Re: [PATCH 3/6] staging:iio:ad2s90: Add max frequency check at probe

[Jonathan Cameron <jic23@kernel.org>]

On Fri,  9 Nov 2018 20:00:41 -0200
Matheus Tavares <matheus.bernardino@usp.br> wrote:

> This patch adds a max frequency check at the beginning of ad2s90_probe
> function so that when it is set to a value above 0.83Mhz, dev_err is
> called with an appropriate message and -EINVAL is returned.
> 
> The defined limit is 0.83Mhz instead of 2Mhz, which is the chip's max
> frequency as specified in the datasheet, because, as also specified in
> the datasheet, a 600ns delay is expected between the application of a
> logic LO to CS and the application of SCLK. Since the delay is not
> implemented in the spi code, to satisfy it, SCLK's period should be at
> most 2 * 600ns, so the max frequency should be 1 / (2 * 6e-7), which
> gives roughly 830000Hz.
> 
> Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
> Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
> ---
> Alex's S-o-B was added because he gave the code suggestion for this
> patch.
If he gave the actual code then he should be the credited author
git commit --amend --author=...

If it was just a suggestion, then an informal tag such as 
Suggested-by is usually used.

Signed-off-by has formal legal meaning to do with the developer
certificate of origin, it's not valid as a way of crediting someone
with a contribution to the patch.


> 
>  drivers/staging/iio/resolver/ad2s90.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/drivers/staging/iio/resolver/ad2s90.c b/drivers/staging/iio/resolver/ad2s90.c
> index 95c118c48400..949ff55ac6b0 100644
> --- a/drivers/staging/iio/resolver/ad2s90.c
> +++ b/drivers/staging/iio/resolver/ad2s90.c
> @@ -19,6 +19,12 @@
>  #include <linux/iio/iio.h>
>  #include <linux/iio/sysfs.h>
>  
> +/*
> + * Although chip's max frequency is 2Mhz, it needs 600ns between CS and the
> + * first falling edge of SCLK, so frequency should be at most 1 / (2 * 6e-7)
> + */
> +#define AD2S90_MAX_SPI_FREQ_HZ  830000
> +
>  struct ad2s90_state {
>  	struct mutex lock;
>  	struct spi_device *sdev;
> @@ -78,6 +84,12 @@ static int ad2s90_probe(struct spi_device *spi)
>  	struct iio_dev *indio_dev;
>  	struct ad2s90_state *st;
>  
> +	if (spi->max_speed_hz > AD2S90_MAX_SPI_FREQ_HZ) {
> +		dev_err(&spi->dev, "SPI CLK, %d Hz exceeds %d Hz\n",
> +			spi->max_speed_hz, AD2S90_MAX_SPI_FREQ_HZ);
> +		return -EINVAL;
> +	}
Now this is interesting as a follow up to the previous and may actually
answer the question I raised.  Let's see what Mark and Rob come
back with. If possible I'd like us to resolve this once and for all
with a thread to point people back to!

> +
>  	indio_dev = devm_iio_device_alloc(&spi->dev, sizeof(*st));
>  	if (!indio_dev)
>  		return -ENOMEM;