linux-kernel - Re: [PATCH v2 3/7] staging:iio:ad2s90: Add max frequency check at probe

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Message-ID: <8d86411bf3267e9d4c72f725d3da87ac4bace203.camel@analog.com>
Date:   Mon, 19 Nov 2018 08:18:53 +0000
From:   "Ardelean, Alexandru" <alexandru.Ardelean@...log.com>
To:     "lars@...afoo.de" <lars@...afoo.de>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "knaack.h@....de" <knaack.h@....de>,
        "jic23@...nel.org" <jic23@...nel.org>,
        "Hennerich, Michael" <Michael.Hennerich@...log.com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "matheus.bernardino@....br" <matheus.bernardino@....br>,
        "pmeerw@...erw.net" <pmeerw@...erw.net>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC:     "victorcolombo@...il.com" <victorcolombo@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
        "kernel-usp@...glegroups.com" <kernel-usp@...glegroups.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 3/7] staging:iio:ad2s90: Add max frequency check at
 probe

On Sun, 2018-11-18 at 02:25 -0200, Matheus Tavares wrote:
> From: Alexandru Ardelean <alexandru.ardelean@...log.com>
> 
> 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: Alexandru Ardelean <alexandru.ardelean@...log.com>

I think you can use "Suggested-by:" instead.
But this is also fine.

> Signed-off-by: Matheus Tavares <matheus.bernardino@....br>
> ---
> Changes in v2:
>  - Correctly credit Alexandru as the patch's author
> 
>  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 913d6fad2d4d..fe90f2056bff 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;
> +	}
> +
>  	indio_dev = devm_iio_device_alloc(&spi->dev, sizeof(*st));
>  	if (!indio_dev)
>  		return -ENOMEM;