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

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Message-Id: <20181109220044.24843-4-matheus.bernardino@usp.br>
Date:   Fri,  9 Nov 2018 20:00:41 -0200
From:   Matheus Tavares <matheus.bernardino@....br>
To:     Lars-Peter Clausen <lars@...afoo.de>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Cc:     linux-iio@...r.kernel.org, devel@...verdev.osuosl.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        kernel-usp@...glegroups.com, victorcolombo@...il.com
Subject: [PATCH 3/6] staging:iio:ad2s90: Add max frequency check at probe

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@....br>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@...log.com>
---
Alex's S-o-B was added because he gave the code suggestion for this
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;
+	}
+
 	indio_dev = devm_iio_device_alloc(&spi->dev, sizeof(*st));
 	if (!indio_dev)
 		return -ENOMEM;
-- 
2.18.0