Re: [PATCH v2 2/2] hwmon: (pmbus/max31785) use access_delay for PMBus-mediated accesses

[Guenter Roeck]

On 3/8/26 17:45, Sanman Pradhan wrote:
> From: Sanman Pradhan <psanman@xxxxxxxxxxx>
>
> The MAX31785 fan controller occasionally NACKs master transactions if
> accesses are too tightly spaced. To avoid this, the driver currently
> enforces a 250us inter-access delay with a private timestamp and wrapper
> functions around both raw SMBus accesses and PMBus helper paths.
>
> Simplify the driver by using pmbus_driver_info.access_delay for normal
> PMBus-mediated accesses instead, and remove the driver-local PMBus
> read/write wrappers.
>
> Keep local delay handling for raw SMBus accesses used before
> pmbus_do_probe(). For the raw i2c_transfer() long-read path, which
> bypasses PMBus core timing, leverage the newly exported pmbus_wait() and
> pmbus_update_ts() core functions. This replaces hardcoded usleep_range()
> calls and ensures the PMBus core tracks the raw transaction. Placing
> pmbus_update_ts() before the error check fixes a bug where a failed
> i2c_transfer() would skip the delay and impact subsequent PMBus commands.
>
> Additionally, update max31785_read_byte_data() so PMBUS_FAN_CONFIG_12
> accesses are only remapped for virtual pages, allowing physical-page
> accesses to fall back to the PMBus core. With that change, we can safely
> drop the custom max31785_update_fan() wrapper in favor of the core
> pmbus_update_fan() helper.
>
> Also use the delayed raw read helper for MFR_REVISION during probe, drop
> the unused to_max31785_data() macro, and rename the local variable
> "virtual" to "vpage".
>
> Signed-off-by: Sanman Pradhan <psanman@xxxxxxxxxxx>

AI review feedback (from https://sashiko.dev/#/patchset/20260309004508.78609-1-sanman.p211993%40gmail.com)
below.

Guenter

> ---
>   drivers/hwmon/pmbus/max31785.c | 187 +++++++++++----------------------
>   1 file changed, 59 insertions(+), 128 deletions(-)
>
> diff --git a/drivers/hwmon/pmbus/max31785.c b/drivers/hwmon/pmbus/max31785.c
> index 50073fe0c5e8..098f43486c45 100644
> --- a/drivers/hwmon/pmbus/max31785.c
> +++ b/drivers/hwmon/pmbus/max31785.c
> @@ -31,8 +31,6 @@ struct max31785_data {
>   	struct pmbus_driver_info info;
>   };
>   
> -#define to_max31785_data(x)  container_of(x, struct max31785_data, info)
> -
>   /*
>    * MAX31785 Driver Workaround
>    *
> @@ -40,9 +38,8 @@ struct max31785_data {
>    * These issues are not indicated by the device itself, except for occasional
>    * NACK responses during master transactions. No error bits are set in STATUS_BYTE.
>    *
> - * To address this, we introduce a delay of 250us between consecutive accesses
> - * to the fan controller. This delay helps mitigate communication problems by
> - * allowing sufficient time between accesses.
> + * Keep minimal local delay handling for raw pre-probe SMBus accesses.
> + * Normal PMBus-mediated accesses use pmbus_driver_info.access_delay instead.
