Date:   Wed, 28 Jun 2017 19:33:33 +0200
From:   Mike Looijmans <mike.looijmans@...ic.nl>
To:     Tom Levens <tom.levens@...n.ch>, Guenter Roeck <linux@...ck-us.net>
CC:     <jdelvare@...e.com>, <robh+dt@...nel.org>, <mark.rutland@....com>,
        <linux-kernel@...r.kernel.org>, <linux-hwmon@...r.kernel.org>,
        <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] hwmon: ltc2990: support all measurement modes

On 28-06-17 19:02, Tom Levens wrote:
> 
> On Wed, 28 Jun 2017, Guenter Roeck wrote:
> 
>> On Wed, Jun 28, 2017 at 05:29:38PM +0200, Tom Levens wrote:
>>>
>> [ ... ]
>>
>>>>
>>>>> Whatever happened to this patch though? It didn't make it to mainline,
>>>>> otherwise I'd have found it sooner...
>>>>>
>>>> I'll have to look it up, but I guess I didn't get an updated version.
>>>
>>> As far as I remember I had a working V3 of this patch, but it is 
>>> entirely
>>> possible that it was never submitted as I have been busy with other 
>>> projects
>>> recently. I'll dig it out and check that it is complete.
>>>
>> FWIW, I don't see it at
>> https://patchwork.kernel.org/project/linux-hwmon/list/?submitter=171225&state=* 
>>
>>
>> Maybe you were waiting for a reply from Rob. Either case, it might make
>> sense to only provide valid modes, ie to abstract the mode bits from the
>> hardware, such as
>>
>> 0 - internal temp only
>> 1 - Tr1
>> 2 - V1
>> 3 - V1-V2
>> 4 - Tr2
>> 5 - V3
>> 6 - V3-V4
>> 7 to 14 - per bit 0..2
>>
>> Guenter
>>
> 
> You are right, there was still an open question about how best to handle 
> the mode selection in DT.
> 
> In the latest version of my patch I have it implemented as an array for 
> setting the two values, for example:
> 
>      lltc,meas-mode = <7 3>;
> 
> This sets bits [2..0] = 7 and [4..3] = 3. Of course these could be split 
> into two DT properties, but I was unsure what to name them as both 
> fields are called "mode" in the datasheet and "mode-43"/"mode-20" (or 
> similar) is ugly.
> 
> With regards to your proposal, it is not clear to me whether the modes 
> which have the same result are exactly equivalent. Does disabling a 
> measurement with the mode[4..3] bits really leaves the part in a safe 
> state for all possible HW connections? With this doubt in my head, I 
> would prefer to keep the option available to the user to select any 
> specific mode. But I am open to suggestions.

Well, the input restrictions always apply, so disabling V3 measurement 
doesn't imply that you can apply 20V to that input safely now.

I'd suggest to set unused input to plain voltage measurement. That is 
"passive" and safe for external components.

So I'd suggest just setting the mode as per device datasheet, I can see 
no real advantage in abstracting it away and forcing users to read yet 
another document to get it right, e.g.:

lltc,mode = <6>;

As for the input disabling, since I doubt anyone would use it (why 
purchase a 4-channel device and use only 2), just add two booleans, e.g. 
"disable-inputs-34" and "disable-inputs-12" which set the command bits 
appropriately, and change the mode such that the disabled inputs are 
voltage readout only.

A case could even be made for changing mode at runtime. This allows 
using it to measure both current and voltage on two inputs, by reading 
V1, and V3, and then switch mode to obtain (accurate) V1-V2 and V4-V3.

That might be a viable way to handle not setting the mode at all. If the 
mode can be selected via sysfs, the driver can keep the device in a 
"safe" mode until the mode has been selected.


> Mike, if you would like to test it, the latest version of my code is here:
> 
> https://github.com/levens/ltc2990/blob/dev/drivers/hwmon/ltc2990.c

Sure, I even have a board with 2 of these devices now :)

-- 
Mike Looijmans


Kind regards,

Mike Looijmans
System Expert

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