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Date: Tue, 28 May 2024 08:50:49 -0700
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Subject: Re: [PATCH v2 1/2] hwmon: add ChromeOS EC driver
To: Stephen Horvath <s.horvath@outlook.com.au>,
 =?UTF-8?Q?Thomas_Wei=C3=9Fschuh?= <linux@weissschuh.net>
Cc: Jean Delvare <jdelvare@suse.com>, Benson Leung <bleung@chromium.org>,
 Lee Jones <lee@kernel.org>, Guenter Roeck <groeck@chromium.org>,
 linux-kernel@vger.kernel.org, linux-hwmon@vger.kernel.org,
 chrome-platform@lists.linux.dev, Dustin Howett <dustin@howett.net>,
 Mario Limonciello <mario.limonciello@amd.com>,
 Moritz Fischer <mdf@kernel.org>
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From: Guenter Roeck <linux@roeck-us.net>
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On 5/27/24 17:15, Stephen Horvath wrote:
> Hi Thomas,
> 
> On 28/5/24 05:24, Thomas Weißschuh wrote:
>> Hi Stephen,
>>
>> 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@weissschuh.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@weissschuh.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.

>> 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.

Guenter

---
[1] https://chromium.googlesource.com/chromiumos/platform/ec