>    */
>   static inline void max31785_wait(const struct max31785_data *data)
>   {
> @@ -54,89 +51,42 @@ static inline void max31785_wait(const struct max31785_data *data)
>   }
>   
>   static int max31785_i2c_write_byte_data(struct i2c_client *client,
> -					struct max31785_data *driver_data,
> -					int command, u8 data)
> +					struct max31785_data *data,
> +					int command, u8 value)
>   {
>   	int rc;
>   
> -	max31785_wait(driver_data);
> -	rc = i2c_smbus_write_byte_data(client, command, data);
> -	driver_data->access = ktime_get();
> +	max31785_wait(data);
> +	rc = i2c_smbus_write_byte_data(client, command, value);
> +	data->access = ktime_get();
>   	return rc;
>   }
>   
>   static int max31785_i2c_read_word_data(struct i2c_client *client,
> -				       struct max31785_data *driver_data,
> +				       struct max31785_data *data,
>   				       int command)
>   {
>   	int rc;
>   
> -	max31785_wait(driver_data);
> +	max31785_wait(data);
>   	rc = i2c_smbus_read_word_data(client, command);
> -	driver_data->access = ktime_get();
> -	return rc;
> -}
> -
> -static int _max31785_read_byte_data(struct i2c_client *client,
> -				    struct max31785_data *driver_data,
> -				    int page, int command)
> -{
> -	int rc;
> -
> -	max31785_wait(driver_data);
> -	rc = pmbus_read_byte_data(client, page, command);
> -	driver_data->access = ktime_get();
> -	return rc;
> -}
> -
> -static int _max31785_write_byte_data(struct i2c_client *client,
> -				     struct max31785_data *driver_data,
> -				     int page, int command, u16 data)
> -{
> -	int rc;
> -
> -	max31785_wait(driver_data);
> -	rc = pmbus_write_byte_data(client, page, command, data);
> -	driver_data->access = ktime_get();
> -	return rc;
> -}
> -
> -static int _max31785_read_word_data(struct i2c_client *client,
> -				    struct max31785_data *driver_data,
> -				    int page, int phase, int command)
> -{
> -	int rc;
> -
> -	max31785_wait(driver_data);
> -	rc = pmbus_read_word_data(client, page, phase, command);
> -	driver_data->access = ktime_get();
> -	return rc;
> -}
> -
> -static int _max31785_write_word_data(struct i2c_client *client,
> -				     struct max31785_data *driver_data,
> -				     int page, int command, u16 data)
> -{
> -	int rc;
> -
> -	max31785_wait(driver_data);
> -	rc = pmbus_write_word_data(client, page, command, data);
> -	driver_data->access = ktime_get();
> +	data->access = ktime_get();
>   	return rc;
>   }
>   
>   static int max31785_read_byte_data(struct i2c_client *client, int page, int reg)
>   {
> -	const struct pmbus_driver_info *info = pmbus_get_driver_info(client);
> -	struct max31785_data *driver_data = to_max31785_data(info);
>   
>   	switch (reg) {
>   	case PMBUS_VOUT_MODE:
>   		return -ENOTSUPP;
>   	case PMBUS_FAN_CONFIG_12:
> -		return _max31785_read_byte_data(client, driver_data,
> -						page - MAX31785_NR_PAGES,
> -						reg);
> +		if (page < MAX31785_NR_PAGES)
> +			return -ENODATA;
> +
> +		return pmbus_read_byte_data(client,
> +					    page - MAX31785_NR_PAGES,
> +					    reg);
>   	}
>   
>   	return -ENODATA;
> @@ -178,7 +128,20 @@ static int max31785_read_long_data(struct i2c_client *client, int page,
>   	if (rc < 0)
>   		return rc;
>   
> +	/* Ensure the raw transfer is properly spaced from the
> +	 * preceding PMBus transaction.
> +	 */
> +	pmbus_wait(client);
> +
>   	rc = i2c_transfer(client->adapter, msg, ARRAY_SIZE(msg));
> +
> +	/*
> +	 * Update PMBus core timing state for the raw transfer, even on error.
> +	 * Pass 0 as the operation mask since this is a raw read, intentionally
> +	 * neither PMBUS_OP_WRITE nor PMBUS_OP_PAGE_CHANGE.
