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

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Message-ID: <f9a296b0ac071eccec02a1c6ef9813b8@codeaurora.org>
Date:   Fri, 17 Nov 2017 16:31:22 +0530
From:   kgunda@...eaurora.org
To:     Lee Jones <lee.jones@...aro.org>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.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 2017-11-17 14:03, Lee Jones wrote:
> 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.

Hi Lee,
Thanks for the feedback! The regmap difference is not confined to couple 
of registers.
Except the one register all the registers have some difference in offset 
or bitmap or
config values as compared to the pm8941. Below is the table for the 
reference. The below
table covers only the registers those exist in the pm8941 driver, if we 
keep adding the
other features the changes may be huge. Apart from this I have mentioned 
other feature
differences between pm8941 and pm8998 as well in response to Bjorn. 
Please refer that
as well.

Register	             Compatibility between 8998 Vs 8941
========                     
===================================================
WLED_MODULE_ENABLE	       Register address offset is same and config 
values match
WLED1_ILED_SYNC_BIT	       Register address offset and config values not 
matching.
WLED1_SWITCHING_FREQUENCY      Register address offset and config values 
are matching.
                                But there is an extra override bit in 
pm8998.
WLED1_WLED_OVP	               Register address offset same. But config 
values are not matching.
WLED1_WLED_ILIM	               Register address offset is same. But 
config values are not matching
WLED1_EN_CURRENT_SINK	       Both register address offset and config 
values are not matching.
WLED1_LED1_MODULATOR_EN	       Both register address offset and config 
values are not matching.
WLED1_LED1_FULL_SCALE_CURRENT  Both register address offset and config 
values are not matching.
WLED1_LED1_MODULATOR_SRC_SE    Register address offset not matching, but 
the config values are matching
WLED1_LED1_CABC_EN	       Register address offset not matching, but the 
config values are not matching.

Thanks,
Kiran