Re: [PATCH 1/2] hwmon: (jc42) optionally try to disable the SMBUS timeout

[Rob Herring]

On Fri, Oct 13, 2017 at 01:35:27PM -0700, Guenter Roeck wrote:
> On Fri, Oct 13, 2017 at 04:26:57PM +0200, Peter Rosin wrote:
> > On 2017-10-13 15:50, Guenter Roeck wrote:
> > > On 10/13/2017 02:27 AM, 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@xxxxxxxxxx>
> > >> ---
> > >>   Documentation/devicetree/bindings/hwmon/jc42.txt |  4 ++++
> > >>   drivers/hwmon/jc42.c                             | 20 ++++++++++++++++++++
> > >>   2 files changed, 24 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.

Is this only for jc24 devices or could be any smbus device?

> > >> +
> > >>   Example:
> > >>   
> > >>   temp-sensor@1a {
> > >> diff --git a/drivers/hwmon/jc42.c b/drivers/hwmon/jc42.c
> > >> index 1bf22eff0b08..fd816902fa30 100644
> > >> --- a/drivers/hwmon/jc42.c
> > >> +++ b/drivers/hwmon/jc42.c
> > >> @@ -45,6 +45,7 @@ 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
> > >> @@ -73,6 +74,9 @@ static const unsigned short normal_i2c[] = {
> > >>   #define ONS_MANID		0x1b09  /* ON Semiconductor */
> > >>   #define STM_MANID		0x104a  /* ST Microelectronics */
> > >>   
> > >> +/* SMBUS register */
> > >> +#define SMBUS_STMOUT		BIT(7)  /* SMBus time-out, active low */
> > >> +
> > >>   /* Supported chips */
> > >>   
> > >>   /* Analog Devices */
> > >> @@ -476,6 +480,22 @@ 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);
> > > 
> > > Looking into the SE97 datasheet, the bit is only writable if the alarm bits
> > > are not locked. Should we take this into account and unlock the alarm bits
> > > if necessary ?
> > 
> > Right. And I thought about the case when the timeout was disabled before
> > probing but with the property not present (perhaps by someone trying things
> > out, like I have). Should the timeout be re-enabled in that case?
> 
> No, because the property only states that the timeout should be disabled.
> It does not say that it should be _enabled_ if the property is not there.
> That would require a different property. A -> B does not imply B -> A.

A not-present/0/1 property is typically used for such cases. Perhaps you 
want that?

Rob