Date:   Thu, 17 Nov 2016 15:40:24 -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 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.

Thanks,
Guenter

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