linux-kernel - Re: [PATCH 2/8] power: add power sequence library

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Date:   Sat, 4 Feb 2017 09:56:18 +0800
From:   Peter Chen <hzpeterchen@...il.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Chen <peter.chen@....com>,
        Alan Stern <stern@...land.harvard.edu>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        Sebastian Reichel <sre@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
        Heiko Stuebner <heiko@...ech.de>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>, p.zabel@...gutronix.de,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Pawel Moll <pawel.moll@....com>,
        Mark Rutland <mark.rutland@....com>,
        "open list:ULTRA-WIDEBAND (UWB) SUBSYSTEM:" 
        <linux-usb@...r.kernel.org>, Arnd Bergmann <arnd@...db.de>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        mail@...iej.szmigiero.name, troy.kisky@...ndarydevices.com,
        Fabio Estevam <festevam@...il.com>, oscar@...andei.net,
        Stephen Boyd <stephen.boyd@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Joshua Clayton <stillcompiling@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        mka@...omium.org, Vaibhav Hiremath <vaibhav.hiremath@...aro.org>,
        Gary Bisson <gary.bisson@...ndarydevices.com>,
        hverkuil@...all.nl, Krzysztof Kozlowski <krzk@...nel.org>
Subject: Re: [PATCH 2/8] power: add power sequence library

On Fri, Feb 03, 2017 at 10:14:31PM +0100, Rafael J. Wysocki wrote:
> On Friday, February 03, 2017 04:16:15 PM Peter Chen wrote:
> > On Wed, Feb 01, 2017 at 09:08:17AM +0100, Greg Kroah-Hartman wrote:
> > > On Wed, Feb 01, 2017 at 12:10:17AM +0100, Rafael J. Wysocki wrote:
> > > > On Tue, Jan 3, 2017 at 7:33 AM, Peter Chen <peter.chen@....com> wrote:
> > > > > We have an well-known problem that the device needs to do some power
> > > > > sequence before it can be recognized by related host, the typical
> > > > > example like hard-wired mmc devices and usb devices.
> > > > >
> > > > > This power sequence is hard to be described at device tree and handled by
> > > > > related host driver, so we have created a common power sequence
> > > > > library to cover this requirement. The core code has supplied
> > > > > some common helpers for host driver, and individual power sequence
> > > > > libraries handle kinds of power sequence for devices. The pwrseq
> > > > > librares always need to allocate extra instance for compatible
> > > > > string match.
> > > > >
> > > > > pwrseq_generic is intended for general purpose of power sequence, which
> > > > > handles gpios and clocks currently, and can cover other controls in
> > > > > future. The host driver just needs to call of_pwrseq_on/of_pwrseq_off
> > > > > if only one power sequence is needed, else call of_pwrseq_on_list
> > > > > /of_pwrseq_off_list instead (eg, USB hub driver).
> > > > >
> > > > > For new power sequence library, it can add its compatible string
> > > > > to pwrseq_of_match_table, then the pwrseq core will match it with
> > > > > DT's, and choose this library at runtime.
> > > > >
> > > > > Signed-off-by: Peter Chen <peter.chen@....com>
> > > > > Tested-by: Maciej S. Szmigiero <mail@...iej.szmigiero.name>
> > > > > Tested-by Joshua Clayton <stillcompiling@...il.com>
> > > > > Reviewed-by: Matthias Kaehlcke <mka@...omium.org>
> > > > > Tested-by: Matthias Kaehlcke <mka@...omium.org>
> > > > 
> > > > Quite honestly, I have a really hard time with trying to follow this
> > > > code and the total lack of documentation makes it even harder. 
> > 
> > Sorry about that, Is it ok I add the design doc at:
> > Documentation/power/power-sequence/design.rst?
> 
> You can do that if you think it will address the request to explain the design.
> 
> > > > particular, the generic power sequence code is not even commented at
> > > > all, 
> > 
> > The generic power sequence code just implements the APIs which are 
> > called at power/pwrseq/core.c, and those API are commented at
> > include/linux/power/pwrseq.h. Anyway, I will add more comments at it.
> 
> It actually seems to be doing more than that and I'm not sure why the code in
> core.c is necessary at all.  The "generic" thing seems to be the only user of
> it anyway and the callbacks seem to be tailored to its needs.
> 

Currently, the "generic" pwrseq is the only use case. But some devices may have
custom power sequence [1], and MMC devices (mmc card/wifi) have two
power sequences (drivers/mmc/core/pwrseq_emmc.c, drivers/mmc/core/pwrseq_simple.c)

I have an example use case at v7 named pwrseq_compatible_sample.c for
those custom use case [2].

[1] https://www.spinics.net/lists/devicetree/msg139756.html
[2] https://lkml.org/lkml/2016/9/19/972

-- 

Best Regards,
Peter Chen