Re: [PATCH v3 09/12] iio: dac: ad5686: implement new sync() op for the spi bus

[Rodrigo Alencar]

On 20/06/26 11:33, David Lechner wrote:
> On 6/16/26 3:21 AM, Rodrigo Alencar via B4 Relay wrote:
> > From: Rodrigo Alencar <rodrigo.alencar@xxxxxxxxxx>
> > 
> > Use of local SPI bus data to manage a collection of SPI transfers and
> > flush them to the SPI platform driver with the sync() operation. This
> > allows for faster handling of multiple channel DAC writes, avoiding kernel
> > overhead per spi_sync() call, which will be helpful when enabling
> > triggered buffer support.

...

> > +/**
> > + * struct ad5686_spi_data - SPI bus specific data
> > + * @msg: SPI message used for transfers
> > + * @size: number of transfers currently in the message
> > + * @capacity: maximum number of transfers that can be added to the message
> > + * @xfers: array of SPI transfers, allocated with the provided capacity
> > + */
> > +struct ad5686_spi_data {
> > +	struct spi_message msg;
> > +	unsigned int size;
> > +	unsigned int capacity;
> > +	struct spi_transfer xfers[] __counted_by(capacity);
> > +};
> > +
> >  static int ad5686_spi_write(struct ad5686_state *st,
> >  			    u8 cmd, u8 addr, u16 val)
> >  {
> > -	struct spi_device *spi = to_spi_device(st->dev);
> > -	u8 tx_len, *buf;
> > +	struct ad5686_spi_data *bus_data = st->bus_data;
> > +	struct spi_transfer *xfer;
> >  
> > +	if (bus_data->size >= bus_data->capacity)
> > +		return -E2BIG;
> > +
> > +	if (bus_data->size)
> > +		bus_data->xfers[bus_data->size - 1].cs_change = 1;
> > +	else
> > +		spi_message_init(&bus_data->msg);
> 
> Seems odd that spi_message_init() is called conditionally. What
> prevents spi_message_add_tail() from growing the message unbounded
> on repeated calls?

The "size >= capacity" check right above this.

> 
> > +
> > +	xfer = &bus_data->xfers[bus_data->size];
> >  	switch (st->chip_info->regmap_type) {
> >  	case AD5310_REGMAP:
> > -		st->data[0].d16 = cpu_to_be16(AD5310_CMD(cmd) |
> > -					      val);
> > -		buf = &st->data[0].d8[0];
> > -		tx_len = 2;
> > +		st->data[bus_data->size].d16 =
> > +			cpu_to_be16(AD5310_CMD(cmd) | val);
> > +		*xfer = (struct spi_transfer) {
> > +			.tx_buf = &st->data[bus_data->size].d16,
> > +			.len = sizeof(st->data[bus_data->size].d16),
> > +		};
> >  		break;
> >  	case AD5683_REGMAP:
> > -		st->data[0].d32 = cpu_to_be32(AD5686_CMD(cmd) |
> > -					      AD5683_DATA(val));
> > -		buf = &st->data[0].d8[1];
> > -		tx_len = 3;
> > +		st->data[bus_data->size].d32 =
> > +			cpu_to_be32(AD5686_CMD(cmd) | AD5683_DATA(val));
> > +		*xfer = (struct spi_transfer) {
> > +			.tx_buf = &st->data[bus_data->size].d8[1],
> > +			.len = sizeof(st->data[bus_data->size].d32) - 1,
> > +		};
> >  		break;
> >  	case AD5686_REGMAP:
> > -		st->data[0].d32 = cpu_to_be32(AD5686_CMD(cmd) |
> > -					      AD5686_ADDR(addr) |
> > -					      val);
> > -		buf = &st->data[0].d8[1];
> > -		tx_len = 3;
> > +		st->data[bus_data->size].d32 =
> > +			cpu_to_be32(AD5686_CMD(cmd) | AD5686_ADDR(addr) | val);
> > +		*xfer = (struct spi_transfer) {
> > +			.tx_buf = &st->data[bus_data->size].d8[1],
> > +			.len = sizeof(st->data[bus_data->size].d32) - 1,
> > +		};
> >  		break;
> >  	default:
> >  		return -EINVAL;
> >  	}
> >  
> > -	return spi_write(spi, buf, tx_len);
> 
> If this function no longer writes, should we change the name of
> the function to something like ad5686_spi_write_prepare_msg()?
> 
> > +	spi_message_add_tail(xfer, &bus_data->msg);
> > +	bus_data->size++;
> > +
> > +	return 0;
> > +}
> > +
> > +static int ad5686_spi_sync(struct ad5686_state *st)
> > +{
> > +	struct spi_device *spi = to_spi_device(st->dev);
> > +	struct ad5686_spi_data *bus_data = st->bus_data;
> > +
> > +	bus_data->size = 0; /* always reset, even on sync failure */
> > +	return spi_sync(spi, &bus_data->msg);
> >  }
> >  
> >  static int ad5686_spi_read(struct ad5686_state *st, u8 addr)
> >  {
> > -	struct spi_transfer t[] = {
> > -		{
> > -			.tx_buf = &st->data[0].d8[1],
> > -			.len = 3,
> > -			.cs_change = 1,
> > -		}, {
> > -			.tx_buf = &st->data[1].d8[1],
> > -			.rx_buf = &st->data[2].d8[1],
> > -			.len = 3,
> > -		},
> > -	};
> >  	struct spi_device *spi = to_spi_device(st->dev);
> > +	struct ad5686_spi_data *bus_data = st->bus_data;
> > +	struct spi_transfer *xfer = &bus_data->xfers[0];
> >  	u8 cmd = 0;
> >  	int ret;
> >  
> > @@ -85,8 +117,21 @@ static int ad5686_spi_read(struct ad5686_state *st, u8 addr)
> >  				      AD5686_ADDR(addr));
> >  	st->data[1].d32 = cpu_to_be32(AD5686_CMD(AD5686_CMD_NOOP));
> >  
> > -	ret = spi_sync_transfer(spi, t, ARRAY_SIZE(t));
> > -	if (ret < 0)
> > +	xfer[0] = (struct spi_transfer) {
> > +		.tx_buf = &st->data[0].d8[1],
> > +		.len = sizeof(st->data[0].d32) - 1,
> 
> Would make more sense to say `sizeof(st->data[0].d8) - 1` since
> the buffer is  &st->data[0].d8[1].

the buffer is d8[1], d8[2] and d8[3], skipping d8[0], for a len = 3.

> > +		.cs_change = 1,
> > +	};
> > +	xfer[1] = (struct spi_transfer) {
> > +		.tx_buf = &st->data[1].d8[1],
> > +		.rx_buf = &st->data[2].d8[1],
> > +		.len = sizeof(st->data[1].d32) - 1,
> 
> And here.
> 
> > +	};
> > +
> > +	spi_message_init_with_transfers(&bus_data->msg, xfer, 2);
> > +
> > +	ret = spi_sync(spi, &bus_data->msg);
> > +	if (ret)
> >  		return ret;
> >  
> >  	return be32_to_cpu(st->data[2].d32);
> > @@ -95,12 +140,26 @@ static int ad5686_spi_read(struct ad5686_state *st, u8 addr)
> >  static const struct ad5686_bus_ops ad5686_spi_ops = {
> >  	.write = ad5686_spi_write,
> >  	.read = ad5686_spi_read,
> > +	.sync = ad5686_spi_sync,
> >  };
> >  

-- 
Kind regards,

Rodrigo Alencar