linux-kernel - Re: of_clk_add_(hw_)providers multipule times for one node?

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Message-ID: <CAK7LNAThC-oBV5qNRNLnkV_tS7JLZDVoeEYrJo_8LsbRftELew@mail.gmail.com>
Date:   Wed, 24 Aug 2016 16:11:41 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Stephen Boyd <sboyd@...eaurora.org>
Cc:     Rob Herring <robh@...nel.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: of_clk_add_(hw_)providers multipule times for one node?

Hi Stephen,


2016-08-12 16:04 GMT+09:00 Masahiro Yamada <yamada.masahiro@...ionext.com>:
> 2016-08-11 8:08 GMT+09:00 Stephen Boyd <sboyd@...eaurora.org>:
>> On 08/10, Masahiro Yamada wrote:
>>> Hi Stephen,
>>>
>>>
>>>
>>> 2016-08-09 8:37 GMT+09:00 Stephen Boyd <sboyd@...eaurora.org>:
>>> > On 08/08, Masahiro Yamada wrote:
>>> >> Hi Stephen,
>>> >>
>>> >>
>>> >> 2016-08-05 6:25 GMT+09:00 Stephen Boyd <sboyd@...eaurora.org>:
>>> >
>>> >>
>>> >> of_clk_add_provider() calls of_clk_del_provider()
>>> >> in its failure path.
>>> >>
>>> >> Notice of_clk_del_provider() unregister
>>> >> all the providers associated with the device node.
>>> >
>>> > Where is that? I see a break statement in the while loop after
>>> > the first matching np is found.
>>>
>>> Ah, I missed the "break".
>>>
>>> So, this works *almost* well.
>>>
>>> I mean *almost* because the of_clk_mutex is released
>>> between of_clk_add_hw_provider() and of_clk_del_provider().
>>>
>>> What if two providers are added concurrently.
>>> I know it never happens in use-cases we assume, though.
>>
>> Agreed, that would be bad. We can definitely do better in that
>> case and properly delete the provider that we have already
>> registered without calling of_clk_del_provider() though. We have
>> everything in the local scope at the time.
>>
>>>
>>>
>>> >>
>>> >> Some platform drivers call of_clk_del_provider() in a .remove callback,
>>> >> so the same problem could happen.
>>> >>
>>> >> Why does of_clk_del_provider() take (struct device_node *np) ?
>>> >> Shouldn't it take (struct of_clk_provider *cp)?
>>> >>
>>> >
>>> > Not sure. Probably someone thought they could hide the structure
>>> > from consumers and just return success or failure.
>>>
>>> consumers?   or did you mean providers?
>>> I think consumers have no chance to call of_clk_del_provider().
>>
>> Sorry, bad choice of words. I meant users of this
>> of_clk_add*_provider() API.
>>
>>>
>>>
>>> > The best we can do is have the framework only return probe defer
>>> > if there isn't a provider registered. Once a provider is
>>> > registered, it needs to do the right thing and return the
>>> > appropriate error (invalid or probe defer for example) at the
>>> > right time.
>>>
>>> Agreed.
>>
>> Ok. I think I will merge my patch then to restore previous
>> behavior.
>>
>>>
>>> Lastly, we have two solutions so far.  Which do you think is better?
>>>
>>> One solution is, as others suggested,
>>> CLK_OF_DECLARE() can allocate a bigger array than it needs,
>>> so that blank entries can be filled by a platfrom_driver later.
>>>
>>>
>>> The other way is,
>>> CLK_OF_DECLARE() and a platfrom_driver
>>> allocate separate of_clk_provider for each of them.
>>>
>>
>> I believe we have precedence for the former case, so there's some
>> momentum around that approach. It doesn't make me feel great
>> though because we have published the provider before all clks are
>> registered, and then we go back and modify the array in place
>> while consumers could potentially be using it. I suppose we're
>> saved because cpus access the pointer in the array and only see
>> the whole pointer and not half of the old one and half of the new
>> one?
>
>
> I am not sure.
>
> But, maybe just filling the blank entries of the array seems safe.
> In this case, filling should be done at the end of the probe callback.
> Otherwise, devm_clk_hw_register() will free the clk_hw when the driver
> is detached.
>

Looks like the whole of my series was rejected,
but I was not sure why the following one was rejected.
https://patchwork.kernel.org/patch/9236563/


Could you explain why -EPROBE_DEFER should be returned
if both .get_hw and .get are missing.


Is there a way to register an OF clk provider without .get(_hw),
but fill it later or something?



-- 
Best Regards
Masahiro Yamada