linux-kernel - Re: [PATCH 1/5] hwmon: pmbus: Implement generic bus access delay

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Message-ID: <114e3fd3-d7d7-41bb-a6b8-babd05ef4eab@roeck-us.net>
Date: Mon, 15 Jul 2024 19:22:58 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Patrick Rudolph <patrick.rudolph@...ements.com>,
 linux-kernel@...r.kernel.org
Cc: Jean Delvare <jdelvare@...e.com>, linux-hwmon@...r.kernel.org
Subject: Re: [PATCH 1/5] hwmon: pmbus: Implement generic bus access delay

On 7/15/24 00:30, Patrick Rudolph wrote:
> Some drivers, like the max15301 or zl6100, are intentionally delaying
> SMBus communications, to prevent transmission errors. As this is necessary
> on additional PMBus compatible devices, implement a generic delay mechanism
> in the pmbus core.
> 
> Introduces two delay settings in the pmbus_driver_info struct, one applies
> to every SMBus transaction and the other is for write transaction only.
> Once set by the driver the SMBus traffic, using the generic pmbus access
> helpers, is automatically delayed when necessary.
> 
> The two settings are:
> access_delay:
>    - Unit in microseconds
>    - Stores the accessed timestamp after every SMBus access
>    - Delays when necessary before the next SMBus access
> 
> write_delay:
>    - Unit in microseconds
>    - Stores the written timestamp after a write SMBus access
>    - Delays when necessary before the next SMBus access
> 
> This allows to drop the custom delay code from the drivers and easily
> introduce this feature in additional pmbus drivers.
> 
> Signed-off-by: Patrick Rudolph <patrick.rudolph@...ements.com>
> ---
>   drivers/hwmon/pmbus/pmbus.h      | 10 ++++
>   drivers/hwmon/pmbus/pmbus_core.c | 92 +++++++++++++++++++++++++++++---
>   2 files changed, 96 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/hwmon/pmbus/pmbus.h b/drivers/hwmon/pmbus/pmbus.h
> index fb442fae7b3e..5d5dc774187b 100644
> --- a/drivers/hwmon/pmbus/pmbus.h
> +++ b/drivers/hwmon/pmbus/pmbus.h
> @@ -466,6 +466,16 @@ struct pmbus_driver_info {
>   
>   	/* custom attributes */
>   	const struct attribute_group **groups;
> +
> +	/*
> +	 * Some chips need a little delay between SMBus communication. When
> +	 * set, the generic PMBus helper functions will wait if necessary
> +	 * to meet this requirement. The access delay is honored after
> +	 * every SMBus operation. The write delay is only honored after
> +	 * SMBus write operations.
> +	 */
> +	int access_delay;		/* in microseconds */
> +	int write_delay;		/* in microseconds */
>   };
>   
>   /* Regulator ops */
> diff --git a/drivers/hwmon/pmbus/pmbus_core.c b/drivers/hwmon/pmbus/pmbus_core.c
> index cb4c65a7f288..5cb093c898a1 100644
> --- a/drivers/hwmon/pmbus/pmbus_core.c
> +++ b/drivers/hwmon/pmbus/pmbus_core.c
> @@ -7,6 +7,7 @@
>    */
>   
>   #include <linux/debugfs.h>
> +#include <linux/delay.h>
>   #include <linux/kernel.h>
>   #include <linux/math64.h>
>   #include <linux/module.h>
> @@ -108,6 +109,8 @@ struct pmbus_data {
>   
>   	int vout_low[PMBUS_PAGES];	/* voltage low margin */
>   	int vout_high[PMBUS_PAGES];	/* voltage high margin */
> +	ktime_t write_time;		/* Last SMBUS write timestamp */
> +	ktime_t access_time;		/* Last SMBUS access timestamp */
>   };
>   
>   struct pmbus_debugfs_entry {
> @@ -158,6 +161,39 @@ void pmbus_set_update(struct i2c_client *client, u8 reg, bool update)
>   }
>   EXPORT_SYMBOL_NS_GPL(pmbus_set_update, PMBUS);
>   
> +/* Some chips need a delay between accesses. */
> +static inline void pmbus_optional_wait(struct i2c_client *client)

I'd suggest to name the function either pmbus_access_wait() or even simply pmbus_wait().

> +{
> +	struct pmbus_data *data = i2c_get_clientdata(client);
> +	const struct pmbus_driver_info *info = data->info;
> +	s64 delta;
> +
> +	if (info->access_delay) {
> +		delta = ktime_us_delta(ktime_get(), data->access_time);
> +
> +		if (delta < info->access_delay)
> +			udelay(info->access_delay - delta);

I am not too happy with this one; the delay can get large.
I'd suggest to use fsleep().

> +	} else if (info->write_delay) {
> +		delta = ktime_us_delta(ktime_get(), data->write_time);
> +
> +		if (delta < info->write_delay)
> +			udelay(info->write_delay - delta);

Same here.

Thanks,
Guenter