linux-kernel - Re: [PATCH v1 1/4] hwmon: (acpi_power_meter) Fix using uninitialized variables

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Message-ID: <77fce1aa-96eb-4c3c-ab0a-a33de46b333b@roeck-us.net>
Date: Tue, 26 Nov 2024 08:19:52 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: "lihuisong (C)" <lihuisong@...wei.com>, linux-hwmon@...r.kernel.org,
 linux-kernel@...r.kernel.org
Cc: jdelvare@...e.com, liuyonglong@...wei.com, zhanjie9@...ilicon.com,
 zhenglifeng1@...wei.com
Subject: Re: [PATCH v1 1/4] hwmon: (acpi_power_meter) Fix using uninitialized
 variables

On 11/25/24 23:03, lihuisong (C) wrote:
> 
> 在 2024/11/26 12:04, Guenter Roeck 写道:
>> On 11/25/24 17:56, lihuisong (C) wrote:
>>> Hi Guente,
>>>
>>> Thanks for your timely review.
>>>
>>> 在 2024/11/26 0:03, Guenter Roeck 写道:
>>>> On 11/25/24 01:34, Huisong Li wrote:
>>>>> The 'power1_alarm' attribute uses the 'power' and 'cap' in the
>>>>> acpi_power_meter_resource structure. However, these two fields are just
>>>>> updated when user query 'power' and 'cap' attribute, or hardware enforced
>>>>> limit. If user directly query the 'power1_alarm' attribute without queryng
>>>>> above two attributes, driver will use the uninitialized variables to judge.
>>>>> In addition, the 'power1_alarm' attribute needs to update power and cap to
>>>>> show the real state.
>>>>>
>>>>> Signed-off-by: Huisong Li <lihuisong@...wei.com>
>>>>> ---
>>>>>   drivers/hwmon/acpi_power_meter.c | 10 ++++++++++
>>>>>   1 file changed, 10 insertions(+)
>>>>>
>>>>> diff --git a/drivers/hwmon/acpi_power_meter.c b/drivers/hwmon/acpi_power_meter.c
>>>>> index 2f1c9d97ad21..4c3314e35d30 100644
>>>>> --- a/drivers/hwmon/acpi_power_meter.c
>>>>> +++ b/drivers/hwmon/acpi_power_meter.c
>>>>> @@ -396,6 +396,9 @@ static ssize_t show_val(struct device *dev,
>>>>>       struct acpi_device *acpi_dev = to_acpi_device(dev);
>>>>>       struct acpi_power_meter_resource *resource = acpi_dev->driver_data;
>>>>>       u64 val = 0;
>>>>> +    int ret;
>>>>> +
>>>>> +    guard(mutex)(&resource->lock);
>>>>>         switch (attr->index) {
>>>>>       case 0:
>>>>> @@ -423,6 +426,13 @@ static ssize_t show_val(struct device *dev,
>>>>>               val = 0;
>>>>>           break;
>>>>>       case 6:
>>>>> +        ret = update_meter(resource);
>>>>> +        if (ret)
>>>>> +            return ret;
>>>>> +        ret = update_cap(resource);
>>>>> +        if (ret)
>>>>> +            return ret;
>>>>> +
>>>>>           if (resource->power > resource->cap)
>>>>>               val = 1;
>>>>>           else
>>>>
>>>>
>>>> While technically correct, the implementation of this attribute defeats its
>>>> purpose. It is supposed to reflect the current status as reported by the
>>>> hardware. A real fix would be to use the associated notification to set or
>>>> reset a status flag, and to report the current value of that flag as reported
>>>> by the hardware.
>>> I know what you mean.
>>> The Notify(power_meter, 0x83) is supposed to meet your proposal IIUC.
>>> It's good, but it depands on hardware support notification.
>>>>
>>>> If there is no notification support, the attribute should not even exist,
>>>> unless there is a means to retrieve its value from ACPI (the status itself,
>>>> not by comparing temperature values).
>>> Currently, the 'power1_alarm' attribute is created just when platform support the power meter meassurement(bit0 of the supported capabilities in _PMC).
>>> And it doesn't see if the platform support notifications.
>>>  From the current implementation of this driver, this sysfs can also reflect the status by comparing power and cap,
>>> which is good to the platform that support hardware limit from some out-of-band mechanism but doesn't support any notification.
>>>
>>
>> The point is that this can also be done from userspace. Hardware monitoring drivers
>> are supposed to provide hardware attributes, not software attributes derived from it.
>>
> So this 'power1_alarm' attribute can be exposed when platform supports hardware enforced limit and notifcations when the hardware limit is enforced, right?
> If so, we have to change the condition that driver creates this sysfs interface.

This isn't about enforcing anything, it is about reporting an alarm
if the power consumed exceeds the maximum configured.

Guenter