linux-kernel - Re: [GPIO]implement sleeping GPIO chip removal

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Message-ID: <4CDDA79F.1060701@o2.pl>
Date:	Fri, 12 Nov 2010 21:46:23 +0100
From:	Maciej Szmigiero <mhej@...pl>
To:	Grant Likely <grant.likely@...retlab.ca>
CC:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org,
	Anton Vorontsov <avorontsov@...mvista.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Uwe Kleine-K?nig <u.kleine-koenig@...gutronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	Jonathan Cameron <jic23@....ac.uk>,
	Ben Nizette <bn@...sdigital.com>
Subject: Re: [GPIO]implement sleeping GPIO chip removal

W dniu 10.11.2010 23:14, Grant Likely pisze:
> On Wed, Nov 10, 2010 at 10:01:40PM +0100, Maciej Szmigiero wrote:
>> W dniu 10.11.2010 21:42, Thomas Gleixner pisze:
>>> On Wed, 10 Nov 2010, Maciej Szmigiero wrote:
>>>> W dniu 10.11.2010 10:49, Thomas Gleixner pisze:
>>>>> Maybe because it open codes a sloppy refcounting with a loop and magic
>>>>> sleeps instead of converting the code to kobjects and proper
>>>>> refcounting ?
>>>>>
>>>>
>>>> The only way to do GPIO chip removal in the current code is to busy-loop.
>>>> "Sloppy" (as you called it) waiting is still more CPU-friendly than looping 
>>>> in hope that somebody will finally release the chip.
>>>> If you would like to implement it as kobject then go ahead and post the code
>>>> so it can be used in drivers.
>>>
>>> Wait a moment. You are getting something backwards here.
>>>
>>> Fact is that the current code is not designed for easy hotunplugging
>>> and therefor requires looping.
>>>
>>> So _you_ propose a work-around to replace the busy-loop by a sleeping
>>> loop with "hope that ....". Hope is the least thing what counts in
>>> programming.
>>>
>>> Now a reviewer tells you that your idea of replacing the busy-loop by
>>> a sleeping in hope loop is flawed, because it does not solve the
>>> underlying design problem of the GPIO code. And you get a suggestion
>>> how to solve it correctly.
>>>
>>> Now you go and request from that reviewer to implement that? That's
>>> not how it works.
>>>
>>> You sent a flawed patch in the first place and people try to tell you
>>> how to do it right. Then it's on you to either go and do it right or
>>> at least ask politely for help and pointers.
>>>
>>> Thanks,
>>>
>>> 	tglx
>>>
>>
>> You misunderstood me.
>> By "looping in hope that somebody will finally release the chip" I meant the only
>> real way to handle a GPIO chip unplugging in the current kernel.
>> Which is way worse that preventing new requests, then waiting for existing one to be released.
>> And this is exactly what my patch does.
>>
>> I understand that it could be simplified by removing redundant code (as Grant Likely had suggested before), and
>> moving it to completion interface instead of manipulating a task structure directly, but this doesn't mean
>> that the whole GPIO code has to be rewritten just to add one functionality.
> 
> BTW, switching to using a kobject != rewriting the whole GPIO code.
> kobjects are not all that scary and the biggest impact is adding
> kobject get/put operation on the gpio get/release apis.  I don't
> expect it to end up being overly complex.
> 
> The Documentation/kobject.txt file should offer some insight.
> 
> g.
> 

I see two approaches to porting GPIO code in current form to krefs:
either per-GPIO kref or per-chip one.

Per GPIO:
- Because every GPIO can be requested only once the refcount will always
be 1 or 2 so refcounting looks a bit like an overkill,

- Every GPIO still needs FLAG_REQUESTED to prevent being requested more
than once at the same time and to support non-blocking chip removal
without reading refcount from inside of the kref structure,

- Still needs some kind of "dead" indication per chip so nobody can
request another GPIO of this chip while it's being removed.
FLAG_REQUESTED cannot be used for this purpose by itself because then it
will be impossible to differentiate between a GPIO which has this flag
to prevent new requests and one which is currently busy.

- After every wakeup the removal code will have to scan all GPIOs under
the chip being removed and go back to sleep if any of them is still
requested.

Per chip:
- This time refcount might go up to chip->ngpio + 1 (number of GPIOs in
this chip).

- Still needs per GPIO FLAG_REQUESTED,

- This time however the removal code will be woken up just once, so it's
more efficient there.

Any other ideas or thoughts?

Maciej Szmigiero
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