Re: [PATCH linux dev-5.10 07/14] pmbus: (max31785) Retry enabling fans after writing MFR_FAN_CONFIG

[Matthew Barth <msbarth@linux.ibm.com>]

On 8/11/21 10:42 AM, Eddie James wrote:
> From: Andrew Jeffery <andrew@aj.id.au>
>
> It has been observed across large fleets of systems that a small subset
> of those systems occasionally loose control of some number of fans
> across a BMC reboot (their hwmon fan attributes are missing from sysfs).
>
> >From extensive testing and tracing it was discovered that writes
> enabling a fan in FAN_CONFIG_1_2 failed to stick on the system under
> test with a frequency of about 1 in 1000 re-binds of the driver.
>
> The MAX31785 datasheet recommends in the documentation for
> MFR_FAN_CONFIG that the asssociated fan(s) be disabled before updating
> the register. The sequence in question implements this suggestion, and
> the observed loss-of-fans symptom occurs when the write to re-enable the
> fan in FAN_CONFIG_1_2 fails to stick.
>
> The trace data suggests a one-shot retry is enough to successfully
> update FAN_CONFIG_1_2. With the workaround, no loss of fans was observed
> in over 20,000 consecutive rebinds of the driver.
>
> OpenBMC-Staging-Count: 1
> Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
> Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Matthew Barth <msbarth@linux.ibm.com>
> ---
>  drivers/hwmon/pmbus/max31785.c | 23 +++++++++++++++++++----
>  1 file changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/hwmon/pmbus/max31785.c b/drivers/hwmon/pmbus/max31785.c
> index 7518fff356f9..b37da2ec1ce4 100644
> --- a/drivers/hwmon/pmbus/max31785.c
> +++ b/drivers/hwmon/pmbus/max31785.c
> @@ -398,6 +398,7 @@ static int max31785_of_fan_config(struct i2c_client *client,
>  	u32 page;
>  	u32 uval;
>  	int ret;
> +	int i;
>  
>  	if (!of_device_is_compatible(child, "pmbus-fan"))
>  		return 0;
> @@ -574,10 +575,24 @@ static int max31785_of_fan_config(struct i2c_client *client,
>  	if (ret < 0)
>  		return ret;
>  
> -	ret = max31785_i2c_smbus_write_byte_data(client, PMBUS_FAN_CONFIG_12,
> -						 pb_cfg);
> -	if (ret < 0)
> -		return ret;
> +	for (i = 0; i < 2; i++) {
> +		ret = max31785_i2c_smbus_write_byte_data(client,
> +							 PMBUS_FAN_CONFIG_12,
> +							 pb_cfg);
> +		if (ret < 0)
> +			continue;
> +
> +		ret = max31785_i2c_smbus_read_byte_data(client,
> +							PMBUS_FAN_CONFIG_12);
> +		if (ret < 0)
> +			continue;
> +
> +		if (ret == pb_cfg)
> +			break;
> +	}
> +
> +	if (i == 2)
> +		return -EIO;
>  
>  	/*
>  	 * Fans are on pages 0 - 5. If the page property of a fan node is