Date:   Fri, 18 Nov 2016 06:16:38 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Tom Levens <tom.levens@...n.ch>
Cc:     Mike Looijmans <mike.looijmans@...ic.nl>,
        "jdelvare@...e.com" <jdelvare@...e.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] hwmon: ltc2990: support all measurement modes

On 11/18/2016 04:23 AM, Tom Levens wrote:
>
>
> On Fri, 18 Nov 2016, Guenter Roeck wrote:
>
>> On Thu, Nov 17, 2016 at 11:25:30PM +0000, Tom Levens wrote:
>>> On 17 Nov 2016, at 22:54, Guenter Roeck <linux@...ck-us.net> wrote:
>>>> On Thu, Nov 17, 2016 at 08:52:12PM +0100, Mike Looijmans wrote:
>>>>> On 17-11-2016 19:56, Guenter Roeck wrote:
>>>>>> On Thu, Nov 17, 2016 at 06:40:17PM +0100, Mike Looijmans wrote:
>>>>>>> On 17-11-16 17:56, Guenter Roeck wrote:
>>>>>>>> On 11/17/2016 04:10 AM, Tom Levens wrote:
>>>>>>>>> Updated version of the ltc2990 driver which supports all measurement
>>>>>>>>> modes available in the chip. The mode can be set through a devicetree
>>>>>>>>> attribute.
>>>>>>>>
>>>>>> [ ... ]
>>>>>>
>>>>>>>>>
>>>>>>>>> static int ltc2990_i2c_probe(struct i2c_client *i2c,
>>>>>>>>>                 const struct i2c_device_id *id)
>>>>>>>>> {
>>>>>>>>>    int ret;
>>>>>>>>>    struct device *hwmon_dev;
>>>>>>>>> +    struct ltc2990_data *data;
>>>>>>>>> +    struct device_node *of_node = i2c->dev.of_node;
>>>>>>>>>
>>>>>>>>>    if (!i2c_check_functionality(i2c->adapter,
>>>>>>>>> I2C_FUNC_SMBUS_BYTE_DATA |
>>>>>>>>>                     I2C_FUNC_SMBUS_WORD_DATA))
>>>>>>>>>        return -ENODEV;
>>>>>>>>>
>>>>>>>>> -    /* Setup continuous mode, current monitor */
>>>>>>>>> +    data = devm_kzalloc(&i2c->dev, sizeof(struct ltc2990_data),
>>>>>>>>> GFP_KERNEL);
>>>>>>>>> +    if (unlikely(!data))
>>>>>>>>> +        return -ENOMEM;
>>>>>>>>> +    data->i2c = i2c;
>>>>>>>>> +
>>>>>>>>> +    if (!of_node || of_property_read_u32(of_node, "lltc,mode",
>>>>>>>>> &data->mode))
>>>>>>>>> +        data->mode = LTC2990_CONTROL_MODE_DEFAULT;
>>>>>>>>
>>>>>>>> Iam arguing with myself if we should still do this or if we should read
>>>>>>>> the mode
>>>>>>>> from the chip instead if it isn't provided (after all, it may have been
>>>>>>>> initialized
>>>>>>>> by the BIOS/ROMMON).
>>>>>>>
>>>>>>> I think the mode should be explicitly set, without default. There's no way
>>>>>>> to tell whether the BIOS or bootloader has really set it up or whether the
>>>>>>> chip is just reporting whatever it happened to default to. And given the
>>>>>>> chip's function, it's unlikely a bootloader would want to initialize it.
>>>>>>>
>>>>>> Unlikely but possible. Even if we all agree that the chip should be configured
>>>>>> by the driver, I don't like imposing that view on everyone else.
>>>>>>
>>>>>>> My advice would be to make it a required property. If not set, display an
>>>>>>> error and bail out.
>>>>>>>
>>>>>> It is not that easy, unfortunately. It also has to work on a non-devicetree
>>>>>> system. I would not object to making the property mandatory, but we would
>>>>>> still need to provide non-DT support.
>>>>>>
>>>>>> My "use case" for taking the current mode from the chip if not specified
>>>>>> is that it would enable me to run a module test with all modes. I consider
>>>>>> this extremely valuable.
>>>>>
>>>>> Good point.
>>>>>
>>>>> The chip defaults to measuring internal temperature only, and the mode
>>>>> defaults to "0".
>>>>>
>>>>> Choosing a mode that doesn't match the actual circuitry could be bad for the
>>>>> chip or the board (though unlikely, it'll probably just be useless) since it
>>>>> will actively drive some of the inputs in the temperature modes (which is
>>>>> default for V3/V4 pins).
>>>>>
>>>>> Instead of failing, one could choose to set the default mode to "7", which
>>>>> just measures the 4 voltages, which would be a harmless mode in all cases.
>>>>>
>>>>> As a way to let a bootloader set things up, I think it would be a good check
>>>>> to see if CONTROL register bits 4:3 are set. If "00", the chip is not
>>>>> acquiring data at all, and probably needs configuration still. In that case,
>>>>> the mode must be provided by the devicetree (or the default "7").
>>>>> If bits 4:3 are "11", it has already been set up to measure its inputs, and
>>>>> it's okay to continue doing just that and use the current value of 2:0
>>>>> register as default mode (if the devicetree didn't specify any mode at all).
>>>>>
>>>>
>>>> At first glance, agreed, though by default b[3:4] are 00, and only the
>>>> internal temperature is measured. Actually, the 5 mode bits are all
>>>> relevant to determine what is measured. Maybe it would be better to take
>>>> all 5 bits into account instead of blindly setting b[34]:=11 and a specific
>>>> setting of b[0:2]. Sure, that would make the mode table a bit larger,
>>>> but then ltc2990_attrs_ena[] could be made an u16 array, and a table size
>>>> of 64 bytes would not be that bad.
>>>
>>> I would tend to agree that it should be possible to configure all 5 mode
>>> bits through the devicetree. What I would propose is as follows.
>>>
>>> If a devicetree node exists, the mode parameter(s?) are required and the
>>> chip is initialised.
>>>
>>> If a devicetree node doesn't exist, it is assumed that the chip has
>>> already been configured (by the BIOS, etc). The mode is read from the
>>> chip to set the visibility of the sysfs attributes. In the worst case, where the
>>> chip has not been configured by another source, it would only be possible
>>> to measure the internal temperature -- but I think this is an acceptable
>>> limitation.
>>>
>> SGTM.
>>
>>> The only case that this does not cover is if the device tree node
>>> exists but the chip is expected to be configured by some other source.
>>> Maybe I am wrong, but I would not expect this to be a terribly common
>>> situation.
>>>
>>> What do you think?
>>>
>> I would not bother about this case. Just make the mode property mandatory.
>
> What do you think about making the devicetree property a list of two integers? Something like
>
> lltc,mode = <7 3>;
>
> which would set mode[2..0]=7 and mode[4..3]=3.
>
I would personally just use a single value for b[4..0]. But that is really up for bikeshedding
(eg should it be <7 3> or <3 7>. I'll leave that up to Rob to decide - he knows better than me
of what makes more sense from a DT perspective.

Guenter

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