linux-kernel - Re: [PATCH v3 2/2] hwmon: ina3221: Read channel input source info from DT

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Message-ID: <20180923003841.GA10071@Asurada-Nvidia.nvidia.com>
Date:   Sat, 22 Sep 2018 17:38:42 -0700
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     jdelvare@...e.com, robh+dt@...nel.org, mark.rutland@....com,
        corbet@....net, afd@...com, linux-hwmon@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v3 2/2] hwmon: ina3221: Read channel input source info
 from DT

On Sat, Sep 22, 2018 at 04:59:55PM -0700, Guenter Roeck wrote:

> > > > +	/* Disable channels if their inputs are disconnected */
> > > > +	for (i = 0, mask = 0; i < INA3221_NUM_CHANNELS; i++) {
> > > > +		if (ina->inputs[i].disconnected)
> > > > +			mask |= INA3221_CONFIG_CHx_EN(i);
> > > > +	}
> > > 
> > > Consequently, you should also _enable_ channels which are not explicitly disabled.
> > 
> > The register has enabled all channels by default. So I felt it'd
> > be neat to have disabling code only. My v1 actually had enabling
> > part as well, but I can add it back if you think it'd be better.
> > 
> > > This can be tricky since you'll have to distinguish non-DT and DT configuration
> > > and retain the original configuration if no channel configuration data is available
> > > from devicetree.

> > For non-DT configurations, input->disconnected is always false by
> > default unless someone adds config for it (through platform_data).
> > If regmap_update_bits only does disabling like this version does,
> > non-DT configurations will not get affected since mask = 0. Or if
> > we change it to do both enabling and disabling, regmap_update_bits
> > will still ignore since there's no register value changed, though
> > it won't really hurt even if regmap writes correct configurations
> > to the register.
> > 
> > For DT configurations (without channel input source defined), it's
> > like the same as non-DT configurations. As we have platforms only
> > enabled ina3221 via DT while they don't have this new DT binding,
> > the driver has to be backward compatible, so my change only sets
> > input->disconnected=true when a status="disabled" is present, i.e.
> > those platforms are treated as all channels getting enabled until
> > they update their DTs.

> I think your assumption may be that the chip is always in its reset state
> when Linux is loaded. This is not necessarily the case; it may be
> preconfigured by BIOS or ROMMON, or even by someone using i2cset before
> loading the driver. If you add enable/disable functionality, you can
> not make an assumption about the original state of the chip at probe time;
> you have to read it from the chip itself.

I see. That made a point. In that case, I think the simplest way
is probably to do software reset before having configurations.

The "disconnected" here is to describe the physical connection,
not exact the switch control status because a channel could be
connected but disabled. However, a channel would not be enabled
if it's disconnected. So I think it'd be safe to just disable
the disconnected channels here as this version does, meanwhile,
I will add a soft reset to make sure all channels are enabled.

Thanks
Nicolin