linux-kernel - Re: [PATCH 5/5] OF: Introduce utility helper functions

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Message-Id: <1F43215E-B68C-4926-832D-CB8DC3533CB2@antoniou-consulting.com>
Date:	Sat, 16 Nov 2013 14:23:15 +0200
From:	Pantelis Antoniou <panto@...oniou-consulting.com>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Rob Herring <robherring2@...il.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Matt Porter <matt.porter@...aro.org>,
	Koen Kooi <koen@...inion.thruhere.net>,
	Alison Chaiken <Alison_Chaiken@...tor.com>,
	Dinh Nguyen <dinh.linux@...il.com>,
	Jan Lubbe <jluebbe@...net.de>,
	Alexander Sverdlin <alexander.sverdlin@....com>,
	Michael Stickel <ms@...able.de>,
	Guenter Roeck <linux@...ck-us.net>,
	Dirk Behme <dirk.behme@...il.com>,
	Alan Tull <delicious.quinoa@...il.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Michael Bohan <mbohan@...eaurora.org>,
	Ionut Nicu <ioan.nicu.ext@....com>,
	Michal Simek <monstr@...str.eu>,
	Matt Ranostay <mranostay@...il.com>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] OF: Introduce utility helper functions

Hi Grant,

On Nov 15, 2013, at 8:27 AM, Grant Likely wrote:

> On Thu, 14 Nov 2013 10:51:05 +0100, Pantelis Antoniou <panto@...oniou-consulting.com> wrote:
>> Hi Grant,
>> 
>> On Nov 14, 2013, at 2:44 AM, Grant Likely wrote:
>> 
>>> On Wed, 13 Nov 2013 10:03:37 +0100, Pantelis Antoniou <panto@...oniou-consulting.com> wrote:
>>>> On Nov 13, 2013, at 2:34 AM, Grant Likely wrote:
>>>>> On Tue, 12 Nov 2013 11:39:08 +0100, Pantelis Antoniou <panto@...oniou-consulting.com> wrote:
>>>>>>> On Tue,  5 Nov 2013 19:50:16 +0200, Pantelis Antoniou <panto@...oniou-consulting.com> wrote:
>>>>>>>> +	} else {
>>>>>>>> +		pr_warn("%s: node %p cannot be freed; memory is gone\n",
>>>>>>>> +				__func__, node);
>>>>>>>> +	}
>>>>>>>> +}
>>>>>>> 
>>>>>>> All of the above is potentially dangerous. There is no way to determine
>>>>>>> if anything still holds a reference to a node. The proper way to handle
>>>>>>> removal of properties is to have a release method when the last
>>>>>>> of_node_put is called.
>>>>>>> 
>>>>>> 
>>>>>> This is safe, and expected to be called only on a dynamically created tree,
>>>>>> that's what all the checks against OF_DYNAMIC guard against.
>>>>>> 
>>>>>> It is not ever meant to be called on an arbitrary tree, created by unflattening
>>>>>> a blob.
>>>>> 
>>>>> I am talking about when being used on a dynamic tree. The problem is
>>>>> when a driver or other code holds a reference to a dynamic nodes, but
>>>>> doesn't release it correctly. The memory must not be freed until all of
>>>>> the references are relased. OF_DYNAMIC doesn't actually help in that
>>>>> case, and it is the reason for of_node_get()/of_node_put()
>>>>> 
>>>> 
>>>> I know, but even that is not enough. of_node_get()/of_node_put() handles the
>>>> case of references to the nodes, but not what happens with references to
>>>> properties. deadprops is mitigating the problem somewhat, but if we're going
>>>> to go to all the trouble of kobjectification let's do the props as well.
>>>> 
>>>> of_get_property could be modified to return a devm_kmalloced copy of the real
>>>> property and that would deal with most of the callers. Of course for
>>>> the small sized scalar data we can avoid the copy.
>>>> 
>>>> By using the devm_* interface we also avoid having to mess too much with the callers.
>>>> 
>>>> I.e. what about something like devm_of_get_property()?
>>> 
>>> Reference counting is already a horrible pain to keep correct. I don't
>>> see a better way to handle it in the dynamic case, so we're stuck with
>>> it, but I don't want to make it any harder. Adding ref counting to
>>> properties will make it harder than it already is to get the code right.
>>> I'm absolutely fine with a little bit of wasted memory in the form of
>>> deadprops when the alternative is so horrible. References at the node
>>> level is enough granularity.
>>> 
>>> I don't think kduping the property is the solution either. I strongly
>>> suspect that will be far more expensive than the deadprop solution.
>>> 
>> 
>> As long as we can live with deadprops all is fine. Perhaps a devm_of_get_property()
>> makes sense for new drivers though? What do you think? Perhaps copying to a
>> user supplied buffer as well?
> 
> I still don't think it is necessary. The device lifetime should always
> be shorter than the node lifetime.
> 
>> It's a kind of drag. That means you get handed a device_node pointer you are not
>> able to free it without having the blob along with the container/accessor of it.
>> I.e. For the normal case where the blob comes from a request_firmware() call
>> You have to keep the firmware structure around.
>> 
>> Depending on what other method you're going to use tends to make the code a little
>> bit messier. 
> 
> Understood. Stick with keeping the blob around for now. It can be
> reworkd in the future if necessary since there are no associated
> userspace ABI issues.
> 
> g.

OK, will do.

Regards

-- Pantelis--
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