Date: Wed, 29 May 2024 10:58:23 +1000
From: Stephen Horvath <s.horvath@...look.com.au>
To: Guenter Roeck <linux@...ck-us.net>,
 Thomas Weißschuh <linux@...ssschuh.net>
Cc: Jean Delvare <jdelvare@...e.com>, Benson Leung <bleung@...omium.org>,
 Lee Jones <lee@...nel.org>, Guenter Roeck <groeck@...omium.org>,
 linux-kernel@...r.kernel.org, linux-hwmon@...r.kernel.org,
 chrome-platform@...ts.linux.dev, Dustin Howett <dustin@...ett.net>,
 Mario Limonciello <mario.limonciello@....com>,
 Moritz Fischer <mdf@...nel.org>
Subject: Re: [PATCH v2 1/2] hwmon: add ChromeOS EC driver

Hi Guenter,

On 29/5/24 09:29, Guenter Roeck wrote:
> On 5/28/24 09:15, Thomas Weißschuh wrote:
>> On 2024-05-28 08:50:49+0000, Guenter Roeck wrote:
>>> On 5/27/24 17:15, Stephen Horvath wrote:
>>>> On 28/5/24 05:24, Thomas Weißschuh wrote:
>>>>> On 2024-05-25 09:13:09+0000, Stephen Horvath wrote:
>>>>>> I was the one to implement fan monitoring/control into Dustin's 
>>>>>> driver, and
>>>>>> just had a quick comment for your driver:
>>>>>>
>>>>>> On 8/5/24 02:29, Thomas Weißschuh wrote:
>>>>>>> The ChromeOS Embedded Controller exposes fan speed and temperature
>>>>>>> readings.
>>>>>>> Expose this data through the hwmon subsystem.
>>>>>>>
>>>>>>> The driver is designed to be probed via the cros_ec mfd device.
>>>>>>>
>>>>>>> Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
>>>>>>> ---
>>>>>>>     Documentation/hwmon/cros_ec_hwmon.rst |  26 ++++
>>>>>>>     Documentation/hwmon/index.rst         |   1 +
>>>>>>>     MAINTAINERS                           |   8 +
>>>>>>>     drivers/hwmon/Kconfig                 |  11 ++
>>>>>>>     drivers/hwmon/Makefile                |   1 +
>>>>>>>     drivers/hwmon/cros_ec_hwmon.c         | 269 
>>>>>>> ++++++++++++++++++++++++++++++++++
>>>>>>>     6 files changed, 316 insertions(+)
>>>>>>>
>>>>>
>>>>> <snip>
>>>>>
>>>>>>> diff --git a/drivers/hwmon/cros_ec_hwmon.c 
>>>>>>> b/drivers/hwmon/cros_ec_hwmon.c
>>>>>>> new file mode 100644
>>>>>>> index 000000000000..d59d39df2ac4
>>>>>>> --- /dev/null
>>>>>>> +++ b/drivers/hwmon/cros_ec_hwmon.c
>>>>>>> @@ -0,0 +1,269 @@
>>>>>>> +// SPDX-License-Identifier: GPL-2.0-or-later
>>>>>>> +/*
>>>>>>> + *  ChromesOS EC driver for hwmon
>>>>>>> + *
>>>>>>> + *  Copyright (C) 2024 Thomas Weißschuh <linux@...ssschuh.net>
>>>>>>> + */
>>>>>>> +
>>>>>>> +#include <linux/device.h>
>>>>>>> +#include <linux/hwmon.h>
>>>>>>> +#include <linux/kernel.h>
>>>>>>> +#include <linux/mod_devicetable.h>
>>>>>>> +#include <linux/module.h>
>>>>>>> +#include <linux/platform_device.h>
>>>>>>> +#include <linux/platform_data/cros_ec_commands.h>
>>>>>>> +#include <linux/platform_data/cros_ec_proto.h>
>>>>>>> +#include <linux/units.h>
>>>>>>> +
>>>>>>> +#define DRV_NAME    "cros-ec-hwmon"
>>>>>>> +
>>>>>>> +struct cros_ec_hwmon_priv {
>>>>>>> +    struct cros_ec_device *cros_ec;
>>>>>>> +    u8 thermal_version;
>>>>>>> +    const char *temp_sensor_names[EC_TEMP_SENSOR_ENTRIES + 
>>>>>>> EC_TEMP_SENSOR_B_ENTRIES];
>>>>>>> +};
>>>>>>> +
>>>>>>> +static int cros_ec_hwmon_read_fan_speed(struct cros_ec_device 
>>>>>>> *cros_ec, u8 index, u16 *speed)
>>>>>>> +{
>>>>>>> +    u16 data;
>>>>>>> +    int ret;
>>>>>>> +
>>>>>>> +    ret = cros_ec->cmd_readmem(cros_ec, EC_MEMMAP_FAN + index * 
>>>>>>> 2, 2, &data);
>>>>>>> +    if (ret < 0)
>>>>>>> +        return ret;
>>>>>>> +
>>>>>>> +    data = le16_to_cpu(data);
>>>>>>> +
>>>>>>> +    if (data == EC_FAN_SPEED_NOT_PRESENT)
>>>>>>> +        return -ENODEV;
>>>>>>> +
>>>>>>
>>>>>> Don't forget it can also return `EC_FAN_SPEED_STALLED`.
>>>>>
>>>>> Thanks for the hint. I'll need to think about how to handle this 
>>>>> better.
>>>>>
>>>>>> Like Guenter, I also don't like returning `-ENODEV`, but I don't 
>>>>>> have a
>>>>>> problem with checking for `EC_FAN_SPEED_NOT_PRESENT` in case it 
>>>>>> was removed
>>>>>> since init or something.
>>>>>
>>>
>>> That won't happen. Chromebooks are not servers, where one might be 
>>> able to
>>> replace a fan tray while the system is running.
>>
>> In one of my testruns this actually happened.
>> When running on battery, one specific of the CPU sensors sporadically
>> returned EC_FAN_SPEED_NOT_PRESENT.
>>
> 
> What Chromebook was that ? I can't see the code path in the EC source
> that would get me there.
> 

