Re: [PATCH 1/3] iio: core: Add devm_ APIs for iio_channel_{get,release}

From: Laxman Dewangan <ldewangan@nvidia.com>
To: Jonathan Cameron <jic23@kernel.org>,
	Daniel Baluta <daniel.baluta@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>,
	Hartmut Knaack <knaack.h@gmx.de>,
	Lars-Peter Clausen <lars@metafoo.de>,
	Peter Meerwald-Stadler <pmeerw@pmeerw.net>,
	<linux-doc@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>
Subject: Re: [PATCH 1/3] iio: core: Add devm_ APIs for iio_channel_{get,release}
Date: Sun, 10 Apr 2016 23:05:10 +0530	[thread overview]
Message-ID: <570A8ECE.8030007@nvidia.com> (raw)
In-Reply-To: <570A5DB3.3080805@kernel.org>

On Sunday 10 April 2016 07:35 PM, Jonathan Cameron wrote:
> On 06/04/16 15:58, Laxman Dewangan wrote:
>> Hi Daniel,
>>
>>
>> On Wednesday 06 April 2016 07:19 PM, Daniel Baluta wrote:
>>> On Wed, Apr 6, 2016 at 1:31 PM, Laxman Dewangan <ldewangan@nvidia.com> wrote:
>>>> Some of kernel driver uses the IIO framework to get the sensor
>>>> value via ADC or IIO HW driver. The client driver get iio channel
>>>> by iio_channel_get() and release it by calling iio_channel_release().
>>>>
>>>> Add resource managed version (devm_*) of these APIs so that if client
>>>> calls the devm_iio_channel_get() then it need not to release it explicitly,
>>>> it can be done by managed device framework when driver get un-binded.
>>>>
>>>> This reduces the code in error path and also need of .remove callback in
>>>> some cases.
>>>>
>>> Please provide at least one example of code that uses this API.
>> Most of client for this APIs are in other subsystem.
>> When I was working on the patch
>> [PATCH 2/2] thermal: generic-adc: Add ADC based thermal sensor driver
>>
>> if I have devm_iio_channel_get() then I can get .remove callback at all.
>>
>> I did not use this new APIs in my patch because they are in different subsystem.
> It's actually worse than that having taken a quick look at the generic-adc thermal patch
> you reference above.
> (perhaps worth cc'ing linux-iio for next version of that).
Sure. I will CC.

>
> Without this devm function set you have a race in remove in which I think you can
> get attempts to access the channels after they have been released...
Yaah, possibly race for very small time possible.

The limitation of devm_ api usage is that, we can keep using this till 
we have devm_ api continuous and if some resource are not there for 
devm_ then we can not use further.
Possibly, I need to wait for the devm_iio_channel_get() to merge and 
available for all subsystem to use (next release) and then only I can 
use devm_thermal_zone_of_sensor_register().