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Date:   Wed, 29 Nov 2017 12:52:19 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Peter Rosin <peda@...ntia.se>
Cc:     linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        Russell King <linux@...linux.org.uk>,
        Jean Delvare <jdelvare@...e.com>,
        Ludovic Desroches <ludovic.desroches@...rochip.com>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-hwmon@...r.kernel.org
Subject: Re: [v2,1/2] hwmon: (jc42) optionally try to disable the SMBUS
 timeout

On Mon, Nov 27, 2017 at 05:31:00PM +0100, Peter Rosin wrote:
> With a nxp,se97 chip on an atmel sama5d31 board, the I2C adapter driver
> is not always capable of avoiding the 25-35 ms timeout as specified by
> the SMBUS protocol. This may cause silent corruption of the last bit of
> any transfer, e.g. a one is read instead of a zero if the sensor chip
> times out. This also affects the eeprom half of the nxp-se97 chip, where
> this silent corruption was originally noticed. Other I2C adapters probably
> suffer similar issues, e.g. bit-banging comes to mind as risky...
> 
> The SMBUS register in the nxp chip is not a standard Jedec register, but
> it is not special to the nxp chips either, at least the atmel chips
> have the same mechanism. Therefore, do not special case this on the
> manufacturer, it is opt-in via the device property anyway.
> 
> Signed-off-by: Peter Rosin <peda@...ntia.se>
> Acked-by: Rob Herring <robh@...nel.org>

Applied to hwmon-next.

Thanks,
Guenter

> ---
>  Documentation/devicetree/bindings/hwmon/jc42.txt |  4 ++++
>  drivers/hwmon/jc42.c                             | 21 +++++++++++++++++++++
>  2 files changed, 25 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/hwmon/jc42.txt b/Documentation/devicetree/bindings/hwmon/jc42.txt
> index 07a250498fbb..f569db58f64a 100644
> --- a/Documentation/devicetree/bindings/hwmon/jc42.txt
> +++ b/Documentation/devicetree/bindings/hwmon/jc42.txt
> @@ -34,6 +34,10 @@ Required properties:
>  
>  - reg: I2C address
>  
> +Optional properties:
> +- smbus-timeout-disable: When set, the smbus timeout function will be disabled.
> +			 This is not supported on all chips.
> +
>  Example:
>  
>  temp-sensor@1a {
> diff --git a/drivers/hwmon/jc42.c b/drivers/hwmon/jc42.c
> index 5f11dc014ed6..e5234f953a6d 100644
> --- a/drivers/hwmon/jc42.c
> +++ b/drivers/hwmon/jc42.c
> @@ -22,6 +22,7 @@
>   * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
>   */
>  
> +#include <linux/bitops.h>
>  #include <linux/module.h>
>  #include <linux/init.h>
>  #include <linux/slab.h>
> @@ -45,6 +46,7 @@ static const unsigned short normal_i2c[] = {
>  #define JC42_REG_TEMP		0x05
>  #define JC42_REG_MANID		0x06
>  #define JC42_REG_DEVICEID	0x07
> +#define JC42_REG_SMBUS		0x22 /* NXP and Atmel, possibly others? */
>  
>  /* Status bits in temperature register */
>  #define JC42_ALARM_CRIT_BIT	15
> @@ -75,6 +77,9 @@ static const unsigned short normal_i2c[] = {
>  #define GT_MANID		0x1c68	/* Giantec */
>  #define GT_MANID2		0x132d	/* Giantec, 2nd mfg ID */
>  
> +/* SMBUS register */
> +#define SMBUS_STMOUT		BIT(7)  /* SMBus time-out, active low */
> +
>  /* Supported chips */
>  
>  /* Analog Devices */
> @@ -495,6 +500,22 @@ static int jc42_probe(struct i2c_client *client, const struct i2c_device_id *id)
>  
>  	data->extended = !!(cap & JC42_CAP_RANGE);
>  
> +	if (device_property_read_bool(dev, "smbus-timeout-disable")) {
> +		int smbus;
> +
> +		/*
> +		 * Not all chips support this register, but from a
> +		 * quick read of various datasheets no chip appears
> +		 * incompatible with the below attempt to disable
> +		 * the timeout. And the whole thing is opt-in...
> +		 */
> +		smbus = i2c_smbus_read_word_swapped(client, JC42_REG_SMBUS);
> +		if (smbus < 0)
> +			return smbus;
> +		i2c_smbus_write_word_swapped(client, JC42_REG_SMBUS,
> +					     smbus | SMBUS_STMOUT);
> +	}
> +
>  	config = i2c_smbus_read_word_swapped(client, JC42_REG_CONFIG);
>  	if (config < 0)
>  		return config;

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