linux-kernel - Re: [PATCH] mfd: qcom-spmi-pmic: Add support for more chips versions

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Date:	Fri, 07 Nov 2014 17:33:10 +0200
From:	"Ivan T. Ivanov" <iivanov@...sol.com>
To:	Bjorn Andersson <bjorn@...o.se>
Cc:	Rob Herring <robh+dt@...nel.org>, Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Lee Jones <lee.jones@...aro.org>,
	Stanimir Varbanov <svarbanov@...sol.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-arm-msm <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH] mfd: qcom-spmi-pmic: Add support for more chips versions


On Thu, 2014-11-06 at 08:55 -0800, Bjorn Andersson wrote:
> On Wed, Nov 5, 2014 at 11:54 PM, Ivan T. Ivanov <iivanov@...sol.com> wrote:
> > On Wed, 2014-11-05 at 17:36 -0800, Bjorn Andersson wrote:
> > > On Wed, Nov 5, 2014 at 10:31 AM, Ivan T. Ivanov <iivanov@...sol.com> wrote:
> [..]
> > > > Some of the child device drivers have to know PMIC chip revision.
> > > > 
> > > 
> > > So your plan is to have a strstr(parent->compatible, "-v2") there?
> > 
> > Actually also PMIC subtype (pm8841, pm8226...) is also required, so
> > the plan is to have something like this:
> > 
> > {
> >         static const struct of_device_id pmic_match_table[] = {
> >                 { .compatible = "qcom,pm8941-v1.0" },
> >                 { .compatible = "qcom,pm8841-v0.0" },
> >                 { }
> > 
> >         };
> > 
> >         const struct of_device_id *match;
> > 
> >         match = of_match_device(pmic_match_table, pdev->dev.parent);
> >         if (match) {
> >                 dev_info(&pdev->dev, "%s chip detected\n", match->compatible);
> >         }
> > }
> > 
> 
> To me this is a hack, you should not alter the devicetree to make it
> "better express the hardware". Either you know these things from boot
> and they go in device tree, or you can probe them and they should not
> go in device tree.
> 
> If you really need these values you should expose them through some api.

I would like to avoid compile time dependency between these drivers.
There are several precedents of using of_update_property() for enhancing
compatible property already.  

> 
> > > Could you be a little bit more elaborate on what you're trying to do
> > > and which child devices that might be?
> > 
> > For example ADC drivers are required temperature compensation based
> > on PMIC variant and chip manufacturer.
> > 
> 
> I see, is that compensation of any practical value? Or is the
> compensation of academic proportions?

It depends of what you mean by academic :-). Attached file have test
application which dump difference between non compensated and compensated
values for different temperature, manufacture and input value.

Output format of the program is:
Column 1: manufacturer GF=0, SMIC=1, TSMC=2
Column 2: chip revision
Column 3: die temperature in mili deg Celsius 
Column 4: input for compensation in micro Volts
Column 5: compensated result in micro Volts
Column 6: difference in micro Volts


Regards,
Ivan

View attachment "temperature-compensation-check.c" of type "text/x-csrc" (14107 bytes)