> +	 */
> +	pmbus_update_ts(client, 0);
> +
>   	if (rc < 0)
>   		return rc;
>   
> @@ -203,19 +166,18 @@ static int max31785_get_pwm(struct i2c_client *client, int page)
>   	return rv;
>   }
>   
> -static int max31785_get_pwm_mode(struct i2c_client *client,
> -				 struct max31785_data *driver_data, int page)
> +static int max31785_get_pwm_mode(struct i2c_client *client, int page)
>   {
>   	int config;
>   	int command;
>   
> -	config = _max31785_read_byte_data(client, driver_data, page,
> -					  PMBUS_FAN_CONFIG_12);
> +	config = pmbus_read_byte_data(client, page,
> +				      PMBUS_FAN_CONFIG_12);
>   	if (config < 0)
>   		return config;
>   
> -	command = _max31785_read_word_data(client, driver_data, page, 0xff,
> -					   PMBUS_FAN_COMMAND_1);
> +	command = pmbus_read_word_data(client, page, 0xff,
> +				       PMBUS_FAN_COMMAND_1);
>   	if (command < 0)
>   		return command;
>   
> @@ -233,8 +195,6 @@ static int max31785_get_pwm_mode(struct i2c_client *client,
>   static int max31785_read_word_data(struct i2c_client *client, int page,
>   				   int phase, int reg)
>   {
> -	const struct pmbus_driver_info *info = pmbus_get_driver_info(client);
> -	struct max31785_data *driver_data = to_max31785_data(info);
>   	u32 val;
>   	int rv;
>   
> @@ -263,7 +223,7 @@ static int max31785_read_word_data(struct i2c_client *client, int page,
>   		rv = max31785_get_pwm(client, page);
>   		break;
>   	case PMBUS_VIRT_PWM_ENABLE_1:
> -		rv = max31785_get_pwm_mode(client, driver_data, page);
> +		rv = max31785_get_pwm_mode(client, page);
>   		break;
>   	default:
>   		rv = -ENODATA;
> @@ -294,35 +254,7 @@ static inline u32 max31785_scale_pwm(u32 sensor_val)
>   	return (sensor_val * 100) / 255;
>   }
>   
> -static int max31785_update_fan(struct i2c_client *client,
> -			       struct max31785_data *driver_data, int page,
> -			       u8 config, u8 mask, u16 command)
> -{
> -	int from, rv;
> -	u8 to;
> -
> -	from = _max31785_read_byte_data(client, driver_data, page,
> -					PMBUS_FAN_CONFIG_12);
> -	if (from < 0)
> -		return from;
> -
> -	to = (from & ~mask) | (config & mask);
> -
> -	if (to != from) {
> -		rv = _max31785_write_byte_data(client, driver_data, page,
> -					       PMBUS_FAN_CONFIG_12, to);
> -		if (rv < 0)
> -			return rv;
> -	}
> -
> -	rv = _max31785_write_word_data(client, driver_data, page,
> -				       PMBUS_FAN_COMMAND_1, command);
> -
> -	return rv;
> -}
> -
> -static int max31785_pwm_enable(struct i2c_client *client,
> -			       struct max31785_data *driver_data, int page,
> +static int max31785_pwm_enable(struct i2c_client *client, int page,
>   			       u16 word)
>   {
>   	int config = 0;
> @@ -351,23 +283,21 @@ static int max31785_pwm_enable(struct i2c_client *client,
>   		return -EINVAL;
>   	}
>   
> -	return max31785_update_fan(client, driver_data, page, config,
> +	return pmbus_update_fan(client, page, 0, config,
>   				   PB_FAN_1_RPM, rate);
>   }
>   
>   static int max31785_write_word_data(struct i2c_client *client, int page,
>   				    int reg, u16 word)
>   {
> -	const struct pmbus_driver_info *info = pmbus_get_driver_info(client);
> -	struct max31785_data *driver_data = to_max31785_data(info);
>   
>   	switch (reg) {
>   	case PMBUS_VIRT_PWM_1:
> -		return max31785_update_fan(client, driver_data, page, 0,
> -					   PB_FAN_1_RPM,
> -					   max31785_scale_pwm(word));
> +		return pmbus_update_fan(client, page, 0, 0,
> +					PB_FAN_1_RPM,
> +					max31785_scale_pwm(word));
>   	case PMBUS_VIRT_PWM_ENABLE_1:
> -		return max31785_pwm_enable(client, driver_data, page, word);
> +		return max31785_pwm_enable(client, page, word);
>   	default:
>   		break;
>   	}
