Re: [PATCH v3 02/10] iio: dac: ad5446: Use DMA safe buffer for transfers

[Andy Shevchenko <andriy.shevchenko@intel.com>]

On Fri, Oct 31, 2025 at 12:31:23PM +0000, Nuno Sá via B4 Relay wrote:
> 
> Make sure to use DMA safe buffer. While for i2c we could be fine without
> them, we need it for spi anyways.
> 
> As we now have DMA safe buffers, use i2c_master_send_dmasafe().

...

> +	union {
> +		__be16 d16;
> +		u8 d24[3];

Why not __be32 d24; ? Yes, it will require explicit size to be provided, but at
least it will look consistent with the above. OR u8 d16[2]; ? But then it becomes
simply a u8 buf[3] __aligned...;

> +	} __aligned(IIO_DMA_MINALIGN);
>  };

...

>  static int ad5660_write(struct ad5446_state *st, unsigned val)
>  {
>  	struct spi_device *spi = to_spi_device(st->dev);
> -	uint8_t data[3];
>  
> -	put_unaligned_be24(val, &data[0]);
> +	put_unaligned_be24(val, &st->d24[0]);
>  
> -	return spi_write(spi, data, sizeof(data));
> +	return spi_write(spi, st->d24, sizeof(st->d24));
>  }

...

>  static int ad5622_write(struct ad5446_state *st, unsigned val)
>  {
>  	struct i2c_client *client = to_i2c_client(st->dev);
> -	__be16 data = cpu_to_be16(val);
> +	st->d16 = cpu_to_be16(val);

Not really, we don't mix code with definitions (with only few exceptions,
mostly related to cleanup.h).

>  	int ret;
>  
> -	ret = i2c_master_send(client, (char *)&data, sizeof(data));
> +	ret = i2c_master_send_dmasafe(client, (char *)&st->d16, sizeof(st->d16));

This will add a quite an overhead to the transfer (not that I²C is super fast,
but rather the processor is going to do _a lot_ of additional work here instead
of doing something more useful.

>  	if (ret < 0)
>  		return ret;

-- 
With Best Regards,
Andy Shevchenko