linux-kernel - Re: [PATCH 2/2] iio: ti_am335x_adc: Add continuous sampling support

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Date:	Wed, 18 Sep 2013 18:05:12 +0100
From:	Jonathan Cameron <jic23@...nel.org>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
CC:	"Zubair Lutfullah :" <zubair.lutfullah@...il.com>,
	linux-iio@...r.kernel.org, linux-input@...r.kernel.org,
	linux-kernel@...r.kernel.org, bigeasy@...utronix.de,
	gregkh@...uxfoundation.org
Subject: Re: [PATCH 2/2] iio: ti_am335x_adc: Add continuous sampling support



Dmitry Torokhov <dmitry.torokhov@...il.com> wrote:
>On Wed, Sep 18, 2013 at 05:12:02PM +0100, Jonathan Cameron wrote:
>> 
>> 
>> Dmitry Torokhov <dmitry.torokhov@...il.com> wrote:
>> >On Wed, Sep 18, 2013 at 10:39:42AM +0100, Jonathan Cameron wrote:
>> >> 
>> >> 
>> >> "Zubair Lutfullah :" <zubair.lutfullah@...il.com> wrote:
>> >> >On Tue, Sep 17, 2013 at 09:27:27PM -0700, Dmitry Torokhov wrote:
>> >> >> Hi Zubair,
>> >> >> 
>> >> >> On Tue, Sep 17, 2013 at 09:44:07AM +0500, Zubair Lutfullah
>wrote:
>> >> >> > +
>> >> >> > +	ret = devm_request_threaded_irq(indio_dev->dev.parent,
>> >> >> > +				irq,
>> >> >> > +				pollfunc_th, pollfunc_bh,
>> >> >> > +				flags, indio_dev->name,
>> >> >> > +				indio_dev);
>> >> >> > +	if (ret)
>> >> >> > +		goto error_kfifo_free;
>> >> >> > +
>> >> >> > +	indio_dev->setup_ops = setup_ops;
>> >> >> > +	indio_dev->modes |= INDIO_BUFFER_HARDWARE;
>> >> >> > +
>> >> >> > +	ret = iio_buffer_register(indio_dev,
>> >> >> > +				  indio_dev->channels,
>> >> >> > +				  indio_dev->num_channels);
>> >> >> > +	if (ret)
>> >> >> > +		goto error_free_irq;
>> >> >> > +
>> >> >> > +	return 0;
>> >> >> > +
>> >> >> > +error_free_irq:
>> >> >> > +	devm_free_irq(indio_dev->dev.parent, irq, indio_dev);
>> >> >> 
>> >> >> What is the point of using devm_* here if you are doing
>explicit
>> >> >> management of the resource anyway (you explicitly release it in
>> >all
>> >> >code
>> >> >> paths)?
>> >> >> 
>> >> >> Thanks.
>> >> >> 
>> >> >> -- 
>> >> >> Dmitry
>> >> >
>> >> >I admit I am unaware at the moment about how it works.
>> >> >
>> >> >I use devm and simply ignore the error path?
>> >> >
>> >> >The devm function header description said something about using
>> >> >devm_free when freeing. And this is the way I am used to seeing 
>> >> >error handling.
>> >> 
>> >
>> >> The devm interfaces ensure this is all cleaned when the device is
>> >> removed thus avoiding the need to free the stuff explicitly. 
>Device
>> >> will get freed on deliberate remove and on an error from probe.
>Hence
>> >> you can drop all calls to devm free. The devm free functions are
>only
>> >> needed if you wish to free in order to reallocate. This might
>happen
>> >> if you want to change a buffer size for instance.
>> >
>> >However in this case such conversion us dangerous. With all but IRQ
>> >resource managed by the traditional methods they will be released
>first
>> >with IRQ handler deregistered very last. Therefore if device is not
>> >properly quiesced IRQ raised during driver unbinding is likely to
>> >result
>> >in kernel oops.
>> >
>> >IOW devm_request_irq() is very often evil (it is still useful if
>_all_
>> >your resources are managed by devm_*).
>> >
>> >In case of your driver I'd recommend switching to
>> >request_irq()/free_irq() instead.
>> >
>> >Thanks.
>> 
>> Pretty much all resources are devm managed in here
>> 
>>
>https://git.kernel.org/cgit/linux/kernel/git/jic23/iio.git/tree/drivers/iio/adc/ti_am335x_adc.c?h=togreg&id=7a1aeba7ed0d5a1e83fd5a8ee2a2869430d40347
>
>
>So we are guaranteed that that new kfifo that is being allocated just
>before we requesting IRQ and will be freed way before we free the IRQ
>will not be used by the IOTQ handler?
>
>Thanks.
Good point. I forgot about that.  The source of interrupts 'should' be disabled before the kfifo is freed but I guess perhaps better to play it safe. Would take a fair bit of review to be sure that is not going to cause grief.

A few more devm handlers need writing before it is truly useful here.

Thanks for pointing this out.

J

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