> @@ -391,6 +321,7 @@ static const struct pmbus_driver_info max31785_info = {
>   	.read_byte_data = max31785_read_byte_data,
>   	.read_word_data = max31785_read_word_data,
>   	.write_byte = max31785_write_byte,
> +	.access_delay = MAX31785_WAIT_DELAY_US,
>   
>   	/* RPM */
>   	.format[PSC_FAN] = direct,
> @@ -438,29 +369,29 @@ static const struct pmbus_driver_info max31785_info = {
>   };
>   
>   static int max31785_configure_dual_tach(struct i2c_client *client,
> -					struct pmbus_driver_info *info)
> +					struct max31785_data *data)
>   {
> +	struct pmbus_driver_info *info = &data->info;
>   	int ret;
>   	int i;
> -	struct max31785_data *driver_data = to_max31785_data(info);
>   
>   	for (i = 0; i < MAX31785_NR_FAN_PAGES; i++) {
> -		ret = max31785_i2c_write_byte_data(client, driver_data,
> +		ret = max31785_i2c_write_byte_data(client, data,
>   						   PMBUS_PAGE, i);
>   		if (ret < 0)
>   			return ret;
>   
> -		ret = max31785_i2c_read_word_data(client, driver_data,
> +		ret = max31785_i2c_read_word_data(client, data,
>   						  MFR_FAN_CONFIG);
>   		if (ret < 0)
>   			return ret;
>   
>   		if (ret & MFR_FAN_CONFIG_DUAL_TACH) {
> -			int virtual = MAX31785_NR_PAGES + i;
> +			int vpage = MAX31785_NR_PAGES + i;
>   
> -			info->pages = virtual + 1;
> -			info->func[virtual] |= PMBUS_HAVE_FAN12;
> -			info->func[virtual] |= PMBUS_PAGE_VIRTUAL;
> +			info->pages = vpage + 1;
> +			info->func[vpage] |= PMBUS_HAVE_FAN12;
> +			info->func[vpage] |= PMBUS_PAGE_VIRTUAL;
>   		}
>   	}
>   
> @@ -471,7 +402,7 @@ static int max31785_probe(struct i2c_client *client)
>   {
>   	struct device *dev = &client->dev;
>   	struct pmbus_driver_info *info;
> -	struct max31785_data *driver_data;
> +	struct max31785_data *data;
>   	bool dual_tach = false;
>   	int ret;
>   
> @@ -480,20 +411,20 @@ static int max31785_probe(struct i2c_client *client)
>   				     I2C_FUNC_SMBUS_WORD_DATA))
>   		return -ENODEV;
>   
> -	driver_data = devm_kzalloc(dev, sizeof(struct max31785_data), GFP_KERNEL);
> -	if (!driver_data)
> +	data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
> +	if (!data)
>   		return -ENOMEM;
>   
> -	info = &driver_data->info;
> -	driver_data->access = ktime_get();
> +	data->access = ktime_get();
> +	info = &data->info;
>   	*info = max31785_info;
>   
> -	ret = max31785_i2c_write_byte_data(client, driver_data,
> -					   PMBUS_PAGE, 255);
> +	ret = max31785_i2c_write_byte_data(client, data,
> +					   PMBUS_PAGE, 0xff);
>   	if (ret < 0)
>   		return ret;
>   
> -	ret = i2c_smbus_read_word_data(client, MFR_REVISION);
> +	ret = max31785_i2c_read_word_data(client, data, MFR_REVISION);
>   	if (ret < 0)
>   		return ret;
>   
> @@ -509,7 +440,7 @@ static int max31785_probe(struct i2c_client *client)
>   	}
>   
>   	if (dual_tach) {
> -		ret = max31785_configure_dual_tach(client, info);
> +		ret = max31785_configure_dual_tach(client, data);
>   		if (ret < 0)
>   			return ret;
>   	}

Is there a missing inter-access delay before the first PMBus core transaction?
During probe, the driver performs raw I2C accesses and enforces the delay
using max31785_wait(data). The driver then calls pmbus_do_probe(), which
initializes the PMBus core delay tracking variable.
When the PMBus core executes its very first command, its internal delay
logic calculates a negative delay against the newly initialized variable and
executes the transaction without delay.
Because the PMBus core is unaware of the driver local timestamp, could
this violate the 250us delay requirement from the last pre-probe driver
access and cause the controller to NACK the first command?
Should there be an explicit max31785_wait(data) right before calling
pmbus_do_probe() to prevent this?