Date:   Mon, 19 Jun 2017 10:09:58 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Peter Chen <hzpeterchen@...il.com>
Cc:     Peter Chen <peter.chen@....com>,
        Mark Rutland <mark.rutland@....com>,
        Heiko Stuebner <heiko@...ech.de>,
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Subject: Re: [PATCH v15 2/7] power: add power sequence library

On 15 June 2017 at 12:06, Peter Chen <hzpeterchen@...il.com> wrote:
> On Thu, Jun 15, 2017 at 11:35:20AM +0200, Ulf Hansson wrote:
>> On 15 June 2017 at 11:11, Peter Chen <hzpeterchen@...il.com> wrote:
>> > On Thu, Jun 15, 2017 at 10:11:45AM +0200, Ulf Hansson wrote:
>> >> > 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.
>> >>
>> >> In general this kind of problems is solved by first parsing the DTB,
>> >> which means you will find out whether there is a resource (a pwrseq)
>> >> required for the device. Then you try to fetch that resource, and if
>> >> that fails, it means the resource is not yet available, and hence you
>> >> want to retry later and should return -EPROBE_DEFER.
>> >>
>> >> In this case, of_pwrseq_on() needs to be converted to start looking
>> >> for a pwrseq compatible in it's child node - I guess. Then if that is
>> >> found, you try to fetch the instance of the corresponding library.
>> >> Failing to fetch the library instance should then cause a return
>> >> -EPROBE_DEFER.
>> >
>> > The most difficulty for this is we can't know whether the requested
>> > pwrseq instance will be registered or not, the kernel configuration
>> > for this pwrseq library may not be chosen at all.
>>
>> In such case it is still correct to return -EPROBE_DEFER, because the
>> driver that tries to probe its device will fail unless it can run the
>> needed pwrseq. Right?
>>
>
> Unlike the MMC design, there is no dts entry to indicate whether this
> device needs pwrseq or not at this design, it will only carry out power
> on sequence after matching. So, return -EPROBE_DEFER may not work since
> this device may never need pwrseq.

Then, how will you really be able to fetch the correct pwrseq library
instance for the device node?

Suppose their is a *list* of pwrseq library instances available. In
pwrseq_find_available_instance() you call of_match_node(table, np).
The "table" there corresponds to the compatible for the pwrseq library
and the np is the device node provided by the caller of
of_pwrseq_on().

Why is this match done?

Why can't the match be done before trying to fetch a library instance
and then in a second step, really try to fetch the instance? If only
the second step fails, returning -EPROBE_DEFER can be done, no?

BTW, I didn't compatible for the generic pwrseq library being
documented in this series.

>
>> >
>> >>
>> >> >
>> >> >> 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.
>> >>
>> >> Another option is to entirely skip to two step approach.
>> >>
>> >> In other words, make the library to cope with multiple users via the
>> >> same registered library instance.
>> >>
>> >
>> > No, the pwrseq instance stores dtb information (clock, gpio, etc), it
>> > needs to be per device.
>>
>> I think you misunderstood my suggestion here. Of course you need to
>> allocate one pwrseq data per device.
>>
>> However, my point is that you shouldn't need more than one instance of
>> the library functions to be registered in the list of available pwrseq
>> libraries.
>>
>
> This additional instance is used to store compatible information for
> this pwrseq library, it is used for the next matching between device
> and pwrseq library, it just likes we need the first pwrseq instance
> registered at boot stage.

Why can't the compatible information be a static table, known by the
pwrseq core library?

Then when of_pwrseq_on() is called, that static table is parsed and
matched, then a corresponding pwrseq library instance tries to be
fetched.

Kind regards
Uffe

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