linux-kernel - Re: [PATCH V1 1/4] qcom: spmi-wled: Add support for qcom wled driver

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Date:   Fri, 17 Nov 2017 08:33:42 +0000
From:   Lee Jones <lee.jones@...aro.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     kgunda@...eaurora.org, linux-arm-msm@...r.kernel.org,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Richard Purdie <rpurdie@...ys.net>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Pavel Machek <pavel@....cz>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        linux-leds@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fbdev@...r.kernel.org,
        linux-arm-msm-owner@...r.kernel.org
Subject: Re: [PATCH V1 1/4] qcom: spmi-wled: Add support for qcom wled driver

On Thu, 16 Nov 2017, Bjorn Andersson wrote:

> On Thu 16 Nov 22:36 PST 2017, kgunda@...eaurora.org wrote:
> 
> > On 2017-11-16 22:25, Bjorn Andersson wrote:
> > > On Thu 16 Nov 04:18 PST 2017, Kiran Gunda wrote:
> > > 
> > > > WLED driver provides the interface to the display driver to
> > > > adjust the brightness of the display backlight.
> > > > 
> > > 
> > > Hi Kiran,
> > > 
> > > This driver has a lot in common with the already upstream pm8941-wled.c,
> > > because it's just a new revision of the same block.
> > > 
> > > Please extend the existing driver rather than providing a new one
> > > (and yes, renaming the file is okay).
> > > 
> > > Regards,
> > > Bjorn
> > 
> > Hi Bjorn,
> > 
> > Yes this driver design is similar to pm8941, however the WLED HW block
> > has undergone quite a few changes in analog and digital from PM8941 to
> > PM8998.
> 
> I can see that, looking at the documentation.
> 
> > Few of them include splitting one module into wled-ctrl and wled-sink
> > peripherals, changes in the register offsets and the bit
> > interpretation.
> 
> This is typical and something we need to handle in all these drivers, to
> avoid having one driver per platform.
> 
> > Hence we concluded that it was better to have a new driver to support
> > this new gen WELD module and decouple it from the pm8941.
> 
> Okay, I can see how it's easier to not have to case about anything but
> pmi8998 in this driver, but where do you add the support for other WLED
> versions? What about PMI8994? Will there not be similar differences
> (registers that has moved around) in the future?
> 
> > Also, going forward this driver will support AMOLED AVDD rail (not
> > supported by pm8941) touching a few more registers/configuration and
> > newer PMICs.
> 
> Is this a feature that was introduced in PMI8998? Will this support not
> be dependent on the pmic version?
> 
> > So spinning off a new driver would make it cleaner and easier to
> > extend further.
> > 
> 
> It's for sure easier at this point in time, but your argumentation
> implies that PMI8998+1 should go into it's own driver as well.
> 
> I suspect that if you're going to reuse this driver for future PMIC
> versions you will have to deal with register layout differences and new
> feature set, and as such I'm not convinced that a new driver is needed.
> 
> Can you give any concrete examples of where it is not possible or
> undesirable to maintain the pm8941 support in the same driver?

I agree with Bjorn.  If you can support multiple devices in a single
driver with a couple of simple ddata struct differences and a slightly
different regmap, you should.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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