I believe Thomas and I both have the Framework 13 AMD, the source code 
is here: 
https://github.com/FrameworkComputer/EmbeddedController/tree/lotus-zephyr

The organisation confuses me a little, but Dustin has previous said on 
the framework forums 
(https://community.frame.work/t/what-ec-is-used/38574/2):

"This one is based on the Zephyr port of the ChromeOS EC, and tracks 
mainline more closely. It is in the branch lotus-zephyr.
All of the model-specific code lives in zephyr/program/lotus.
The 13"-specific code lives in a few subdirectories off the main tree 
named azalea."


Also I just unplugged my fan and you are definitely correct, the EC only 
generates EC_FAN_SPEED_NOT_PRESENT for fans it does not have the 
capability to support. Even after a reboot it just returns 0 RPM for an 
unplugged fan. I thought about simulating a stall too, but I was mildly 
scared I was going to break one of the tiny blades.

>>>>> Ok.
>>>>>
>>>>>> My approach was to return the speed as `0`, since the fan probably 
>>>>>> isn't
>>>>>> spinning, but set HWMON_F_FAULT for `EC_FAN_SPEED_NOT_PRESENT` and
>>>>>> HWMON_F_ALARM for `EC_FAN_SPEED_STALLED`.
>>>>>> No idea if this is correct though.
>>>>>
>>>>> I'm not a fan of returning a speed of 0 in case of errors.
>>>>> Rather -EIO which can't be mistaken.
>>>>> Maybe -EIO for both EC_FAN_SPEED_NOT_PRESENT (which should never 
>>>>> happen)
>>>>> and also for EC_FAN_SPEED_STALLED.
>>>>
>>>> Yeah, that's pretty reasonable.
>>>>
>>>
>>> -EIO is an i/o error. I have trouble reconciling that with
>>> EC_FAN_SPEED_NOT_PRESENT or EC_FAN_SPEED_STALLED.
>>>
>>> Looking into the EC source code [1], I see:
>>>
>>> EC_FAN_SPEED_NOT_PRESENT means that the fan is not present.
>>> That should return -ENODEV in the above code, but only for
>>> the purpose of making the attribute invisible.
>>>
>>> EC_FAN_SPEED_STALLED means exactly that, i.e., that the fan
>>> is present but not turning. The EC code does not expect that
>>> to happen and generates a thermal event in case it does.
>>> Given that, it does make sense to set the fault flag.
>>> The actual fan speed value should then be reported as 0 or
>>> possibly -ENODATA. It should _not_ generate any other error
>>> because that would trip up the "sensors" command for no
>>> good reason.
>>
>> Ack.
>>
>> Currently I have the following logic (for both fans and temp):
>>
>> if NOT_PRESENT during probing:
>>    make the attribute invisible.
>>
>> if any error during runtime (including NOT_PRESENT):
>>    return -ENODATA and a FAULT
>>
>> This should also handle the sporadic NOT_PRESENT failures.
>>
>> What do you think?
>>
>> Is there any other feedback to this revision or should I send the next?
>>
> 
> No, except I'd really like to know which Chromebook randomly generates
> a EC_FAN_SPEED_NOT_PRESENT response because that really looks like a bug.
> Also, can you reproduce the problem with the ectool command ?

I have a feeling it was related to the concurrency problems between ACPI 
and the CrOS code that are being fixed in another patch by Ben Walsh, I 
was also seeing some weird behaviour sometimes but I *believe* it was 
fixed by that.

Thanks,
Steve

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