linux-kernel - Re: [PATCH v5 3/4] pmbus (core): Add virtual page config bit

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Date:   Sun, 19 Nov 2017 08:49:19 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Andrew Jeffery <andrew@...id.au>, linux-hwmon@...r.kernel.org
Cc:     robh+dt@...nel.org, mark.rutland@....com, jdelvare@...e.com,
        corbet@....net, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        joel@....id.au, openbmc@...ts.ozlabs.org
Subject: Re: [PATCH v5 3/4] pmbus (core): Add virtual page config bit

On 11/17/2017 07:26 PM, Andrew Jeffery wrote:
> Some circumstances call for virtual pages, to expose multiple values
> packed into an extended PMBus register in a manner non-compliant with
> the PMBus standard. An example of this is the Maxim MAX31785 controller, which
> extends the READ_FAN_SPEED_1 PMBus register from two to four bytes to support
> tach readings for both rotors of a dual rotor fan. This extended register
> contains two word-sized values, one reporting the rate of the fastest rotor,
> the other the rate of the slowest. The concept of virtual pages aids this
> situation by mapping the page number onto the value to be selected from the
> vectored result.
> 
> We should not try to set virtual pages on the device as such a page explicitly
> doesn't exist; add a flag so we can avoid doing so.
> 
> Signed-off-by: Andrew Jeffery <andrew@...id.au>
> ---
>   drivers/hwmon/pmbus/pmbus.h      |  2 ++
>   drivers/hwmon/pmbus/pmbus_core.c | 10 +++++++---
>   2 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/hwmon/pmbus/pmbus.h b/drivers/hwmon/pmbus/pmbus.h
> index b54d7604d3ef..d39d506aa63e 100644
> --- a/drivers/hwmon/pmbus/pmbus.h
> +++ b/drivers/hwmon/pmbus/pmbus.h
> @@ -372,6 +372,8 @@ enum pmbus_sensor_classes {
>   #define PMBUS_HAVE_PWM12	BIT(20)
>   #define PMBUS_HAVE_PWM34	BIT(21)
>   
> +#define PMBUS_PAGE_VIRTUAL	BIT(31)
> +
>   enum pmbus_data_format { linear = 0, direct, vid };
>   enum vrm_version { vr11 = 0, vr12, vr13 };
>   
> diff --git a/drivers/hwmon/pmbus/pmbus_core.c b/drivers/hwmon/pmbus/pmbus_core.c
> index 631db88526b6..ba97230fcde7 100644
> --- a/drivers/hwmon/pmbus/pmbus_core.c
> +++ b/drivers/hwmon/pmbus/pmbus_core.c
> @@ -164,14 +164,18 @@ int pmbus_set_page(struct i2c_client *client, int page)
>   	int rv = 0;
>   	int newpage;
>   
> -	if (page >= 0 && page != data->currpage) {
> +	if (page < 0 || page == data->currpage)
> +		return 0;
> +
> +	if (!(data->info->func[page] & PMBUS_PAGE_VIRTUAL)) {
>   		rv = i2c_smbus_write_byte_data(client, PMBUS_PAGE, page);
>   		newpage = i2c_smbus_read_byte_data(client, PMBUS_PAGE);
>   		if (newpage != page)
>   			rv = -EIO;

This should probably be something like
		rv = i2c_smbus_write_byte_data(client, PMBUS_PAGE, page);
		if (rv < 0)
			return rv;
		newpage = i2c_smbus_read_byte_data(client, PMBUS_PAGE);
		if (newpage < 0)
			return newpage;
		if (newpage != page)
			return -EIO;

We can actually drop 'newpage' and just use 'rv'.

> -		else
> -			data->currpage = page;
>   	}
> +
> +	data->currpage = page;
> +

... otherwise currpage is set even on error.

>   	return rv;

this can then be
	return 0;

>   }
>   EXPORT_SYMBOL_GPL(pmbus_set_page);
>