Re: [PATCH v7 9/9] iio: ssp_sensors: reuse embedded RX buffer for SPI transfers

Jonathan Cameron
2026-04-26T14:32:04Z

On Sun, 26 Apr 2026 14:47:10 +0530
Sanjay Chitroda <sanjayembeddedse@gmail.com> 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.
>
> 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@gmail.com>
> ---
> 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@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.
> - v3 change: 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
> - v2 change: 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
> - v1 change: https://lore.kernel.org/all/20260310200513.2162018-3-sanjayembedded@gmail.com/
> ---
> drivers/iio/common/ssp_sensors/ssp.h | 3 +++
> drivers/iio/common/ssp_sensors/ssp_spi.c | 20 ++------------------
> 2 files changed, 5 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/iio/common/ssp_sensors/ssp.h b/drivers/iio/common/ssp_sensors/ssp.h
> index f649cdecc277..8295bb7062a3 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];

Whilst the define exists for this size, do we have anything to confirm it is right?
It wasn't previously used for anything so is a bit high risk.

Also, why doesn't it need to be DMA safe? I'd expect it either after the following buffer
or with a __aligned(IIO_DMA_MINALIGN) marking in which case the one below isn't used.

> +
> __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 870214551f0b..e41da88bf96d 100644
> --- a/drivers/iio/common/ssp_sensors/ssp_spi.c
> +++ b/drivers/iio/common/ssp_sensors/ssp_spi.c
> @@ -339,7 +339,7 @@ static int ssp_parse_dataframe(struct ssp_data *data, char *dataframe, int len)
> /* threaded irq */
> int ssp_irq_msg(struct ssp_data *data)
> {
> - char *buffer;
> + char *buffer = data->rx_buf;

Put this in inline. The local variable may reduce code churn but
it also hides the important info that this is not locally allocated.

> u8 msg_type;
> int ret;
> u16 length, msg_options;
> @@ -383,19 +383,12 @@ 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;
> - }
>
> /* 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 +421,13 @@ 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;
> -
> 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);

This now makes this path different from the other switch legs.
I would precede this patch with one doing direct returns from
all the legs in here. Then this just becomes an obvious update
in this patch.

> default:
> dev_err(SSP_DEV, "unknown msg type\n");
> return -EPROTO;