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

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Message-ID: <20180923005019.GA11764@Asurada-Nvidia.nvidia.com>
Date:   Sat, 22 Sep 2018 17:50:19 -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 05:38:41PM -0700, Nicolin Chen wrote:
> 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.

Actually...just a couple of lines before this, the probe() does
soft reset:

537         ret = regmap_field_write(ina->fields[F_RST], true);
538         if (ret) {
539                 dev_err(dev, "Unable to reset device\n");
540                 return ret;
541         }

So I think it'd be okay to just keep disabling disconnected
channels only as this version?