Date:   Sat, 22 Sep 2018 20:39:46 -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 08:33:00PM -0700, Nicolin Chen wrote:
> On Sat, Sep 22, 2018 at 07:07:02PM -0700, Guenter Roeck wrote:
> > On 09/22/2018 05:38 PM, 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.
> > >
> > No. If the chip was configured by the BIOS/ROMMON, it is supposed
> > to be that way. We can not just override that.
> 
> For this driver, it does soft reset in the probe() so we're
> sure that all channels are enabled at the moment of calling
> this regmap_update_bits. So there's no assumption anymore.
> 
> But the case that you mentioned is a good one. It does give
> me some insight about the use case and the things that will
> need to be careful when adding in[123]_enable. Just it'd be
> also possible that BIOS could disable a channel that is not
> explicitly disabled in the DT -- then the driver should not
> enable it.
> 
> Therefore, for both cases, it seems that disabling only the
> disconnected channels as this version is a safe solution.
> 
> And the driver actually won't update input->disconnected as
> this is a physical hardware status that won't be changed. I

[...]
> probably could define the input->disconnected in const type.
[...]

Please ignore this line. It'd be read-only then. I got myself
confused as it's not a pointer like label.

> For in[123]_enable, it'd be safer to read/write the register
> directly.
> 
> Thanks
> Nicolin
> 
> > >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
> > >
> > 

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