Re: [PATCH 5/5] iio: adc: ad7380: use spi_optimize_message()

[Jonathan Cameron]

On Mon, 12 Feb 2024 17:26:45 -0600
David Lechner <dlechner@xxxxxxxxxxxx> wrote:

> This modifies the ad7380 ADC driver to use spi_optimize_message() to
> optimize the SPI message for the buffered read operation. Since buffered
> reads reuse the same SPI message for each read, this can improve
> performance by reducing the overhead of setting up some parts the SPI
> message in each spi_sync() call.
> 
> Signed-off-by: David Lechner <dlechner@xxxxxxxxxxxx>
> ---
>  drivers/iio/adc/ad7380.c | 52 +++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 45 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/iio/adc/ad7380.c b/drivers/iio/adc/ad7380.c
> index abd746aef868..5c5d2642a474 100644
> --- a/drivers/iio/adc/ad7380.c
> +++ b/drivers/iio/adc/ad7380.c
> @@ -133,6 +133,7 @@ struct ad7380_state {
>  	struct spi_device *spi;
>  	struct regulator *vref;
>  	struct regmap *regmap;
> +	struct spi_message *msg;
>  	/*
>  	 * DMA (thus cache coherency maintenance) requires the
>  	 * transfer buffers to live in their own cache lines.
> @@ -231,19 +232,55 @@ static int ad7380_debugfs_reg_access(struct iio_dev *indio_dev, u32 reg,
>  	return ret;
>  }
>  
> +static int ad7380_buffer_preenable(struct iio_dev *indio_dev)
> +{
> +	struct ad7380_state *st = iio_priv(indio_dev);
> +	struct spi_transfer *xfer;
> +	int ret;
> +
> +	st->msg = spi_message_alloc(1, GFP_KERNEL);

As it only ever has one element, is there a clear advantage over
just embedding the spi_message in the structure rather than
as a separate allocation? You'd need the transfer as well.

	spi_message_init_with_transfers(st->msg, &st->trans, 1);

The transfer is then also available without walking the list (though
obviously you don't walk very far ;).

> +	if (!st->msg)
> +		return -ENOMEM;
> +
> +	xfer = list_first_entry(&st->msg->transfers, struct spi_transfer,
> +				transfer_list);
> +
> +	xfer->bits_per_word = st->chip_info->channels[0].scan_type.realbits;
> +	xfer->len = 4;
> +	xfer->rx_buf = st->scan_data.raw;
> +
> +	ret = spi_optimize_message(st->spi, st->msg);
> +	if (ret) {
> +		spi_message_free(st->msg);
Would avoid freeing explicitly here or later if it was embedded in
struct ad7380_state

Also, this doesn't seem very dynamic in general. Anything stopping this
being done at probe() as a one time thing?

> +		return ret;
> +	}
> +
> +	return 0;
> +}
> +
> +static int ad7380_buffer_postdisable(struct iio_dev *indio_dev)
> +{
> +	struct ad7380_state *st = iio_priv(indio_dev);
> +
> +	spi_unoptimize_message(st->msg);
> +	spi_message_free(st->msg);
> +
> +	return 0;
> +}
> +
> +static const struct iio_buffer_setup_ops ad7380_buffer_setup_ops = {
> +	.preenable = ad7380_buffer_preenable,
> +	.postdisable = ad7380_buffer_postdisable,
> +};
> +
>  static irqreturn_t ad7380_trigger_handler(int irq, void *p)
>  {
>  	struct iio_poll_func *pf = p;
>  	struct iio_dev *indio_dev = pf->indio_dev;
>  	struct ad7380_state *st = iio_priv(indio_dev);
> -	struct spi_transfer xfer = {
> -		.bits_per_word = st->chip_info->channels[0].scan_type.realbits,
> -		.len = 4,
> -		.rx_buf = st->scan_data.raw,
> -	};
>  	int ret;
>  
> -	ret = spi_sync_transfer(st->spi, &xfer, 1);
> +	ret = spi_sync(st->spi, st->msg);
>  	if (ret)
>  		goto out;
>  
> @@ -420,7 +457,8 @@ static int ad7380_probe(struct spi_device *spi)
>  
>  	ret = devm_iio_triggered_buffer_setup(&spi->dev, indio_dev,
>  					      iio_pollfunc_store_time,
> -					      ad7380_trigger_handler, NULL);
> +					      ad7380_trigger_handler,
> +					      &ad7380_buffer_setup_ops);
>  	if (ret)
>  		return ret;
>  
>