Date:   Wed, 26 Sep 2018 13:44:55 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     jdelvare@...e.com, corbet@....net, afd@...com,
        linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH 2/2] hwmon: ina3221: Add enable sysfs nodes

On Wed, Sep 26, 2018 at 01:25:20PM -0700, Nicolin Chen wrote:
> Hello,
> 
> On Wed, Sep 26, 2018 at 12:58:17PM -0700, Guenter Roeck wrote:
> > On Wed, Sep 26, 2018 at 11:02:44AM -0700, Nicolin Chen wrote:
> > > On Wed, Sep 26, 2018 at 06:06:32AM -0700, Guenter Roeck wrote:
> > > > On 09/25/2018 11:42 PM, Nicolin Chen wrote:
> > > > > The inX_enable interface allows user space to enable or disable
> > > > > the corresponding channel. Meanwhile, according to hwmon ABI, a
> > > > > disabled channel/sensor should return -ENODATA as a read result.
> > > > > 
> > > > > However, there're configurable nodes sharing the same __show()
> > > > > functions. So this change also adds to check if the attribute is
> > > > > read-only to make sure it's not reading a configuration but the
> > > > > sensor data.
> > >  
> > > > One necessary high level change I don't see below: With this change,
> > > > we should no longer drop a channel entirely if it is disabled from
> > > > devicetree. All channels should be visible but report -ENODATA if
> > > > disabled. In other words, it should be possible for the 'enable' flag
> > > > to override settings in DT.
> > > 
> > > Hmm...I don't feel so convinced here. The status in DT binding isn't
> > > exactly a setting but a physical status: if a hardware design leaves
> > > a channel to be disconnected, I don't really see a point in enabling
> > > it in the runtime. Or maybe you can shed some light on it?
> > > 
> > 
> > You are making an assumption from your use case. It might as well be that
> > some or all channels are disabled in DT by default to conserve power,
> > not because they are disconnected.
> 
> I think I probably should update my DT binding somehow to say it
> explicitly that the property should be only used in cases of the
> physical disconnections, although I feel the current binding "no
> input source" already has the same meaning.
> 
> In my opinion, disabling channels in DT to save power isn't very
> plausible, because it sounds more likely a user decision, while,
> as we know, DT merely describes the hardware design.
> 

I try to avoid making such assumptions. All I know is that I'll have
to deal with the fallout if a single person wants to use the property
differently. This is similar to disabling an entire subsystem in DT
because it isn't used in a specific system - say, a SPI or I2C bus
which has nothing connected on some shipping hardware, even though
the board has a connector for it. Your argument is that one shall
not use the status property do disable loading the driver, and that
one must not remove a set of properties for unused hardware either.
That doesn't sound very realistic to me.

Point is that I don't _know_ how this is going to be used, so I'd
rather keep it flexible.

Guenter

> Otherwise, if we want something like a setting for this purpose,
> we should probably use a different property for DT binding, bool
> type "disable-on-boot" for example.
> 
> > > Meanwhile, I believe the enable nodes are necessary in either way as
> > > users could decide to disable the connected channels, based on their
> > > use cases, to save power.
> > > 
> > Agreed, though I would not say "necessary". "Useful" seems to be more
> > appropriate.
> 
> Yea..

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