PACKET_SIZE;
> +	data->rx_buf = devm_kzalloc(&spi->dev,
> +				    data->rx_buf_size,
> +				    GFP_KERNEL | GFP_DMA);

Since this is a fixed size, we don't need a separate alloc here. We can
just embed the array in the data struct.

> +
> +	if (!data->rx_buf) {
> +		dev_err(&spi->dev,
> +			"Failed to allocate memory for rx_buf\n");
> +		ret = -ENOMEM;
> +		goto err_setup_spi;
> +	}
> +
>  	for (i = 0; i < SSP_SENSOR_MAX; ++i) {
>  		data->delay_buf[i] = SSP_DEFAULT_POLLING_DELAY;
>  		data->batch_latency_buf[i] = 0;
> diff --git a/drivers/iio/common/ssp_sensors/ssp_spi.c b/drivers/iio/common/ssp_sensors/ssp_spi.c
> index 7c1780e15acf..2f7445e8b4d1 100644
> --- a/drivers/iio/common/ssp_sensors/ssp_spi.c
> +++ b/drivers/iio/common/ssp_sensors/ssp_spi.c
> @@ -383,19 +383,13 @@ 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) {
> -				ret = -ENOMEM;
> -				goto _unlock;
> -			}
> +			buffer = data->rx_buf;


I don't think it is helpful to keep the buffer local variable.

>  
>  			/* 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);
>  
> @@ -428,22 +422,15 @@ int ssp_irq_msg(struct ssp_data *data)
>  		mutex_unlock(&data->pending_lock);
>  		break;
>  	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);
>  			break;
>  		}
>  
> -		ret = ssp_parse_dataframe(data, buffer, length);
> -
> -		kfree(buffer);
> -		break;
> -
> +		return ssp_parse_dataframe(data, buffer, length);
>  	default:
>  		dev_err(SSP_DEV, "unknown msg type\n");
>  		return -EPROTO;