linux-kernel - Re: [PATCH v15 2/7] power: add power sequence library

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Date:   Thu, 15 Jun 2017 14:58:54 +0800
From:   Peter Chen <hzpeterchen@...il.com>
To:     Ulf Hansson <ulf.hansson@...aro.org>
CC:     Peter Chen <peter.chen@....com>,
        Mark Rutland <mark.rutland@....com>,
        Heiko Stuebner <heiko@...ech.de>,
        Stephen Boyd <stephen.boyd@...aro.org>, <frank.li@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Gary Bisson <gary.bisson@...ndarydevices.com>,
        Fabio Estevam <festevam@...il.com>,
        Joshua Clayton <stillcompiling@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
        Vaibhav Hiremath <vaibhav.hiremath@...aro.org>,
        Krzysztof Kozlowski <krzk@...nel.org>, <mka@...omium.org>,
        Alan Stern <stern@...land.harvard.edu>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>,
        Pawel Moll <pawel.moll@....com>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        <troy.kisky@...ndarydevices.com>, Rob Herring <robh+dt@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>, <hverkuil@...all.nl>,
        <oscar@...andei.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux USB List <linux-usb@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Sebastian Reichel <sre@...nel.org>,
        Mark Brown <broonie@...nel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Shawn Guo <shawnguo@...nel.org>, <jun.li@....com>
Subject: Re: [PATCH v15 2/7] power: add power sequence library

On Wed, Jun 14, 2017 at 10:53:29AM +0200, Ulf Hansson wrote:
> On 14 June 2017 at 03:53, Peter Chen <hzpeterchen@...il.com> wrote:
> > On Tue, Jun 13, 2017 at 12:24:42PM +0200, Ulf Hansson wrote:
> >> [...]
> >>
> >> > +
> >> > +/**
> >> > + * of_pwrseq_on - Carry out power sequence on for device node
> >> > + *
> >> > + * @np: the device node would like to power on
> >> > + *
> >> > + * Carry out a single device power on.  If multiple devices
> >> > + * need to be handled, use of_pwrseq_on_list() instead.
> >> > + *
> >> > + * Return a pointer to the power sequence instance on success,
> >> > + * or an error code otherwise.
> >> > + */
> >> > +struct pwrseq *of_pwrseq_on(struct device_node *np)
> >> > +{
> >> > +       struct pwrseq *pwrseq;
> >> > +       int ret;
> >> > +
> >> > +       pwrseq = pwrseq_find_available_instance(np);
> >> > +       if (!pwrseq)
> >> > +               return ERR_PTR(-ENOENT);
> >>
> >> In case the pwrseq instance hasn't been registered yet, then there is
> >> no way to deal with -EPROBE_DEFER properly here.
> >>
> >> I haven't been following the discussions in-depth during all
> >> iterations, so perhaps you have already discussed why doing it like
> >> this.
> >
> > Yes, it has been discussed. In order to compare with compatible string
> > at dts, we need to have one registered pwrseq instance for each
> > pwrseq library, this pre-registered one is allocated using
> > postcore_initcall, and the new (eg, second) instance is registered
> > after pwrseq_get has succeeded.
> 
> I understand you need one compatible per pwrseq library, but how does
> that have anything to do with -EPROBE_DEFER?
> 
> My point is that, if a driver calls of_pwrseq_on() (which calls
> pwrseq_find_available_instance()), but the corresponding pwrseq
> library and instance has not yet been registered for that device. Then
> how will you handle -EPROBE_DEFER? I guess you simply can't, which is
> why *all* pwrseq libraries needs to be registered in early boot phase,
> like at postcore_initcall(). Right?
> 
> If that is the case, I really don't like it.
> 

Yes, you are right. This is the limitation for this power sequence
library, the registration for the 1st power sequence instance must
be finished before device driver uses it. I am appreciated that
you can supply some suggestions for it.

> Moreover, I have found yet another severe problem but reviewing the code:
> In the struct pwrseq, you have a "bool used", which you are setting to
> "true" once the pwrseq has been hooked up with the device, when a
> driver calls of_pwrseq_on(). Setting that variable to true, will also
> prevent another driver from using the same instance of the pwrseq for
> its device. So, to cope with multiple users, you register a new
> instance of the same pwrseq library that got hooked up, once the
> ->get() callback is about to complete.
> 
> The problem the occurs, when there is another driver calling
> of_pwrseq_on() in between, meaning that the new instance has not yet
> been registered. This will simply fail, won't it?

Yes, you are right, thanks for pointing that, I will add mutex_lock for
of_pwrseq_on.

> 
> Sorry for jumping in late, however to me it seems like there is still
> some pieces missing to make this work.
> 
> [...]
> 
> Kind regards
> Uffe

-- 

Best Regards,
Peter Chen