linux-kernel - Re: [PATCH v3] iio: adc: xilinx-xadc: Push interrupts into threaded context

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Date:	Tue, 04 Aug 2015 10:05:00 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Shubhrajyoti Datta <shubhrajyoti.datta@...il.com>
CC:	Xander Huff <xander.huff@...com>, jic23@...nel.org,
	bigeasy@...utronix.de, knaack.h@....de,
	Peter Meerwald <pmeerw@...erw.net>,
	Michal Simek <michal.simek@...inx.com>,
	Sören Brinkmann 
	<soren.brinkmann@...inx.com>, linux-iio@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	joe.hershberger@...com, joshc@...com, nathan.sullivan@...com,
	jaeden.amero@...com
Subject: Re: [PATCH v3] iio: adc: xilinx-xadc: Push interrupts into threaded
 context

On 08/04/2015 07:34 AM, Shubhrajyoti Datta wrote:
> On Fri, Jul 24, 2015 at 6:08 PM, Lars-Peter Clausen <lars@...afoo.de> wrote:
>> Hi,
>>
>> Sorry, but I don't think this patch has been sufficiently tested against a
>> mainline kernel. The driver wont even probe the way it is right now.
>>
>> On 07/21/2015 01:14 AM, Xander Huff wrote:
>>>
>>> The driver currently registers a pair of irq handlers using
>>> request_threaded_irq(), however the synchronization mechanism between the
>>> hardirq and the threadedirq handler is a regular spinlock.
>>
>>
>> If everything runs in threaded context we don't really need the spinlock
>> anymore and can use the mutex throughout.
> 
> that should be better from the performance point of view.
> 
>>
>>>
>>> Unfortunately, this breaks PREEMPT_RT builds, where a spinlock can sleep,
>>> and is thus not able to be acquired from a hardirq handler. This patch
>>> gets
>>> rid of the hardirq handler and pushes all interrupt handling into the
>>> threaded context.
>>
>>
>> We actually might as well run everything in the hardirq handler (which will
>> be threaded in PREEMPT_RT). The reason why we have the threaded handler is
>> because xadc_handle_event() used to sleep, but it doesn't do this anymore.
> 
> The point is why have the hard irq. If we use hardirq then not mutex
> can be used and spinlock will
> be busy.

Well there is no need to use a threaded IRQ. The interrupt handler is quite
small and doesn't take too much time and doesn't have any delays or sleeps
in it either.


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