linux-kernel - Re: [PATCH 2/2] hwmon: iio: Add labels from IIO channels

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Date: Fri, 21 Jun 2024 09:45:59 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Sean Anderson <sean.anderson@...ux.dev>,
 Jonathan Cameron <jic23@...nel.org>, Jean Delvare <jdelvare@...e.com>,
 linux-iio@...r.kernel.org, linux-hwmon@...r.kernel.org
Cc: Lars-Peter Clausen <lars@...afoo.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] hwmon: iio: Add labels from IIO channels

On 6/21/24 08:31, Sean Anderson wrote:
> On 6/21/24 11:22, Sean Anderson wrote:
>> On 6/21/24 11:08, Guenter Roeck wrote:
>>> On 6/20/24 14:13, Sean Anderson wrote:
>>>> Add labels from IIO channels to our channels. This allows userspace to
>>>> display more meaningful names instead of "in0" or "temp5".
>>>>
>>>> Signed-off-by: Sean Anderson <sean.anderson@...ux.dev>
>>>> ---
>>>>
>>>>    drivers/hwmon/iio_hwmon.c | 33 ++++++++++++++++++++++++++++++---
>>>>    1 file changed, 30 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/hwmon/iio_hwmon.c b/drivers/hwmon/iio_hwmon.c
>>>> index 4c8a80847891..588b64c18e63 100644
>>>> --- a/drivers/hwmon/iio_hwmon.c
>>>> +++ b/drivers/hwmon/iio_hwmon.c
>>>> @@ -33,6 +33,17 @@ struct iio_hwmon_state {
>>>>        struct attribute **attrs;
>>>>    };
>>>>    +static ssize_t iio_hwmon_read_label(struct device *dev,
>>>> +                  struct device_attribute *attr,
>>>> +                  char *buf)
>>>> +{
>>>> +    struct sensor_device_attribute *sattr = to_sensor_dev_attr(attr);
>>>> +    struct iio_hwmon_state *state = dev_get_drvdata(dev);
>>>> +    struct iio_channel *chan = &state->channels[sattr->index];
>>>> +
>>>> +    return iio_read_channel_label(chan, buf);
>>>
>>> This can return -EINVAL if there is no label. Since the label attribute
>>> is created unconditionally, every affected system would end up with
>>> lots of error messages when running the "sensors" command.
>>> This is not acceptable.
>>
>> The sensors command gracefully handles this. There are no errors, and the label is unused.
> 
> For example, without IIO labels I get:
> 
> $ sensors hwmon_ams_ps-isa-0000
> hwmon_ams_ps-isa-0000
> Adapter: ISA adapter
> in1:         825.00 mV
> in2:         826.00 mV
> in3:           1.81 V
> in4:           1.18 V
> in5:           1.80 V
> in6:           1.80 V
> in7:           3.27 V
> in8:           1.81 V
> in9:         825.00 mV
> in10:          1.81 V
> in11:          1.80 V
> temp1:        +79.8 C
> temp2:        +80.9 C
> 
> and with labels I get
> 
> $ sensors hwmon_ams_ps-isa-0000
> hwmon_ams_ps-isa-0000
> Adapter: ISA adapter
> VCC_PSINTLP: 824.00 mV
> VCC_PSINTFP: 822.00 mV
> VCC_PSAUX:     1.81 V
> VCC_PSDDR:     1.18 V
> VCC_PSIO3:     1.80 V
> VCC_PSIO0:     1.80 V
> VCC_PSIO1:     3.27 V
> VCC_PSIO2:     1.80 V
> PS_MGTRAVCC: 822.00 mV
> PS_MGTRAVTT:   1.81 V
> VCC_PSADC:     1.80 V
> Temp_LPD:     +79.5 C
> Temp_FPD:     +80.2 C
> 

You are correct, the sensors code ignores access errors on label attributes.
Still, I am not even going there. If there is no label, don't create
the attribute, period. The ABI says

     Should only be created if the driver has hints about what
     this voltage channel is being used for, and user-space
     doesn't. In all other cases, the label is provided by
     user-space.

It should be obvious that this doesn't mean that you can create random
attributes and then just return an error if it isn't actually supported.

Guenter